In recent years, new x-ray radiographic systems based on large area flat panel technology have revolutionized our capability to produce digital x-ray radiographic images. However, these active matrix flat panel imagers (AMFPI) are extraordinarily expensive compared to the systems they are replacing. Thus there is a clear need for a low cost digital imaging system for general applications in radiology. Different approaches have been considered to make lower cost xray imaging devices for digital radiography, including: reducing the cost of existing flat panel systems, scanned projection x-ray, an approach based on computed radiography (CR) technology, and optically demagnified x-ray screen/camera systems. All of these approaches are quite expensive and none have the image quality of AMFPIs. We have identified a new approach - the X-ray Light Valve (XLV). It combines three well-established technologies: an a- Se layer to convert x-rays to image charge, a liquid crystal (LC) cell, and a scanned digital readout. This device achieves our goal of immediate readout with image quality comparable to an AMFPI, while keeping costs low. The XLV system has been shown to have all the properties required for general radiography.

In this study, we report a multi-beam x-ray imaging system that can generate a scanning x-ray beam to image an object from multiple projection angles without mechanical motion. The key part of this imaging system is a multi-beam field emission x-ray (MBFEX) source which comprises a linear array of gated electron emitting pixels. The pixels are individually addressable via a MOSFET (metal-oxide-semiconductor field effect transistor) based electronic circuit. The device can provide a tube current of 0.1-1 mA at 40 kVp with less than 300 μm focal spot size from each of the emitting pixels. Multilayer images of different phantoms were reconstructed to demonstrate its potential applications in tomographic imaging. Since no mechanical motion is needed and the electronic switching time is generally negligible the MBFEX system has the potential to simplify the system design and lead to a fast data acquisition for tomographic imaging.

The need to assure the image quality of digital systems for mammography screening applications is now widely recognized. One approach is embodied in Part B of the European Protocol for the Quality Control of the Physical and Technical Aspects of Mammography Screening (EPQCM), which prescribes criteria for several interconnected image quality metrics. The focus of this study is on the "threshold contrast visibility" (TCV) protocol (section 2.4.1 of the EPQCM), in which human observers score images of a CDMAM or similar 4-AFC phantom. This section of the EPQCM currently omits many critical experimental details, which must be gleaned from ancillary documents. Given these, the purpose of this study is to quantify the effects of several remaining experimental variables, including phantom design, and the methods used for scoring and analysis, on the measured results. Preliminary studies of two CDMAM version 3.4 (CDMAM 3.4) phantoms have revealed a 17% difference in TCV when averaged over all target diameters from 0.1 to 2.0 mm. This indicates phantom variability may affect results at some sites. More importantly, we have shown that the current CDMAM phantom design, methods for scoring, and analysis, substantially limit the ability to measure system performance accurately and precisely. An improved phantom design has been shown to avoid these limitations. Viewing environment and presentation context affect the performance and efficiency of visual scoring of phantom images. An automated display tool has been developed that isolates individual 4-AFC targets of CDMAM phantom images, automatically optimizes window/level, and automatically records observers' scores. While not substantially changing TCV, the tool has increased scoring efficiency while mitigating several of the limitations associated with unassisted visual scoring. For example, learning bias and navigational issues are completely avoided. Ultimately, software-based ideal observer scoring will likely prove to be a better approach. Statistical-decision-theory-based (SDT) analysis has been shown to mitigate limitations associated with the current CDMAM phantom and the ad hoc nearest-neighbor correcting (NNC) scoring method. NNC analysis is sensitive to the degree of incomplete scoring (stopping criteria). However, SDT substantially mitigates this problem, using all of the available data to derive thresholds that are more interpretable. Bootstrap sampling was used to provide an estimate of the standard error for SDT analysis. In conclusion, the current EPQCM section 2.4.1 protocol fails to measure TCV accurately and precisely enough to qualify digital mammography systems. This paper presents a series of recommendations that supplement section 2.4.1 of the EPQCM and that provide a stable and accurate measure of TCV.

European Guidelines for quality control in digital mammography specify minimum and achievable standards of image quality in terms of threshold contrast, based on readings of images of the CDMAM test object by human observers. However this is time-consuming and has large inter-observer error. To overcome these problems a software program (CDCOM) is available to automatically read CDMAM images, but the optimal method of interpreting the output is not defined. This study evaluates methods of determining threshold contrast from the program, and compares these to human readings for a variety of mammography systems. The methods considered are (A) simple thresholding (B) psychometric curve fitting (C) smoothing and interpolation and (D) smoothing and psychometric curve fitting. Each method leads to similar threshold contrasts but with different reproducibility. Method (A) had relatively poor reproducibility with a standard error in threshold contrast of 18.1 ± 0.7%. This was reduced to 8.4% by using a contrast-detail curve fitting procedure. Method (D) had the best reproducibility with an error of 6.7%, reducing to 5.1% with curve fitting. A panel of 3 human observers had an error of 4.4% reduced to 2.9 % by curve fitting. All automatic methods led to threshold contrasts that were lower than for humans. The ratio of human to program threshold contrasts varied with detail diameter and was 1.50 ± .04 (sem) at 0.1mm and 1.82 ± .06 at 0.25mm for method (D). There were good correlations between the threshold contrast determined by humans and the automated methods.

This paper will present new anatomically adaptable automatic exposure control (AEC), called Flex-AEC⁺, for amorphous selenium (a-Se) full field digital mammography (FFDM) system. The AEC operation is based on a principle where the imaging chain components are all modelled into the system software. Once the imaging parameters are all known it enables the system to exactly define the tissue composition imaged and utilize exposure parameters optimal for it. Based on the detected object composition together with the other imaging parameters the amount of signal produced by the amorphous selenium flat panel is exactly calculated and the desired dose of the exposure on the detector is thereby reached accurately. The AEC consists of 48 individual detection areas that cover a selenium flat panel area of 100 cm². It is therefore able to measure a well representative sample of the tissue to be exposed and adjust the exposure parameters optimal for the breast tissue composition. Clinical benefits of AEC are found because of fully understanding the behaviour of the x-ray beam together with the calculation models of the AEC. This gives better understanding of breast anatomy in all mammography screening and diagnostic cases, and responses to various tissue compositions by optimizing the image quality and dose. The spectrum of the x-ray radiation changes remarkably when passing through the various materials on its path. Optimal image quality and dose requires anatomically adjusted imaging parameters, which will represent the true breast tissue composition taking account in all different glandular tissue in the breast. Based on the detected object composition together with the other imaging parameters the amount of signal produced by the selenium flat panel is exactly calculated, and the desired image quality and dose is reached accurately.

We use a previously reported, optimized quasi-monochromatic beam technique together with unique complex acquisition trajectories made possible with a novel, dedicated cone-beam transmission computed mammotomography (CmT) system to investigate effects of low dose imaging of pendant, uncompressed breasts. Investigators have used a guideline of dose for CmT type applications as that used for dual-view mammography (4-6 mGy for average breast size). This dose is somewhat arbitrary, and it may be possible to reduce this significantly without sacrificing image quality using our quasi-monochromatic x-ray beam, 3D complex acquisition orbits, and iterative reconstruction techniques. A low-scatter acrylic resolution phantom in various media, a breast phantom with sponge and oil-filled lesions, and a cadaver breast are used to evaluate the effect of lowered dose on resolution and image artifacts. Complex saddle acquisition trajectories (necessary to overcome cone-beam distortion) are carried out for total exposures of 96, 300, and 600 mAs over 240 projections. These exposures relate approximately to 1/10^th, 1/3^rd, and 2/3^rd of the standard dual view mammography dose for an average sized 50% adipose/glandular breast. Iterative reconstruction uses an OSTR algorithm with 0.125 mm³ voxels. Image artifacts increased as dose was reduced but did not appear to greatly degrade image quality except at the lowest contrast tested (1% absolute contrast). As expected, noise increased as dose was reduced. However, this did not appear to affect resolution for rods in air (high contrast), nor rods in oil (20% absolute contrast). Resolution was reduced for rods in water (1% absolute contrast) due to increased prevalence of image artifacts as well as increased noise. Breast phantom imaging of soft lesions in a highly glandular breast (6% absolute contrast) clearly yielded the 60uL and all larger volume lesions. Preliminary biological breast tissue results illustrate excellent subjective image quality at all dose levels tested. Results indicate that our quasi-monochromatic beam together with complex orbit capability and iterative reconstruction has the potential to provide sufficient image quality for practical 3D mammotomography of uncompressed breasts at significantly lower dose than dual view mammography. This is nominally a 2-fold improvement over other approaches using circular orbits and broader spectral x-ray beams. While simple image filtering (post-reconstruction smoothing) could improve noise quality, improvements in image artifact correction and scatter correction are required to more accurately determine the lower limits on dose. A contrast-detail study is also warranted with a greater variety of lesion sizes and contrasts.

Monochromatic X-rays have been proposed for medical imaging, especially in the mammographic energy range. Our previous investigations have shown that the contrast of objects such as lesions or contrast media can be enhanced considerably by using monochromatic X-rays instead of the common polychromatic spectra. Admittedly, only one specific polychromatic spectrum and one monochromatic energy have been compared so far. In this work, we investigated the contrast yielded by a series of different X-ray spectra obtained by varying tube voltage and beam filtering. This resulted in spectra of different mean energies and spectral widths. The objects under examination were aqueous solutions containing different chemical elements such as I, Gd, Dy, Yb, and Bi. A monoenergetic spectrum at 17.5 keV was obtained using a mammographic X-ray tube with a Mo anode and a monochromator equipped with a HOPG crystal. Moreover, we simulated quasi-monoenergetic spectra at different energies and with different widths. As a result, we demonstrated that in many cases spectra with an energetic width of some keV yield an equivalent contrast to monoenergetic radiation at the same energy. Therefore, the advantage in image contrast of monochromatic X-rays at 17.5 keV over narrow-band polychromatic X-ray spectra obtained by appropriate filtering is only slight. Thus, the additional expenditure on a mammography system with HOPG monochromator that can deliver only a small X-ray dose and the unfavorable slot-scan geometry can be avoided. Moreover, we carried out simulations of monochromatic versus polychromatic spectra throughout the whole radiographic energy range. We found advantages in using monochromatic X-rays at higher energies and thicker objects that will justify their application for diagnostic imaging in a number of specific cases.

Measurements and simulations of the signal-difference-to-noise ratio (SDNR) and average glandular dose (AGD) have been performed on a photon counting full-field digital mammography system to determine the optimal operating conditions. Several beam qualities were experimentally evaluated by using different combinations of tube voltage, added filters and thickness of BR12 with a tungsten target x-ray tube. The SDNR and AGD were also calculated theoretically for an extended number of operating conditions and a more accurate breast model. As figure of merit for each operating condition, a spectral quantum efficiency (SQE) was calculated as the polychromatic SDNR squared over the optimal monochromatic SDNR squared at the same AGD. The theoretical model agreed within ±4% relative the measured SDNR throughout the evaluated breast thickness (30-70 mm) and tube voltage range (26-38 kV). The optimization was performed with a constant SDNR-rate as compared to using a fixed filter thickness. The optimal combinations of tube voltage-filter material were: 32 kV-Ag, 34 kV-Cd, 36 kV-Sn for a breast thickness of 30, 50 and 70 mm respectively. These K-edge filter materials increased the SQE by less than 4% compared to the optimal Al filtration.

Digital breast tomosynthesis promises solutions to many of the problems associated with projection mammography, including elimination of artifactual densities due to the superposition of normal tissues and increasing the conspicuity of true lesions that would otherwise be masked by superimposed normal tissue. We have investigated tomosynthesis using a digital camera containing 48 photon counting, orientation sensitive, linear detectors which are precisely aligned with the focal spot of the x-ray source. The x-ray source and the digital detectors are scanned in a continuous motion across the object (patient), each linear detector collecting an image at a distinct angle. A preliminary assessment of tomosynthesis image quality has been performed with both qualitative and quantitative methods. Measured values of MTF and NPS appear concordant with theoretical values. The MTF in the scanning direction is dominated by scanning unsharpness and geometric factors, while the NPS is white. The MTF and NPS in the strip direction are somewhat lower than in the scan direction. The NPS of tomographic images show a slight decrease with increasing spatial frequency, related to the sampling and interpolation in the reconstruction process. A phase I clinical trial is ongoing; 9 women have been recruited. Breast positioning is comparable to other imaging systems. The visualization of breast anatomy appears to be superior to screen-film mammography, at the same average glandular dose. Examination of images reconstructed with a sub-sampled set of projection images appears to support the hypothesis that image quality is superior when more projection images are used in the reconstruction.

A new mammography tomosynthesis prototype system that acquires 21 projection images over a 60 degree angular range in approximately 8 seconds has been developed and characterized. Fast imaging sequences are facilitated by a high power tube and generator for faster delivery of the x-ray exposure and a high speed detector read-out. An enhanced a-Si/CsI flat panel digital detector provides greater DQE at low exposure, enabling tomo image sequence acquisitions at total patient dose levels between 150% and 200% of the dose of a standard mammographic view. For clinical scenarios where a single MLO tomographic acquisition per breast may replace the standard CC and MLO views, total tomosynthesis breast dose is comparable to or below the dose in standard mammography. The system supports co-registered acquisition of x-ray tomosynthesis and 3-D ultrasound data sets by incorporating an ultrasound transducer scanning system that flips into position above the compression paddle for the ultrasound exam. Initial images acquired with the system are presented.

Digital breast tomosynthesis (DBT) is a tomographic technique in which individual slices through the breast are reconstructed from x-ray projection images acquired over a limited angular range. In contrast-enhanced DBT (CE-DBT) functional information can be observed by administration of an x-ray contrast agent. We have investigated the technical requirements necessary to quantitatively analyze CE-DBT exams. Using a simplified physiological model, a maximum aerial concentration of approximately 2.2 mg iodine/cm² in a 0.5 cm thick breast lesion is expected when administering 70 ml of 320 mg iodine/ml Visipaque-320^®. This corresponds to a small change in x-ray transmission; up to 5% for a 4 cm thick compressed breast. We have modeled CE-DBT acquisition by simulating Rh target x-ray spectra from 40 to 49 kV. Comparison of attenuation data of our simulated and measured spectra were found to agree well. We investigated the effect of scatter, patient motion and temporal stability of the detector on quantifying iodine uptake. These parameters were evaluated by means of experiments and theoretical modeling.

Breast cancer is a major problem and the most common cancer among women. The nature of conventional mammpgraphy makes it very difficult to distinguish a cancer from overlying breast tissues. Digital Tomosynthesis refers to a three-dimensional imaging technique that allows reconstruction of an arbitrary set of planes in the breast from limited-angle series of projection images as the x-ray source moves. Several tomosynthesis algorithms have been proposed, including Matrix Inversion Tomosynthesis (MITS) and Filtered Back Projection (FBP) that have been investigated in our lab. MITS shows better high frequency response in removing out-of-plane blur, while FBP shows better low frequency noise propertities. This paper presents an effort to combine MITS and FBP for better breast tomosynthesis reconstruction. A high-pass Gaussian filter was designed and applied to three-slice "slabbing" MITS reconstructions. A low-pass Gaussian filter was designed and applied to the FBP reconstructions. A frequency weighting parameter was studied to blend the high-passed MITS with low-passed FBP frequency components. Four different reconstruction methods were investigated and compared with human subject images: 1) MITS blended with Shift-And-Add (SAA), 2) FBP alone, 3) FBP with applied Hamming and Gaussian Filters, and 4) Gaussian Frequency Blending (GFB) of MITS and FBP. Results showed that, compared with FBP, Gaussian Frequency Blending (GFB) has better performance for high frequency content such as better reconstruction of micro-calcifications and removal of high frequency noise. Compared with MITS, GFB showed more low frequency breast tissue content.

Digital breast tomosynthesis is a new technique intended to overcome the limitations of conventional projection mammography by reconstructing slices through the breast from projection views acquired from different angles with respect to the breast. We formulate a general theory of filtered backprojection reconstruction for linear tomosynthesis. The filtering step consists of an MTF inversion filter, a spectral filter, and a slice thickness filter. In this paper the method is applied first to simulated data to understand the basic effects of the various filtering steps. We then demonstrate the impact of the filter functions with simulated projections and with clinical data acquired with a research breast tomosynthesis system.** With this reconstruction method the image quality can be controlled regarding noise and spatial resolution. In a wide range of spatial frequencies the slice thickness can be kept constant and artifacts caused by the incompleteness of the data can be suppressed.

Inverse-geometry CT (IGCT) is a promising new scanning geometry. Employing a scanned-anode x-ray source array the system is expected to provide sub-second volumetric imaging with isotropic resolution and no conebeam effects. Three detector arrays spaced apart laterally can achieve a 50 cm in-plane FOV with a 31 cm source. However, when three separate detector arrays are used, motion artifacts are expected to be different than in conventional CT and need to be assessed. Simulations were performed for two objects representing slow and fast motion as well as periodic and non-periodic motion. The simulations were repeated at different points in the FOV to study motion effects in three regions: 1) the inner 15 cm region which is sampled only by the central detector array, 2) the transition between the inner and outer regions, and 3) the outer region which is sampled by all three detector arrays. 2D simulations assumed 125 "superviews" acquired in step-and-shoot mode over 360 degrees. A gridding algorithm was used to resample the data into parallel rays which were then filtered and backprojected. Artifacts from the inner region are exactly like those that arise in a traditional CT system. The most significant artifacts caused by the multi-detector nature of the system are in the outer region, at the angles where the object sampling transitions between detector arrays. These streaking artifacts are comparable to motion artifacts in conventional CT and can be reduced by increasing the overlap region at the expense of FOV size and SNR uniformity.

In cardiac CT temporal resolution is directly related to the gantry rotation time of 3^rd generation CT scanners. This time cannot be substantially reduced below current standards of 0.33 s - 0.35 s due to mechanical limitations. As an alternative we present a dual source CT (DSCT) system. The system is equipped with two X-ray tubes and two corresponding detectors that are mounted onto the rotating gantry with an angular offset of 90°. Due to the simultaneous data acquisition and the angular offset, complementary quarter-scan data are measured at the same phase in the cardiac cycle. Hence, the exposure time of any image slice is reduced by a factor of two and the temporal resolution is improved by the same factor. In contrast to single source cardiac CT with multi-segment image reconstruction, the temporal resolution does not depend on the heart rate. Since multi-segment reconstruction techniques applied in single source cardiac CT, which limit the table speed, are no longer needed, faster volume coverage in cardiac spiral imaging can be achieved. As a consequence of these concepts, patient dose in cardiac CT can be significantly reduced. ECG correlated image reconstruction is based on 3D backprojection of the Feldkamp type. Data truncation coming from the fact that one detector (A) covers the entire scan field of view (50 cm in diameter), while the other detector (B) is restricted to a smaller, central field of view (26 cm in diameter), has to be treated. We evaluate temporal resolution and dose efficiency by means of phantom scans and computer simulations. We present first patient scans to illustrate the performance of DSCT for ECG correlated cardiac imaging.

We investigate the effect of heart rate on the quality and artifact generation in coronary artery images obtained using multi-slice computed tomography (MSCT) with the purpose of finding the optimal time resolution for data acquisition. To perform the study, we used the 4D NCAT phantom, a computer model of the normal human anatomy and cardiac and respiratory motions developed in our laboratory. Although capable of being far more realistic, the 4D NCAT cardiac model was originally designed for low-resolution imaging research, and lacked the anatomical detail to be applicable to high-resolution CT. In this work, we updated the cardiac model to include a more detailed anatomy and physiology based on high-resolution clinical gated MSCT data. To demonstrate its utility in high-resolution dynamic CT imaging research, the enhanced 4D NCAT was then used in a pilot simulation study to investigate the effect of heart rate on CT angiography. The 4D NCAT was used to simulate patients with different heart rates (60-120 beats/minute) and with various cardiac plaques of known size and location within the coronary arteries. For each simulated patient, MSCT projection data was generated with data acquisition windows ranging from 100 to 250 ms centered within the quiet phase (mid-diastole) of the heart using an analytical CT projection algorithm. CT images were reconstructed from the projection data, and the contrast of the plaques was then measured to assess the effect of heart rate and to determine the optimal time resolution required for each case. The 4D NCAT phantom with its realistic model for the cardiac motion was found to provide a valuable tool from which to optimize CT cardiac applications. Our results indicate the importance of optimizing the time resolution with regard to heart rate and plaque location for improved CT images at a reduced patient dose.

Coronary artery imaging with multi-slice helical computed tomography is a promising noninvasive imaging technique. The current major issues include the insufficient temporal resolution and large patient dose. We propose an image reconstruction method which provides a solution to both of the problems. The method uses an iterative approach repeating the following four steps until the difference between the two projection data sets falls below a certain criteria in step-4: 1) estimating or updating the cardiac motion vectors, 2) reconstructing the time-resolved 4D dynamic volume images using the motion vectors, 3) calculating the projection data from the current 4D images, 4) comparing them with the measured ones. In this study, we obtain the first estimate of the motion vector. We use the 4D NCAT phantom, a realistic computer model for the human anatomy and cardiac motions, to generate the dynamic fan-beam projection data sets as well to provide a known truth for the motion. Then, the halfscan reconstruction with the sliding time-window technique is used to generate cine images: f(t, r r). Here, we use one heart beat for each position r so that the time information is retained. Next, the magnitude of the first derivative of f(t, r r) with respect to time, i.e., |df/dt|, is calculated and summed over a region-of-interest (ROI), which is called the mean-absolute difference (MAD). The initial estimation of the vector field are obtained using MAD for each ROI. Results of the preliminary study are presented.

Optimal time windowing in cardiac CT reconstruction would permit a reduction of motion artifacts. We used Doppler Tissue Imaging (DTI) in order to acquire the speed and displacement of region of interest in one direction with a high temporal resolution. Applied on different segments of coronary arteries, 3D information is recovered in three quasi orthogonal acquisitions (abdominal, parasternal and apical views). Their reproducibility was carried on five healthy subjects. This experiment will allow finding the best time window for reconstruction. A window of half the rotation time (250ms for our CT) was estimated based on minimization of the motion variation in 3D. Since we obtained a high reproducibility between the two sessions (generally < 2mm) DTI could be an interesting approach to evaluate motion in 3D. This yields more information as compared to 2D data. Future work will require additional acquisitions and experiments in order to analyze more in depth the variability and the influence of the heart rate. The collected data will be used also as input for our motion platform and CT simulation.

In this contribution, we introduce a retrospectively gated cardiac cone-beam reconstruction scheme for continuous circular acquisition with parallel ECG recording. The technique is capable of handling variable angular ranges and enables high resolution 3D and 4D reconstruction of objects covered by the cone at a high temporal resolution. Though the detector coverage of nowadays CT scanners is not large enough to cover the complete human heart, it is sufficient to image smaller objects like conventional stents. We applied the proposed reconstruction method to the visualization of an in-stent re-stenoses phantom covered by a clinical stent, attached to a dynamic heart phantom. The method delivers images of stents in vitro at an excellent visibility and is able to rule out in-stent occlusions.

Diffuse optical tomography (DOT) uses the transmission of near-infrared light through tissue to image absorption and scattering. Especially for mammography applications DOT might become of clinical use. In this work we present simulation results on the influence of noise on the detectability of lesions for the Philips mammoscope system. Noise can have a significant impact on the image quality. It can prevent the detection of even high contrast lesions. The influence of noise can be reduced by a proper treatment in the reconstruction algorithm. But it changes also the effective sampling pattern of the imaging system if noisy data are not used (or used with a lower weight). In the case of this optical tomography system this means that lesions are only detectable up to a certain depth. This depth depends on where the signal power gets close to the constant noise floor, and on the volume and contrast of the lesion. Our simulation results show that the detection of lesions with 10 mm diameter and 100% absorption contrast should be possible with the mammoscope system, even in the worst case where the lesion is located in the center of the breast.

Multiphoton optical tomography based on NIR (near-infrared) femtosecond laser pulses provides non-invasive optical sectioning of skin with high spatial intracellular resolution and high tissue penetration. The imaging system DermaInspect was used to perform this technology in clinical studies in vivo on patients with suspicious melanoma. Pigmented cell clusters based on non-linear luminescence were clearly distinguished from non-pigmented cells in the epidermis using the autofluorescence of endogenous fluorophores like NAD(P)H, flavins, keratin, elastin, collagen and melanin. Some of the investigated tissues showed differences in the structure of the epidermal layers and the presence of dendritic cells compared to normal skin. Multiphoton laser microscopy was used to visualize extracellular matrix (ECM) structures of native and tissueengineered heart valves. The quality of the resulting 3-D images allowed an exact differentiation between collagenous and elastic fibers. The analysis of heart valve tissues of patients with cardiomyopathy revealed a dramatic loss of its capability to generate SH (second harmonic), indicating a structural deformation of the collagenous fibers, which was virtually impossible to obtain by routine histological or immunohistological staining. These results indicate that NIR femtosecond laser scanning systems can be employed as novel non-invasive optical technology for 3-D resolved ECM component imaging and in vitro and in vivo tissue diagnosis.

MRI with multiple receiver coils (parallel MRI) has been extensively used to achieve higher spatial and temporal resolution, suppress imaging artifacts, and reduced scan time. A number of techniques have been proposed to reconstruct images from reduced (undersampled) k-space datasets acquired by multiple coils. All the techniques require some type of calibration information to describe image encoding by spatially varying coil sensitivities. This information can be derived from supplementary calibration scans. However, this approach increases scan time and can be erroneous due to patient motion between calibration and imaging scans. Auto-calibrating techniques such as the commonly used GRAPPA, do not require calibration scans and estimate reconstruction coefficients directly from acquired k-space data. GRAPPA typically gives good quality results for low undersampling rates. However, strong noise amplification and non-resolved aliasing artifacts makes the technique less applicable in cases of high undersampling. In this work, we have proposed a novel auto-calibrating technique for image reconstruction from sensitivity encoded MRI data that overcomes limitations of the existing auto-calibrating techniques. In the proposed technique (GARSE), specifics of coil sensitivity representation in the image and k-space domains are utilized in the reconstruction in such a way that more trustworthy reconstruction coefficients can be identified resulting in improved image quality. GARSE reconstruction coefficients are spatially variable and adjusted according to local coil sensitivities characteristics, whereas GRAPPA reconstruction coefficients are spatially invariant and, therefore, sub-optimal. Results from MRI studies of phantoms and humans demonstrate substantial advantages of GARSE in comparison with GRAPPA, especially for high undersampling rates.

Multi-channel acquisition is employed in MRI to decrease total imaging time. In this paper, artifact free images are calculated by utilizing the difference in spatial encoding of the MR signal from neighboring channels. The encoding functions are estimated in the presence of noise and motion. For fMRI studies, the temporal stability of the signal is essential, since neuronal activity in the brain is detected by probing subtle BOLD (blood oxygen level dependent) signal changes. To ensure artifact free noise representation a new type of weight is used. By effectively selecting and eliminating low SNR pixels, increased temporal stability is achieved. Using the parallel imaging method SENSE the proposed method is tested with in-vivo data to ensure noise suppression and demonstrate correct assignment of fMRI activation.

The underlying phenomena in Magnetic Resonance imaging (MRI) physics, reconstruction, and analysis can be described by their statistics. This paper reports some new developments and focuses on two aspects: (1) new insights into statistical theory of MRI beyond its conclusions, and (2) three new applications of this theory spanned from image analysis to imaging physics. Why MR signals as well as k-space samples are statistically independent? When k-space samples are independent, why pixels in the reconstructed images are correlated? How does this correlation arise? Are spatially asymptotic independence and exponential correlation coefficient consistent? Why the homogeneity and scales can be characterized by stationarity and ergodicity? This paper provides answers to these questions. The first application of statistical theory of MRI is a stochastic model-based image analysis approach. The second application is a sensor array processing approach for detecting distinctive object regions. The third application is the Parallel Magnetic Resonance imaging (P-MRI) which shows by combining statistics of MRI and sensor array processing, P-MRI can be easily formulated. This paper shows that statistical theory of MRI not only provides basis for MR imaging, reconstruction, and analysis, but also offers means to integrate signal processing methods and image processing methods for solving various MRI related problems.

Positron Emission Tomography (PET) scanners dedicated to small animal studies have seen a swift development in recent years. Higher spatial resolution, greater sensitivity and faster scanning procedures are the leading factors driving further improvements. The new LabPET^TM system is a second-generation APD-based animal PET scanner that combines avalanche photodiode (APD) technology with a highly integrated, fully digital, parallel electronic architecture. This work reports on the performance characteristics of the LabPET quad detector module, which consists of LYSO/LGSO phoswich assemblies individually coupled to reach-through APDs. Individual crystals 2×2×~10 mm³ in size are optically coupled in pair along one long side to form the phoswich detectors. Although the LYSO and LGSO photopeaks partially overlap, the good energy resolution and decay time difference allow for efficient crystal identification by pulse-shape discrimination. Conventional analog discrimination techniques result in significant misidentification, but advanced digital signal processing methods make it possible to circumvent this limitation, achieving virtually error-free decoding. Timing resolution results of 3.4 ns and 4.5 ns FWHM have been obtained for LYSO and LGSO, respectively, using analog CFD techniques. However, test bench measurements with digital techniques have shown that resolutions in the range of 2 to 4 ns FWHM can be achieved.

Various direct and indirect active matrix flat-panel imagers (AMFPI) are being investigated for x-ray imaging. In both direct AMFPI and indirect AMFPI with avalanche gain, a bias potential up to several thousand volts is required to operate the photoconductor. Under the condition of a large amount of radiation exposure between subsequent readout, a potential >80 V could appear across the amorphous silicon (a-Si) thin film transistor (TFT) and cause permanent damage. The purpose of this paper is to investigate a simple pixel design for high voltage protection. The pixel electrode acts as an additional gate for the top channel of an a-Si TFT to drain excess image charge from the pixel electrode until an equilibrium is reached where the TFT channel current equals the detector signal current at a predetermined safe maximum value V_Pmax for the pixel potential. This "dual-gate" TFT structure without additional protective device simplifies the TFT array design and improves yield. However special care is required to understand the characteristics of both the top and the bottom gates to ensure sufficient detector dynamic range as well as reliable high voltage protection. A physical model for dual-gate a-Si TFTs was developed and device parameters were determined by fitting the model to measured characteristics from a dual-gate TFT array. Our results showed that compared to the bottom (normal) gate, the protective gate has a shallower transfer characteristics (i.e. channel current as a function of gate voltage) due to a higher density of states in the top interface. Nevertheless it provides adequate protection of the TFT with V_Pmax of ~40 V for typical radiographic exposures.

The most widely used architecture in large-area amorphous silicon (a-Si) flat panel imagers is a passive pixel sensor (PPS), which consists of a detector element and a readout switch. While the PPS has the advantage of being compact and amenable toward high-resolution imaging, reading small PPS output signals requires external column charge amplifiers that produce additional noise and reduce the minimum readable sensor input signal. In contrast, on-pixel amplifiers in a-Si technology reduce readout noise by decoupling off-pixel noise sources, such as external charge amplifier and data line noise, from the sensor input. The off-pixel noise is reduced by the charge gain of the pixel amplifier, allowing for low-noise performance. Theoretical calculations and simulations of gain, linearity, metastability, pixel area requirements and noise indicate the applicability of the amplified a-Si pixel architectures for low-exposure, real-time fluoroscopy. In addition, the detailed noise results allow for the computation of noise performance as a function of transistor dimensions for both amorphous silicon and polysilicon technologies, allowing the designer to choose appropriate device dimensions when designing flat-panel imaging circuits.

The tremendous increase in speed with which the body can now be scanned using multislice CT has improved the diagnostic ability of the modality, especially in time critical applications involving contrast injection. Advances in photodiode and front-end electronics technology now allow a CT detector module to be made that can be tiled in two dimensions. An array of such modules can be used to easily make a CT scanner with hundreds of slices with the promise of scanning whole organs with a single revolution and further improving diagnostic ability. Recently, a back-illuminated photodiode for CT has been developed which has its electrical connections on the underside. With all four sides of the silicon chip free, the photodiodes can be tiled in two dimensions. In addition, improvements in front-end electronics now allow the A/D converters for all photodiode elements to be placed completely behind the photodiode. A prototype detector module has been constructed and tested. Measurements of DQE, MTF, dynamic range and temporal response are presented showing that the module has the same high performance as detectors found in current diagnostic CT scanners. A dynamic range of 250,000:1 at a frame rate of 10,000 fps has been achieved. Alternatively a dynamic range of 1,000,000:1 can be achieved at 2,500 fps. This new compact 2D tiled detector with digital data output can be used as a basic building block for future multislice detection systems enabling larger coverage and the promise of improved diagnostic ability.

Detectors working in photon counting mode offer an interesting alternative to the common charge integrating detectors for computed tomography (CT), because they can potentially measure the energy of every detected X-ray photons and achieve better image contrast sensitivity for a given dose. Unfortunately, most current X-ray detectors suffer from limited count rate capability, due either to a long charge migration time in semiconductor and gas detectors, or to a slow decay time in ceramic scintillators. To overcome these difficulties, we propose to use pixel detectors based on fast light emitting inorganic scintillators individually coupled to avalanche photodiodes with parallel, low-noise, fast digital processing electronics to provide real time single event detection and recording. The proposed detector was investigated with 2 × 2 × 10 mm³ Lu_1.9Y_0.1SiO₅ (LYSO), a fast decay time (40 ns), heavy (7.19 g/cc) scintillator that is also suitable for coincidence detection of annihilation radiation (511 keV) in positron emission tomography (PET). Therefore, the detector characteristics make it a good candidate for implementation in a combined PET/CT dual-modality scanner. Although only coarse spectral analysis is possible in the X-ray energy range, it is demonstrated that appropriate CT images for anatomical localization can be obtained at very low dose in counting mode using a PET/CT simulator set up for small animal imaging. Data are reported on CT image resolution, noise, contrast and dose.

In previous work, we and others have focused the validation of detector models for computational simulations of imaging system performance on matching, to a small difference, specific aspects of the detector's performance, i.e., modulation transfer function or light output. In this work, instead, we selected three parameters that, together, represent a more complete description of the imaging properties of the phosphor screen to be modeled. The three performance parameters are the information or Swank factor determined from pulse-height spectra, the light output (either in absolute or relative scale), and the point-response function. Using this general methodology, we created screen models that exhibit good agreement with recent experimental measurements available in the literature, over a wide x-ray energy range (18-75 keV), and for front- and back-screen configurations. The models are being used in conjunction with MANTIS, a Monte Carlo code for simulating imaging systems that tracks x rays, electrons, and optical photons in the same geometric model, with x-ray and electron physics models from the PENELOPE package, and optical physics models from DETECT-II. This study allows us to incorporate realistic detector models into a detailed and complete Monte Carlo simulation of the entire imaging system, including the object and its absorbed dose map, and the properties of the imaging acquisition.

In this work we study the resolution and noise properties of penalized-likelihood sinogram smoothing and restoration algorithms that we have been developing for preprocessing of computed tomography (CT) data. We have formulated CT sinogram preprocessing as a statistical restoration problem in which the goal is to obtain the best estimate of the line integrals needed for reconstruction from the set of noisy, degraded measurements. The degradations that afflict raw CT data include beam hardening, off-focal radiation, detector afterglow, and electronic crosstalk, among others. We estimate the line integrals by maximizing a roughness-penalized Poisson likelihood-based objective function. The maximization algorithm is based on the separable paraboloidal surrogates strategy and image reconstruction can then proceed by use of existing, non-iterative approaches. In the case of sinogram smoothing, the only degradation modeled directly is noise; the other effects can then be corrected for by standard means. In the case of sinogram restoration, a number of different degradations are included directly in the objective function. We demonstrate that the approaches can correct for sinogram degradations, eliminating the image artifacts caused by beam hardening and off-focal radiation. We also evaluate the local modulation transfer function, local noise power spectrum, and local noise equivalent quanta in a numerical test phantom and find that the proposed approaches outperform standard approaches based on deconvolution and shift-invariant filtration.

The raw data acquired during a computed tomography (CT) scan carry the unwanted traces of a number of adverse effects connected with the measurement setup and the acquisition process. To name a few, these include systematic errors like detector crosstalk and afterglow, fluctuations in tube power during the scan, but also statistical effects like photon noise. Most systematic effects can be cast into a linear model, providing a way for neutralizing the influence of these errors through deconvolution. However, this deconvolution process inevitably increases the image noise content. For low-dose scans, application of some kind of noise suppression algorithm is mandatory, in order to keep its disturbing influence on the reconstructed images in check. Since resolution and noise are antagonizing properties, noise suppression usually has the side effect of decreasing resolution. The interest in finding an algorithm that deals with this quandary in an optimal way is obvious. This work compares three deconvolution/denoising methods, identifying the one that performs best on a set of simulated data. The tested methods of combined sinogram deconvolution/denoising are based on (1) regularized matrix inversion, (2) straight matrix inversion plus adaptive filtering, and (3) deconvolution by a penalized maximum likelihood approach. In-plane and axial noise/resolution measurements identified the penalized maximum-likelihood method as best suited for low-dose applications. The adaptive filter approach performed well, but did not retain as much resolution when going to higher smoothing levels. The analytic deconvolution, however, could not compete against the other two methods.

A cone beam CT (CBCT) imaging system has been developed based on a high-speed flat panel detector specially designed for cone beam CT. This system has shown great potential for early breast cancer detection with high contrast and spatial resolution, and the potential for high-speed dynamic studies. Image lag always plays an important role in cone beam CT imaging by affecting the image quality. This paper investigated the relationship of the image lag with various parameters of this new system, which include frame number, detector mode, frame rate, detector signal strength and kVp value. A simulated breast phantom was designed to demonstrate how image lag causes artifacts in the image and affects the reconstructed linear attenuation coefficient of tumor in different tissues. Experimental results illustrate that lag on this system is less than 3%, and is independent of kVp value and detector mode. When the detector signal strength increases from 10% to 80% saturation, lag decreases by 10%. Lag is also a function of the frame rate increasing by 20% when the frame rate changed from 3.75 fps to 30 fps. Computer simulations reveal that lag on this new system caused less than 6 HU reduction in the CT# for simulated tumors and soft tissue and 20 HU reduction for high contrast objects. This work tells us that lag on this new CBCT system for breast imaging can be ignored. However, when this system is used for dynamic study, which requires a high readout speed from the detector, image lag correction will need to be considered to achieve good image quality.

X-ray cone-beam computed tomography (CBCT) is of importance in image-guided intervention (IGI) and image-guided radiation therapy (IGRT). In this paper, we present a cone-beam CT data acquisition system using a GE INNOVA 4100 (GE Healthcare Technologies, Waukesha, Wisconsin) clinical system. This new cone-beam data acquisition mode was developed for research purposes without interfering with any clinical function of the system. It provides us a basic imaging pipeline for more advanced cone-beam data acquisition methods. It also provides us a platform to study and overcome the limiting factors such as cone-beam artifacts and limiting low contrast resolution in current C-arm based cone-beam CT systems. A geometrical calibration method was developed to experimentally determine parameters of the scanning geometry to correct the image reconstruction for geometric non-idealities. Extensive phantom studies and some small animal studies have been conducted to evaluate the performance of our cone-beam CT data acquisition system.

The clinical goal of breast imaging is to detect tumor masses when they are as small as possible, preferably less than 10 mm in diameter. Conventional screen-film mammography is the most effective tool for the early detection of breast cancer currently available. However, conventional mammography has relatively low sensitivity for the detection of small breast cancers (under several millimeters). Specificity and the positive predictive value of mammography remain limited owing to an overlap in the appearance of benign and malignant lesions, and surrounding structure. We propose to address the limitations accompanying conventional mammography by incorporating a cone beam CT reconstruction technique with a recently developed flat panel detector (FPD). We have performed a computer simulation study and preliminary phantom studies to prove the feasibility of developing an FPD-based cone beam CT breast imaging technique for a small size normal breast phantom. In this study, we report the design and construction of a novel FPD-based cone beam breast CT scanner prototype. In addition, we present the results of phantom studies performed on our current FPD-based cone beam CT scanner prototype, which uses the same flat panel detector proposed for the cone beam breast CT scanner prototype, to predict the image performance of the novel cone beam breast CT scanner, while we are completing the construction of the system.

When an imaging task is specified, the design of a cone-beam CT scanner includes specifications of the scanning trajectory and corresponding image reconstruction algorithms, requirements on the detector size, and requirements on the x-ray tubes. Given the limited flat-panel detector readout speed and the need of short scanning time in a clinical setting, the available number of total view angles is normally limited to several hundred. It is known that when all the focal spots are distributed along a circular trajectory, the cone-beam artifacts are present in the reconstructed out-of-plane images when the cone-angle is relatively large. In order to mitigate or eliminate the cone-beam artifacts, the source trajectory should be complete in the sense of satisfying the so-called Tuy data sufficiency condition. However, assuming a constant number of view angles, a complete source trajectory will potentially lead to a lower view sampling rate and cause view aliasing artifacts. Therefore, for a given imaging task and a given total number of view angles, it is important to study the tradeoff between the view sampling rate and the completeness of the scanning source trajectories. In this paper, we numerically and experimentally studied the above tradeoff. Specifically, numerical simulations were conducted to study this tradeoff using three different source trajectories: (1) a circular trajectory, (2) a helical trajectory, and (3) a two-concentric-orthogonal-circle trajectory. A single x-ray tube and a flat panel imager mounted on an optical bench was utilized to experimentally study the tradeoff between the circular source trajectory and the helical source trajectory. For the complete source trajectories, some novel cone-beam image reconstruction algorithms have been utilized to reconstruct images and compare image quality in numerical simulations and benchtop experiments.

We have developed X-ray refraction based computed tomography (CT) which is able to visualize soft tissue in between hard tissue. The experimental system consists of Si(220) diffraction double-crystals called the DEI (diffraction-enhanced imaging) method, object locating in between them and a CCD camera to acquire data of 900 x-ray images. The x-ray energy used was 17.5 keV. The algorithm used to reconstruct CT images has been invented by A. Maksimenko et al.. We successfully visualized calcification and distribution of breast cancer nest which are the inner structure. It has much higher contrast which in comparison with the conventional absorption based CT system.

Vascular disease is a leading cause of death and disability in the western world. Diagnosis and staging of atherosclerosis is a challenge, especially with regards to the identification of plaque vulnerability. We are developing imaging methods based upon MRI and intravascular microcoils. In order to rigorously validate our MRI imaging methods and algorithms, we have developed a new cryo-imaging system that allows one to alternately section and image the block face of tissue. We obtain 3D pathology of vessel segments excised from cadaver and we characterize the tissues of atheroma using episcopic autofluorescence and bright field microscopy images. After embedding the vessel, the block is frozen, and block face microscopic images are taken every 200μm with an image resolution of 30μm×30μm. The series of images is then corrected for uneven illumination, serially registered to one another, and the 3D vessel segment is reconstructed. Some sections are recovered and processed with histological staining for validation. Seven tissue types can be readily identified from the cryo-images: necrotic core, calcification, lipid pool, media, adventitia, fibrosis, thrombus, and normal intima. Since the whole vessel segment is available, we could register 3D data to images from MR, or other modalities, for validation. In addition, visualization tools such as multi-planar reformatting 3D rendering can be used to study 3D plaque morphology, in microscopic detail.

Optical imaging of fluorescent probes is an essential tool for investigation of molecular events in small animals for drug developments. In order to get localization and quantification information of fluorescent labels, CEA-LETI has developed efficient approaches in classical reflectance imaging as well as in diffuse optical tomographic imaging with continuous and temporal signals. This paper presents an overview of the different approaches investigated and their performances. High quality fluorescence reflectance imaging is obtained thanks to the development of an original "multiple wavelengths" system. The uniformity of the excitation light surface area is better than 15%. Combined with the use of adapted fluorescent probes, this system enables an accurate detection of pathological tissues, such as nodules, beneath the animal's observed area. Performances for the detection of ovarian nodules on a nude mouse are shown. In order to investigate deeper inside animals and get 3D localization, diffuse optical tomography systems are being developed for both slab and cylindrical geometries. For these two geometries, our reconstruction algorithms are based on analytical expression of light diffusion. Thanks to an accurate introduction of light/matter interaction process in the algorithms, high quality reconstructions of tumors in mice have been obtained. Reconstruction of lung tumors on mice are presented. By the use of temporal diffuse optical imaging, localization and quantification performances can be improved at the price of a more sophisticated acquisition system and more elaborate information processing methods. Such a system based on a pulsed laser diode and a time correlated single photon counting system has been set up. Performances of this system for localization and quantification of fluorescent probes are presented.

Small animal imaging has recently become an area of increased interest because more human diseases can be modeled in transgenic and knockout rodents. Current micro-CT systems are capable of achieving spatial resolution on the order of 10 μm, giving highly detailed anatomical information. However, the speed of data acquisition of these systems is relatively slow, when compared with clinical CT systems. Dynamic CT perfusion imaging has proven to be a powerful tool clinically in detecting and diagnosing cancer, stroke, pulmonary and ischemic heart diseases. In order to perform this technique in mice and rats, quantitative CT images must be acquired at a rate of at least 1 Hz. Recently, a research pre-clinical CT scanner (eXplore Ultra, GE Healthcare) has been designed specifically for dynamic perfusion imaging in small animals. Using an amorphous silicon flat-panel detector and a clinical slip-ring gantry, this system is capable of acquiring volumetric image data at a rate of 1 Hz, with in-plane resolution of 150 μm, while covering the entire thoracic region of a mouse or whole organs of a rat. The purpose of this study was to evaluate the principal imaging performance of the micro-CT system, in terms of spatial resolution, image uniformity, linearity, dose and voxel noise for the feasibility of imaging mice and rats. Our investigations show that 3D images can be obtained with a limiting spatial resolution of 2.7 line pairs per mm and noise of 42 HU, using an acquisition interval of 8 seconds at an entrance dose of 6.4 cGy.

Certain elements (such as Fe, Cu, Zn, etc.) are vital to the body and an imbalance of such elements can either be a symptom or cause of certain pathologies. Neutron Stimulated Emission Computed Tomography (NSECT) is a spectroscopic imaging technique whereby the body is illuminated via a beam of neutrons causing elemental nuclei to become excited and emit characteristic gamma radiation. Acquiring the gamma energy spectra in a tomographic geometry allows reconstruction of elemental concentration images. Previously we have demonstrated the feasibility of NSECT using first generation CT approaches; while successful, the approach does not scale well and has limited resolution. Additionally, current gamma cameras operate in an energy range too low for NSECT imaging. However, the orbiting Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) captures and images gamma rays over the high-energy range equivalent to NSECT's (3 keV to 17 MeV) by utilizing Collimator-based Fourier transform imaging. A High Purity Germanium (HPGe) detector counts the number of energy events per unit of time, providing spectroscopic data. While a pair of rotating collimators placed in front of the detector modulates the number of gamma events, providing spatial information. Knowledge of the number of energy events at each discrete collimator angle allows for 2D image reconstruction. This method has proven successful at a focus of infinity in the RHESSI application. Our goal is to achieve similar results at a reasonable near-field focus. Here we describe the results of our simulations to implement a rotating modulation collimator (RMC) gamma imager for use in NSECT using simulations in Matlab. To determine feasible collimator setups and the stability of the inverse problem a Matlab environment was created that uses the geometry of the system to generate 1D observation data from 2D images and then to reconstruct 2D images using the MLEM algorithm. Reasonable collimator geometries were determined, successful reconstruction was achieved and the inverse problem was found to be stable.

Regional x-ray exposure management is a class of techniques for x-ray dose reduction and image quality improvement in radiography and fluoroscopy. X-ray intensity is modulated spatially prior to the beam's interaction with the imaged objects for optimal dose efficiency. The optimal intensity field may be determined from the imaged objects' morphology, local dose sensitivities, regions of clinical interest, and expected information. We present the derivation and simulation results for a method for automated feedback-controlled real-time spatial modulation of the x-ray intensity field. The method employs spatial x-ray gating, a technique in which various beam regions are blocked for controlled portions of the frame integration period while the beam intensity is modulated in time. The method promises to provide controllable smooth x-ray intensity fields with precise and sensitive controls operating, if desired, automatically in a feedback loop.

Digital radiographs are currently characterized in terms of a variety of incompatible, vendor-specific dose metrics, which make it difficult for users to monitor receptor dose. AAPM Task Group #116 has been formed to "Standardize an Image Receptor Dose Index for Digital Radiography," prompting the publication of a proposed definition for receptor dose that can be readily implemented on any system that acquires a projection x-ray image and produces a display-ready image.¹ This paper carries that work forward by demonstrating the applicability of that proposal to a range of digital detector types and to a new set of clinical images. Digital receptor dose can be usefully defined for the four ISO beam qualities, in terms of the exposure needed to produce a specified display response in a displayed image, analogous to the ISO speed definition for screen-film images. It requires that the system produce an original image (calibrated in terms of the relationship between system response and exposure for the standard set of x-ray beam qualities defined by ISO-9236-1) and a display-ready image (represented as DICOM GSDF p-values). The receptor dose is computed in terms of the median of pixels in the original image that corresponds to near midrange pixels in the display-ready image. The exposure responses of Gd₂O₂S-, CsI(Tl)-, a-Se-, and BaFBrI-based digital radiography acquisition systems, as well as a commonly used Gd₂O₂S screen-film combination, have been measured for the four ISO beam qualities. The proposed receptor dose metric was computed for a sample of 602 clinical images for which body-part thickness technique factors (kV, mAs, and SID) were known. Analysis of this data demonstrates that the proposed receptor dose can be a useful predictor of exposure adequacy over a wide range of body parts and thickness. In conclusion, the proposed vendor-independent receptor dose metric has been shown to produce consistent results across a representative range of digital receptor technologies for a wide range of clinical images. It has also been shown to produce results that are consistent with existing standards for screen-film technology and thereby provides a measurement capable of bridging the transition to digital imaging.

In this paper, we evaluate the performance of biplane correlation imaging (BCI) using a set of off-angle projections acquired from an anthropomorphic chest phantom. BCI reduces the effect of anatomical noise, which would otherwise impact the detection subtle lesions in planar images. BCI also minimizes the number of false positives (FPs) when used in conjunction with computer aided diagnosis (CAD) applied to a set of coronal chest x-ray projections by eliminating non-correlated nodule candidates. In BCI, two digital images of the chest are acquired within a short time interval from two slightly different posterior projections. The image data are then incorporated into the CAD algorithm in which nodules are detected by examining the geometrical correlation of the detected signals in the two views, thus largely "canceling" the impact of anatomical noise. Seventy-one low exposure posterior projections were acquired of an anthropomorphic chest phantom containing tissue equivalent lesions with small angular separations (0.32 degree) over a range of 20 degrees, [-10°, +10°], along the vertical axis. The data were analyzed to determine the accuracy of the technique as a function of angular separation. The results indicated that the best performance was obtained when the angular separation of the projection pair was greater than 6 degrees. Within the range of optimum angular separation, the number of FPs per image, FPpI, was ~1.1 with average sensitivity around 75% (supported by a grant from the NIH R01CA109074).

Cone beam Computed Tomography (CBCT) enables three-dimensional imaging with isotropic resolution. X-rays scatter estimation is a big challenge for quantitative CBCT imaging of thorax: scatter level is significantly higher on cone beam systems compared to collimated fan beam systems. The effects of this scattered radiation are cupping artifacts, streaks, and quantification inaccuracies. In this paper, an original scatter management process on tomographic projections without supplementary on-line acquisitions is presented. The correction method is based on scatter calibration through off-line acquisitions, combined to an on-line analytical transformation issued from physical equations to adapt calibration to the observed object. Evaluations of the method were performed on an anthropomorphic thorax phantom. First, tomographic acquisitions were performed with a flat panel detector. Reconstructed volume obtained with the proposed scatter correction method has been compared with the one obtained through a classical beam stops method. Secondly, reconstructed volume has been compared with the one obtained through a fan beam system (Philips multi slice CT scanner). The new method provided results in good agreement with the beam stops approach and with the multi slice CT scanner, suppressing cupping artifacts and improving quantification significantly. Compared to the beam stops method, lower X-rays doses (divided by a factor 9) and shorter acquisition times were needed.

Scattered radiation is a major source of artifacts in flat detector based cone-beam computed tomography. In this paper, a novel software-based method for retrospective scatter correction is described and evaluated. The method is based on approximation of the imaged object by a simple geometric model (e.g., a homogeneous water-like ellipsoid) that is estimated from the set of acquired projections. This is achieved by utilizing a numerical optimization procedure to determine the model parameters for which there is maximum correspondence between the measured projections and the projections of the model. Monte-Carlo simulations of this model are used for calculation of scatter estimates for the acquired projections. Finally, using the scatter-corrected projections, tomographic reconstruction is conducted by means of cone-beam filtered back-projection. The correction method is evaluated using simulated and experimentally acquired projection data sets of geometric and physical head phantoms. It is found that the method is able to accurately estimate mean scatter levels in X-ray projections, allowing to significantly reduce scatter-caused artifacts in 3D reconstructed images.

The design principles that drove the development of a new cardiovascular x-ray digital flat panel (DFP) detector system are presented, followed by assessments of imaging and dose performance achieved relative to other state of the art FPD systems. The new system (GE Innova 2100^IQTM) incorporates a new detector with substantially improved DQE at fluoroscopic (73%@1μR) and record (79%@114μR) doses, an x-ray tube with higher continuous fluoro power (3.2kW), a collimator with a wide range of copper spectral filtration (up to 0.9mm), and an improved automatic x-ray exposure management system. The performance of this new system was compared to that of the previous generation GE product (Innova 2000) and to state-of-the art cardiac digital x-ray flat panel systems from two other major manufacturers. Performance was assessed with the industry standard Cardiac X-ray NEMA/SCA&I phantom, and a new moving coronary artery stent (MCAS) phantom, designed to simulate cardiac clinical imaging conditions, composed of an anthropomorphic chest section with stents moving in a manner simulating normal coronary arteries. The NEMA/SCA&I phantom results showed the Innova 2100IQ to exceed or equal the Innova 2000 in all of the performance categories, while operating at 28% lower dose on average, and to exceed the other DFP systems in most of the performance categories. The MCAS phantom tests showed the Innova 2100IQ to be significantly better (p << 0.05) than the Innova 2000, and significantly better than the other DFP systems in most cases at comparable or lower doses, thereby verifying excellent performance against design goals.

The application of high-performance flat-panel detectors (FPDs) to dual-energy (DE) imaging offers the potential for dramatically improved detection and characterization of subtle lesions through reduction of "anatomical noise," with applications ranging from thoracic imaging to image-guided interventions. In this work, we investigate DE imaging performance from first principles of image science to preclinical implementation, including: 1.) generalized task-based formulation of NEQ and detectability as a guide to system optimization; 2.) measurements of imaging performance on a DE imaging benchtop; and 3.) a preclinical system developed in our laboratory for cardiac-gated DE chest imaging in a research cohort of 160 patients. Theoretical and benchtop studies directly guide clinical implementation, including the advantages of double-shot versus single-shot DE imaging, the value of differential added filtration between low- and high-kVp projections, and optimal selection of kVp pairs, filtration, and dose allocation. Evaluation of task-based NEQ indicates that the detectability of subtle lung nodules in double-shot DE imaging can exceed that of single-shot DE imaging by a factor of 4 or greater. Filter materials are investigated that not only harden the high-kVp beam (e.g., Cu or Ag) but also soften the low-kVp beam (e.g., Ce or Gd), leading to significantly increased contrast in DE images. A preclinical imaging system suitable for human studies has been constructed based upon insights gained from these theoretical and experimental studies. An important component of the system is a simple and robust means of cardiac-gated DE image acquisition, implemented here using a fingertip pulse oximeter. Timing schemes that provide cardiac-gated image acquisition on the same or successive heartbeats is described. Preclinical DE images to be acquired under research protocol will afford valuable testing of optimal deployment, facilitate the development of DE CAD, and support comparison of DE diagnostic imaging performance to low-dose CT and radiography.

The detection of coronary calcifications with CT is generally accepted as a useful method for predicting early onset of coronary artery disease. Film-screen X-ray and fluoroscopy have also been shown to have high predictive value for coronary disease diagnosis, but have minimal sensitivity. Recently, flat-panel detectors capable of dual-energy techniques have enabled the separation of soft-tissue and bone from images. Clinical studies report substantially improved sensitivity for the detection of coronary calcifications using these techniques. However, heart motion causes minor artefacts from misregistration of both calcified and soft-tissue structures, resulting in inconsistent detection of calcifications. This research examines whether cardiac gating improves the reliability of calcification detection. Single-energy, gated, and non-gated dual-energy imaging techniques are examined in a dynamic phantom model. A gating system was developed to synchronize two dual-energy exposures to a specified phase of the cardiac cycle. The performance and repeatability of the gating system was validated with the use of a cyclical phantom. An anthropomorphic phantom was developed to simulate both cardiac and soft-tissue motion, and generate ECG-like output signals. The anthropomorphic phantom and motion artefact accuracy was verified by comparison with clinical images of patients with calcifications. The ability of observers to detect calcifications in non-gated, and gated techniques was compared through the use of an ROC experiment. Gating visibly reduces the effect of motion artifacts in the dual-energy images. Without gating, motion artefacts cause greater variability in calcification detection. Comparison of the average area-under-the-curve of the ROC curves show that gating significantly increases the accuracy of calcification detection. The effects of motion and gating on DE cardiac calcification detection have been demonstrated and characterized in a phantom model that mimics the clinical scenario for dual-energy examinations. There exists significant potential for reliable cardiac calcification detection with gated dual-energy radiography.

We present the analysis of the accuracy and precision of dual energy material basis decomposition for the quantification of tissue fat content in computed tomography. We compare the benefits of a pre-reconstruction (sinogram-based) dual energy imaging technique versus a post-reconstruction (image) based dual energy decomposition technique using a numerical simulation. A phantom containing plastics of known composition is measured to validate the technique. The accuracy of the image based dual energy decomposition technique is contingent on the amount of beam hardening encountered in the phantom. The accuracy of the pre-reconstruction dual energy technique depends on how accurately the system spectral response can be modeled. In both cases the precision of the dual energy imaging is determined by the photon flux.

This paper presents a dual-energy imaging technique optimized for contrast-enhanced mammography using a photon counting detector. Each photon pulse is processed separately in the detector and the addition of an electronic threshold near the middle of the energy range of the x-ray spectrum allows discrimination of high and low energy photons. This effectively makes the detector energy sensitive, and allows the acquisition of high- and low-energy images simultaneously. These high- and low-energy images can be combined to dual-energy images where the anatomical clutter has been suppressed. By setting the electronic threshold close to 33.2 keV (the k-edge of iodine) the system is optimized for dual-energy contrast-enhanced imaging of breast tumors. Compared to other approaches, this method not only eliminates the need for separate exposures that might lead to motion artifacts, it also eliminates the otherwise deteriorating overlap between high- and low-energy spectra. We present phantom dual-energy images acquired on a prototype system to illustrate that the technique is already operational, albeit in its infancy. We also present a theoretical estimation of the potential gain in tumor signal-difference-to-noise ratio when using this electronic spectrum-splitting method as opposed to acquiring the high- and low-energy images separately with double exposures with separate x-ray spectra. Assuming ideal energy sensitive photon counting detectors, we arrive at the conclusion that the signal-difference-to-noise ratio could be increased by 145% at constant dose. We also illustrate our results on synthetic images.

This paper describes a system for multi-spectral single-scan lung imaging. The proposed approach relies on a low noise detector sampled at a high rate. The proposed method overcomes limitations of CCD time-and-delay integration slot-scanning systems. The system design and preliminary specifications are described. The results of initial spectral and system simulations in support of system feasibility per the outlined specifications are described. Initial investigations support the potential of the proposed approach to alleviate four shortcomings of the current digital flat-panel approach to chest radiography: (i) by enabling dynamic multi-spectral imaging in a single scan, the approach reduces the time delay between exposures, thus reducing sensitivity to motion; (ii) the approach enables dynamic technique feedback and technique adaptation, eliminating the need for a pre-exposures and reducing the likelihood of poor x-ray techniques in local image areas; (iii) by enabling direct measurement of the scatter field, the proposed method allows further scatter correction resulting in image quality improvements; (iv) finally, full-frame sampling of a digital detector allows imaging of the beam penumbra, thereby reclaiming the detection quantum efficiency loss due to over-collimation in current TDI slot-scanning approach; the resulting DQE potentially exceeds that of flat-panel detectors by a factor up to two.

With growing clinical acceptance of dual-energy chest radiography, there is increased interest in the application of dual-energy techniques to other clinical areas. This paper describes the creation and experimental validation of a poly-energetic signal-propagation model for technique optimization of new dual-energy clinical applications. The model is verified using phantom experiments simulating typical abdominal radiographic applications such as Intravenous Urography (IVU) and the detection of pelvic and sacral bone lesions or kidney stones in the presence of bowel gas. The model is composed of a spectral signal propagation component and an image-processing component. The spectral propagation component accepts detector specifications, X-ray spectra, phantom and imaging geometry as inputs, and outputs the detected signal and estimated noise. The image-processing module performs dual-energy logarithmic subtraction and returns figures-of-merit such as contrast and contrast-to-noise ratio (CNR), which are evaluated in conjunction with Monte Carlo calculations of dose. Phantoms assembled from acrylic, aluminum, and iodinated contrast-agent filled tubes were imaged using a range of kVp's and dose levels. Simulated and experimental results were compared by dose, clinical suitability, and system limitations in order to yield technique recommendations that optimize one or more figures-of-merit. The model accurately describes phantom images obtained in a low scatter environment. For the visualization of iodinated vessels in the abdomen and the detection of pelvic bone lesions, both simulated and experimental results indicate that dual-energy techniques recommended by the model yield significant improvements in CNR without significant increases in patient dose as compared to conventional techniques. For example the CNR of iodinated vessels can be doubled using two-thirds of the dose of a standard exam. Alternatively, in addition to a standard dose image, the clinician can obtain a dual-energy bone image with greater than 8-fold increase in CNR, with the addition of just 15% higher dose. It is expected that this tool will enable the rapid clinical utilization of new applications of dual-energy radiography.

Light transport in trabecular bone is not well understood despite its clinical interest. Recent experimental studies on optical bone biopsy are lacking models that relate their measurements to the underlying morphology and thus to tissue condition. Laser surgery can also benefit from a better understanding of energy distribution in cancellous bone. A Monte Carlo (MC) simulation environment, able to efficiently compute complex geometries and account for refraction and reflection on tissue boundaries has been developed to provide the missing insight. The geometry description is based on a 3D triangle mesh organised in a bounding-volume hierarchy. This efficient structure allows a fast photon-surface intersection test, ensuring a sufficient number of photon paths and thus a good signal-to-noise ratio. The simulation program has been validated against well-known problems of refractive optics and turbid media. This new tool has been applied to a set of numerical phantoms indicating that morphology may have a fundamental impact on long-range light transport. The simulation environment has also been used on high-resolution models of trabecular bone, based on micro-CT scans. Calculation of time resolved signals in transmission and reflectance geometries has been demonstrated, paving the way to numerical evaluation of new minimally invasive diagnostic techniques, and offering a link to evaluation of Optical Coherence Tomography (OCT) in complex heterogeneous geometries. Preliminary experimental results in support of the mentioned effects are presented.

The detection of lesions in conventional mammography is a difficult task, predominantly due to the masking effect of superimposed parenchymal breast patterns. Breast tomosynthesis is a technique that has been proposed to reduce this masking effect, by providing the radiologist with tomographic image slices through the breast. The goal of this research was to investigate the impact of varying x-ray spectra on image quality of breast tomosynthesis using an indirect CsI based detector. The ideal observer SNR was used as a figure-of-merit, under the assumption that the imaging system is linear and shift-invariant. Computations of the ideal observer SNR used a serial cascade model to predict signal and noise propagation through the detector, as well as a model of the lesion detection task in breast imaging. An indirect detector breast tomosynthesis prototype system was modeled which acquires 11 projection views by rotating the x-ray tube over a 50° angular range, with the breast and detector remaining stationary. Specific attention was focused on the impact of electronic noise for indirect detector breast tomosynthesis. Three different target/filters were studied including Mo/Mo, Mo/Rh, and W/Rh. Spectra were scaled to give a total of 2.4 mGy average glandular dose to the breast. It was observed that theW/Rh target/filter exhibited the best performance. In addition, electronic noise was observed to have a moderate effect on the SNR with more impact for thicker breasts and lower kVp settings.

A detailed four-dimensional model of the coronary artery tree has great potential in a wide variety of applications especially in biomedical imaging. We developed a computer generated three-dimensional model for the coronary arterial tree based on two datasets: (1) gated multi-slice computed tomography (MSCT) angiographic data obtained from a normal human subject and (2) statistical morphometric data obtained from porcine hearts. The main coronary arteries and heart structures were segmented from the MSCT data to define the initial segments of the vasculature and geometrical details of the boundaries. An iterative rule-based computer generation algorithm was then developed to extend the coronary artery tree beyond the initial segmented branches. The algorithm was governed by the following factors: (1) the statistical morphometric measurements of the connectivities, lengths, and diameters of the arterial segments, (2) repelling forces from other segments and boundaries, and (3) optimality principles to minimize the drag force at each bifurcation in the generated tree. Using this algorithm, the segmented coronary artery tree from the MSCT data was optimally extended to create a 3D computational model of the largest six orders of the coronary arterial tree. The new method for generating the 3D model is effective in imposing the constraints of anatomical and physiological characteristics of coronary vasculature. When combined with the 4D NCAT phantom, a computer model for the human anatomy and cardiac and respiratory motions, the new model will provide a unique tool to study cardiovascular characteristics and diseases through direct and medical imaging simulation studies.

We are developing a breast specific scatter correction method for digital beast tomosynthesis (DBT). The 3D breast volume was initially reconstructed from 15 projection images acquired from a GE prototype tomosynthesis system without correction of scatter. The voxel values were mapped to the tissue compositions using various segmentation schemes. This voxelized digital breast model was entered into a Monte Carlo package simulating the prototype tomosynthesis system. One billion photons were generated from the x-ray source for each projection in the simulation and images of scattered photons were obtained. A primary only projection image was then produced by subtracting the scatter image from the corresponding original projection image which contains contributions from the both primary photons and scatter photons. The scatter free projection images were then used to reconstruct the 3D breast using the same algorithm. Compared with the uncorrected 3D image, the x-ray attenuation coefficients represented by the scatter-corrected 3D image are closer to those derived from the measurement data.

Neutron Stimulated Emission Computed Tomography (NSECT) was evaluated as a potential technique for breast cancer diagnosis. NSECT can form a 3D tomographic image with an elemental (isotopic) spectrum provided at each reconstructed voxel. The target is illuminated (in vivo) by a neutron beam that scatters in-elastically producing characteristic gamma emission that is acquired tomographically with a spectrograph. Images are reconstructed of each element in the acquired spectrum. NSECT imaging was simulated for benign and malignant breast masses. A range of the number of incident neutrons was simulated from 19 million to 500k neutrons. Simulation included all known primary and secondary physical interactions in both the breast as well as in the spectrometer. Characteristic energy spectra were acquired by simulation and were analyzed for statistically significant differences between benign and malignant breasts. For 1 million incident neutrons, there were 61 differences in the spectra that were statistically significant (p < 0.05). Of these, 23 matched known characteristic emission from 6 elements that have been found in the breast (Br, Cs, K, Mn, Rb, Zn). The dose to two breasts was less than 3% of the dose of a 4 view screening mammogram. Increasing the dose to 52% of the mammogram (19 million neutrons) provided 89 significant spectral differences that matched 30 known emissions from 7 elements that have been found in the breast (Br, Co, Cs, K, Mn, Rb, Zn). Decreasing the dose to 1.4% (500K neutrons) eliminated all statistically significant matches to known elements. This study suggests that NSECT may be a viable technique for detecting human breast cancer in vivo at a reduced dose compared to 4 view screening mammography.

Megavoltage cone-beam computed tomography (CBCT) using active matrix flat-panel imagers (AMFPIs) is a promising candidate for providing image guidance in radiation therapy. Unfortunately, the practical clinical implementation of this technique is limited by the relatively low detective quantum efficiency (DQE) of conventional megavoltage AMFPIs. This limitation is due to the modest thickness of the phosphor screen employed to convert incident x-rays to optical photons and the trade-off that exists between phosphor thickness and spatial resolution. Recently, our group has begun pursuing the development of thick crystalline segmented scintillating detectors as x-ray converters for AMFPIs so as to circumvent this limitation. In order to examine the potential of such detectors for providing soft-tissue visualization by means of CBCT at megavoltage energies, a Monte Carlo-based method was used to simulate the acquisition of projection images of a contrast phantom. These images were used to perform CT reconstructions by means of a Feldkamp-based algorithm. In this study, various detector configurations involving CsI and BGO scintillators at thicknesses of 10 mm and 40 mm were evaluated. In addition, since the simulations only considered energy deposition, and did not include optical phenomena, both segmented and non-segmented (continuous) detector configurations were evaluated. For the segmented CsI detectors, septal wall materials with densities lower, equivalent and higher than that of the scintillator were considered. Performance was quantified in terms of the contrast-to-noise ratio obtained for lowcontrast, soft-tissue-equivalent objects (i.e., liver, brain, and breast) embedded in the phantom. The results obtained from these early studies suggest that such segmented converters can provide visualization of soft-tissue contrast in tomographic images at clinically practical doses. It is anticipated that the realization of optimized segmented detector designs will lead to clinically useful megavoltage AMFPIs exhibiting impressive performance.

While full-field digital mammography (FFDM) technology is gaining clinical acceptance, the overwhelming majority (96%) of the installed base of mammography systems are conventional film-screen (FSM) systems. A high performance, and economical digital cassette based product to conveniently upgrade FSM systems to FFDM would accelerate the adoption of FFDM, and make the clinical and technical advantages of FFDM available to a larger population of women. The planned FFDM cassette is based on our commercial Digital Radiography (DR) cassette for 10 cm x 10 cm field-of-view spot imaging and specimen radiography, utilizing a 150 micron columnar CsI(Tl) scintillator and 48 micron active-pixel CMOS sensor modules. Unlike a Computer Radiography (CR) cassette, which requires an external digitizer, our DR cassette transfers acquired images to a display workstation within approximately 5 seconds of exposure, greatly enhancing patient flow. We will present the physical performance of our prototype system against other FFDM systems in clinical use today, using established objective criteria such as the Modulation Transfer Function (MTF), Detective Quantum Efficiency (DQE), and subjective criteria, such as a contrast-detail (CD-MAM) observer performance study. Driven by the strong demand from the computer industry, CMOS technology is one of the lowest cost, and the most readily accessible technologies available for FFDM today. Recent popular use of CMOS imagers in high-end consumer cameras have also resulted in significant advances in the imaging performance of CMOS sensors against rivaling CCD sensors. This study promises to take advantage of these unique features to develop the first CMOS based FFDM upgrade cassette.

New cone-beam computed tomographic (CBCT) mammography system designs are presented where the detectors provide high spatial resolution, high sensitivity, low noise, wide dynamic range, negligible lag and high frame rates similar to features required for high performance fluoroscopy detectors. The x-ray detectors consist of a phosphor coupled by a fiber-optic taper to either a high gain image light amplifier (LA) then CCD camera or to an electron multiplying CCD. When a square-array of such detectors is used, a field-of-view (FOV) to 20 x 20 cm can be obtained where the images have pixel-resolution of 100 μm or better. To achieve practical CBCT mammography scan-times, 30 fps may be acquired with quantum limited (noise free) performance below 0.2 μR detector exposure per frame. Because of the flexible voltage controlled gain of the LA's and EMCCDs, large detector dynamic range is also achievable. Features of such detector systems with arrays of either generation 2 (Gen 2) or 3 (Gen 3) LAs optically coupled to CCD cameras or arrays of EMCCDs coupled directly are compared. Quantum accounting analysis is done for a variety of such designs where either the lowest number of information carriers off the LA photo-cathode or electrons released in the EMCCDs per x-ray absorbed in the phosphor are large enough to imply no quantum sink for the design. These new LA- or EMCCD-based systems could lead to vastly improved CBCT mammography, ROI-CT, or fluoroscopy performance compared to systems using flat panels.

Direct conversion flat panel detectors (FPD) often experience a loss of sensitivity to x rays caused by previous exposure of the panel to radiation-a phenomenon known as ghosting. Bulk charge trapping and recombination, collectively referred to as incomplete collection of charges, are one of the major causes of ghosting in FPDs. In our investigation, the effects of incomplete charge collection on the modulation transfer function (MTF) of an a-Se direct conversion FPD were studied. The approach used was to repeatedly ghost the panel by uniformly exposing it to a high dose of radiation to force bulk trapping of charges, measure the MTF of the panel after each exposure, and compare this MTF to that of a non-ghosted panel by taking their ratio. MTF ratios allow us to isolate charge collection dependent components of the MTF from all other components. Our results show that ghosting brings about an increase in the MTF of the panel which we interpret as being due to an increase in the incomplete collection of holes. This involves the recombination of free holes with electrons that were trapped from prior exposures, as well as additional trapping of holes caused by the formation of new radiation induced trap states.

We examined the spatial resolution of a columnar CsI(Tl), single-photon imaging system using an approach that estimates the interaction position to better than the spread of the light distribution. A columnar scintillator was directly coupled to a 512×512 electron multiplying CCD (EMCCD) camera (16 μm pixels) binned at 2×2 to sample at 32 μm pixels. Optical photons from gamma-ray/scintillator interactions are sampled over multiple pixels. Resultant images show clusters of signal at the original interaction site, clusters from Cs and I K x-rays up to several hundred microns away, and clusters from collimator K x-rays. Also evident are depth-of-interaction effects which result in a broadening of the light distribution. These effects result in a degradation of spatial and energy resolution. Cluster pixel data was processed to better estimate the interaction position within the initial interaction cluster. Anger (centroid) estimation of individual gamma-ray events yielded spatial resolutions better than 100 μm; a result previously achievable only with pixellated semiconductor detector arrays. After proper calibration, depth-of-interaction (DOI) effects are corrected by performing maximum-likelihood 3D position and energy estimation of individual gamma-ray interactions.

Digital clinical imaging systems designed for radiography or cone-beam computed-tomography are highly shift-variant. The x-ray cone angle of such systems varies between 0° and 15°, resulting in large variations of the focal spot projection across the image field. Additionally, the variable x-ray beam incidence across the detector field creates a location-dependent asymmetric detector response function. In this paper we propose a practical method for the measurement of the angle of incidence dependent two-dimensional presampled detector response function. We also present a method for the measurement of the source radiance at the center of the detector, and provide a geometric transformation for reprojecting given any location in object space. The measurement procedure involves standard, readily available tools such as a focal-spot/pinhole camera, and an edge. Using the measured data and a model based on smooth functions derived from Monte Carlo simulations we obtain the location-dependent detector response function. In this paper we ignore scatter, therefore the resulting location dependent system response is a function of the focal spot and detector response. The system matrix, a representation of the full deterministic point response of the system for all positions in object space, can then be calculated. The eigenvalues and eigenvectors of the system matrix are generated and interpreted.

A compact, dual modality computed mammotomography (CmT) and single photon emission computed tomography (SPECT) system for dedicated 3D breast imaging is in development. The CmT component utilizes novel, heavy K-edge filtration to practicably narrow the energy spectrum of the cone-shaped x-ray beam incident on the patient's pendant, uncompressed breast. This quasi-monochromatic beam in CmT is expected to improve discrimination of tissue with very similar attenuation coefficients while restraining dose levels to below that of existing dual view mammography. Our previous extensive simulation studies showed the optimal energy range that provides maximum dose efficiency for a 50/50 adipose/glandular breast is in the 35-40keV range. This current study aims to experimentally validate previous simulation results. Here, experimental pre-breast and post-breast collimated x-ray beam spectral measurements are made under tube operating voltages between 40-100kVp using filter materials from Z=13-74, with K-edge values spanning that of Ce (K=40.4keV), and using different attenuating thicknesses of filter material, approximately equivalent to the 200^th and 500^th attenuating value layer (VL) thickness. Ce-filtered post breast spectra for 8cm to 18cm breasts are measured for a range of breast adipose/glandular compositions. Evaluated figures of merit include mean beam energy, spectral full-width at tenth-maximum, beam hardening and dose for the range of breast sizes. Measurements are shown to corroborate the simulations, and both indicate that for a given dose a 200^th VL of Ce filtration may have the most optimal performance in the dedicated mammotomography paradigm.

The risk of radiation exposure is greatest for pediatric patients and, thus, there is a great incentive to reduce the radiation dose used in diagnostic procedures for children to "as low as reasonably achievable" (ALARA). Testing of low-dose protocols presents a dilemma, as it is unethical to repeatedly expose patients to ionizing radiation in order to determine optimum protocols. To overcome this problem, we have developed a computed-radiography (CR) dose-reduction simulation tool that takes existing images and adds synthetic noise to create realistic images that correspond to images generated with lower doses. The objective of our study was to determine the extent to which simulated, low-dose images corresponded with original (non-simulated) low-dose images. To make this determination, we created pneumothoraces of known volumes in five neonate cadavers and obtained images of the neonates at 10 mR, 1 mR and 0.1 mR (as measured at the cassette plate). The 10-mR exposures were considered "relatively-noise-free" images. We used these 10 mR-images and our simulation tool to create simulated 0.1- and 1-mR images. For the simulated and original images, we identified regions of interest (ROI) of the entire chest, free-in-air region, and liver. We compared the means and standard deviations of the ROI grey-scale values of the simulated and original images with paired t tests. We also had observers rate simulated and original images for image quality and for the presence or absence of pneumothoraces. There was no statistically significant difference in grey-scale-value means nor standard deviations between simulated and original entire chest ROI regions. The observer performance suggests that an exposure ≥0.2 mR is required to detect the presence or absence of pneumothoraces. These preliminary results indicate that the use of the simulation tool is promising for achieving ALARA exposures in children.

Fluoroscopic procedures can result in significant radiation exposures to patients. To maximize the patient benefit-to-risk ratio, systems must be designed to produce the highest possible image quality for a given patient exposure, and quality assurance programs must be designed to ensure these standards are maintained. While the detective quantum efficiency (DQE) is often used in radiography to quantify "dose efficiency," attempts to measure the DQE of fluoroscopic systems have produced nonsensical results due to system lag reducing measured noise power spectrum (NPS) values. Methods involving the use of the system temporal modulation transfer function (MTF) have been proposed to remove this effect. However, these methods are not easily implemented in a clinical setting and as a result, the DQE of fluoroscopic systems is rarely measured. We have developed a novel method to measure system temporal MTF using a moving slanted-edge method and acquiring image data while the edge is translated across the detector with constant velocity. Each pixel from a video frame is mapped to a spatiotemporal coordinate based on the distance and time from passage of the edge at that pixel. Using data acquired with both stationary and moving (45 cm/s) edges, we calculate both the spatial and temporal presampling MTF. The method has been demonstrated using a bench-top image-intensifier-based fluoroscopic system using detector exposures representative of clinical procedures. Image data was acquired by digitizing the fluoroscopic video signal. The method was validated by comparison with a direct measure of the optical decay curve of the image intensifier. After correction for the temporal effects of the video integration time, excellent agreement was obtained between the two methods. It is concluded that the moving slanted-edge method provides a practical method for measuring the temporal presampling MTF of a fluoroscopic system.

A novel phantom has been developed to measure the modulation transfer function (MTF) in 3D for x-ray computed tomography. The phantom consists of three tungsten wires, positioned nearly orthogonal to each other. Simultaneous measurements of the MTF are taken at various locations along the three orthogonal reconstructed planes. Our computed mammotomography (CmT) system uses a Varian Paxscan 2520 digital x-ray detector which can be positioned anywhere in ~2pi steradian band and can have arbitrary trajectories. With a half-cone beam geometry and with the phantom positioned near the center of rotation, projection images are acquired over 360 degrees. Various 3D orbits are evaluated including vertical axis of rotation and saddle. Reconstructions were performed using an iterative ordered-subsets transmission algorithm on rebinned projection images, using various numbers of iterations. Rotation of reconstructed slices isolated each wire into its own plane. At various locations along the length of each wire, corresponding MTFs were calculated from 1D line spread functions. Through measurement, accuracy of wire method was verified by comparison of the projection MTFs computed from a wire and a standard edge device. Results indicated minor variations in MTF among the three orthogonal planes, which imply a high degree of uniform sampling in the imaged volume. Findings indicate that the phantom can be used to assess the intrinsic image resolution in 3D as well as potential degradative effects of measurements in various media.

The rapid development of modern multi-slice computed tomography (MSCT) scanners has provided imaging systems with cone-beam geometry, sub-millimetre slice thickness, and gantry rotation speeds approaching 0.3 seconds per revolution. Clinical MSCT scanners routinely generate volume data sets yet the methods used to quantify spatial resolution remain relatively unchanged from those used to evaluate single slice scanners. In this paper, we describe a method for quantifying the spatial resolution of an MSCT scanner, with cone-beam, geometry using a sphere phantom. By scanning a Teflon sphere embedded in a uniform silicone cylinder, the plane spread function (PlSF) and modulation transfer function (MTF) may be determined. Furthermore, the spatial resolution in the axial and trans-axial directions may be independently quantified, as well as the effects of Azimuthal blur and spiral scanning. To illustrate the utility of the sphere method, the spatial resolution of two commercially available MSCT scanners was measured.

Sinogram truncation is a common problem in tomographic reconstruction; it occurs when a scanned object or patient extends outside the scan field-of-view. The truncation artifact propagates from the edge of truncation towards the center, resulting in degraded image quality. Several methods have been proposed recently to reconstruct the image artifact-free within the scan FOV; however it is often necessary to recover image outside the scan FOV. We propose a novel truncation correction algorithm that accurately completes unmeasured data outside of the scan field-of-view, which allows us to extend the reconstruction field-of-view. Contrary to 1D extrapolation, we perform interpolation along the so-called sinogram curves. First, we propose an approach to parameterize the family of sinogram curves for efficient sinogram decomposition. Secondly, we propose two ways to estimate the truncated data outside the field-of-view. Both methods are combined for more accurate sinogram completion. Our evaluation shows the validity of our approach. Even objects completely outside the FOV can be accurately reconstructed using the proposed method. The proposed method can be used with any modality where sinogram truncation occurs, such as CT, C-arm, PET/CT, and SPECT.

Usage of the backprojection filtration (BPF) algorithm for reconstructing images from motion-contaminated fan-beam data may result in motion-induced streak artifacts, which appear in the direction of the chords on which images are reconstructed. These streak artifacts, which are most pronounced along chords tangent to the edges of the moving object, may be suppressed by use of the weighted BPF (WBPF) algorithm, which can exploit the inherent redundancies in fan-beam data. More specifically, reconstructions using full-scan and short-scan data can allow for substantial suppression of these streaks, whereas those using reduced-scan data can allow for partial suppression. Since multiple different reconstructions of the same chord can be obtained by varying the amount of redundant data used, we have laid the groundwork for a possible method to characterize the amount of motion encoded within the data used for reconstructing an image on a particular chord. Furthermore, since motion artifacts in WBPF reconstructions using full-scan and short-scan data appear similar to those in corresponding fan-beam filtered backprojection (FFBP) reconstructions for the cases performed in this study, the BPF and WBPF algorithms potentially may be used to arrive at a more fundamental characterization of how motion artifacts appear in FFBP reconstructions.

A formula was recently described by Clackdoyle et. al. for image reconstruction within a region of interest (ROI) from knowledge of its truncated 2D Radon transform. In this work, we present an alternative, simple derivation of the formula by using the well-known relationship between the parallel-beam and fan-beam geometries. Based upon our derivation, the role of parameter t in the formula in ROI-image reconstruction can be clearly identified. We show that the parameter t determines the size of a reconstructible ROI from parallel-beam data containing truncations. Numerical studies were performed to by use of the formula with different t. We show that the formula yields ROI images with smaller sizes and lower quality than does our backprojection filtration algorithm.

Images reconstructed for transmission tomography with iterative Ordered Subsets Maximum Likelihood (OSML) algorithms have a higher signal-to-noise ratio than images reconstructed with filtered back-projection type algorithms. However, a drawback of OSML reconstruction is the requirement that a field-of-view (FOV) has to be reconstructed that covers the whole volume, which contributed to the absorption. In the case of a high resolution reconstruction, this demands a huge number of voxels. This paper presents a solution, how an iterative OSML reconstruction can be limited to a region of interest without loosing the advantages of a OSML reconstruction. Compared with a full FOV OSML reconstruction, the reconstruction speed mainly increases by the number of voxels, which are saved. In addition, less iterations are needed to achieve the same result.

The capabilities of flat panel interventional x-ray systems continue to expand, enabling a broader array of medical applications to be performed in a minimally invasive manner. Although CT is providing pre-operative 3D information, there is a need for 3D imaging of low contrast soft tissue during interventions in a number of areas including neurology, cardiac electro-physiology, and oncology. Unlike CT systems, interventional angiographic x-ray systems provide real-time large field of view 2D imaging, patient access, and flexible gantry positioning enabling interventional procedures. However, relative to CT, these C-arm flat panel systems have additional technical challenges in 3D soft tissue imaging including slower rotation speed, gantry vibration, reduced lateral patient field of view (FOV), and increased scatter. The reduced patient FOV often results in significant data truncation. Reconstruction of truncated (incomplete) data is known an "interior problem", and it is mathematically impossible to obtain an exact reconstruction. Nevertheless, it is an important problem in 3D imaging on a C-arm to address the need to generate a 3D reconstruction representative of the object being imaged with minimal artifacts. In this work we investigate the application of an iterative Maximum Likelihood Transmission (MLTR) algorithm to truncated data. We also consider truncated data with limited views for cardiac imaging where the views are gated by the electrocardiogram(ECG) to combat motion artifacts.

With the introduction of volumetric computed tomography (VCT), many studies have been conducted on the cone beam reconstruction algorithms. Majority of the research efforts was devoted to the reduction of cone beam artifacts by designing special weighting functions that treat the projection samples differently based on their cone angles. The selection of weighting function can be quite complex due to the cone beam helical geometry and is often designed through trial-and-error. In this paper, we present a conjugate backprojection approach (CBA) that is capable of significantly reducing cone beam artifacts without the application of a pre-backprojection weighting function. Our approach utilizes CBA operation which simultaneously backprojects and interpolates conjugate sampling pairs. Unlike the conventional backprojection, the final backprojected value depends not only on the distance of these samples to the point-of-intersection, but also on the cone angles of these samples. Consequently, optimization can be performed on a pixel by pixel basis and is independent of the acquisition protocols, such as helical pitches or acquisition modes. The proposed approach is applicable to cone beam helical as well as step-and-shoot acquisitions. Extensive computer simulation and phantom experiments were conducted to demonstrate the efficacy of our approach. Results have shown that the proposed approach significantly reduces the cone beam artifacts.

We develop a reconstruction algorithm for local cone-beam X-ray tomography for use with generalized scanning trajectories. The algorithm is based upon an extension of a recently-developed chord-based theory for exact conebeam image reconstruction. Being chord-based, it is distinct mathematically and conceptually from conventional local tomography reconstruction algorithms. A computer-simulation study is conducted to demonstrate the algorithm, and compare its performance to an existing algorithm.

A three-dimensional weighted cone beam filtered backprojection (CB-FBP) algorithm (namely 3D weighted CB-FBP algorithm) has been proposed to reconstruct images from the projection data acquired along a helical trajectory in angular ranges up to [0, 2π]. However, an over scan is usually employed in the clinic to provide premium image qualities for an accurate diagnosis at the most challenging anatomic structures, such as head, spine and extremities. In an over scan, the corresponding normalized helical pitch is usually smaller than 1:1, under which the projection data acquired along angular range larger than [0, 2π] can be utilized to reconstruct an image. To improve noise characteristics or dose efficiency in an over scan, we extended the 3D weighted CB-FBP algorithm to handle helical pitches that are smaller than 1:1, while the algorithm's other advantages, such as reconstruction accuracy and computational efficiency, are maintained. The novelty of the extended 3D weighted CB-FBP algorithm is the decomposition of an over scan with an angular range corresponding to [0, 2π + Δβ] (0 < Δβ < 2π) into a union of full scans with an angular range corresponding to [0, 2π]. As a result, the extended 3D weighted function is a weighted sum of all 3D weighting functions corresponding to each overlapped full scan. An experimental evaluation shows that, the extended 3D weighted CB-FBP algorithm can significantly improve noise characteristics or dose efficiency of the 3D weighted CB-FBP algorithm at helical pitch smaller than 1:1, while its reconstruction accuracy and computational efficiency are maintained. It is imortant to indicate that, the extended 3D weighting function is still applied on projection data before 3D backporjection, resulting in the computational efficiency of the extended 3D weighted CB-FBP algorithm comparable to that of the 3D weighted CB-FBP algorithm. It is believed that, such an efficient CB reconstruction algorithm that can provide premium image qualities at low helical pitches will find its extensive applications in CT medical imaging.

In this work, we investigate exact image reconstruction within a 3D region of interest from data acquired with a circle-arc trajectory. In particular, the data may contain both longitudinal and transverse truncations. This work may find applications in lung or heart imaging using a C-arm scanner. When the arc portion of the trajectory is posterior or anterior to the patient, exact images within the lung or heart region can be reconstructed from truncated data.

Image reconstruction from few-view CT is of interest because of the potential to reduce scanning time and radiation dose. The challenge of few-view CT for image reconstruction is essentially a problem of interpolation from under-sampled data. Recently, a new algorithm for inverting the Fourier transform from under-sampled data has been developed by Candes et al. IEEE Trans. Inf. Theory , 52 489 (2006). This algorithm can be directly applied to image reconstruction in 2D parallel-beam CT because of the central slice theorem. This article presents a discussion of the new algorithm, showing examples for different degrees of under-sampling.

A new reconstruction algorithm for Radon data is introduced. We call the new algorithm OPED as it is based on Orthogonal Polynomial Expansion on the Disk. OPED is fundamentally different from the filtered back projection (FBP) method. It allows one to use fan beam geometry directly without any additional procedures such as interpolation or rebinning. It reconstructs high degree polynomials exactly and works for smooth functions without the assumption that functions are band- limited. Our initial tests indicate that the algorithm is stable, provides high resolution images, and has a small global error. Working with the geometry specified by the algorithm and a new mask, OPED could also lead to a reconstruction method that works with reduced x-ray dose (see the paper by Tischenko et al in these proceedings).

In order to obtain accurate quantitative results, flat panel detectors require specific calibration and correction of acquisitions. Main artefacts are due to bad pixels, variations of photodiodes characteristics and inhomogeneity of X-rays sensitivity of the scintillator layer. Other limitations for quantification are the non-linearity of the detector due to charge trapping in the transistors and the scattering generated inside the detector, called detector scattering. Based on physical models of artefacts generation, this paper presents an unified framework for the calibration and correction of these artefacts. The following specific algorithms have been developed to correct them. A new method for correction of deviation to linearity is based on the comparison between experimental and simulated data. A method of detector scattering correction is performed in two steps: off-line characterization of detector scattering by considering its spatial distribution through a convolution model and on-line correction based on a deconvolution approach. Radiographic results on an anthropomorphic thorax phantom imaged with a flat panel detector, that convert X-rays into visible light using scintillator coupled to an amorphous silicon transistor frame for photons to electrons conversion, demonstrate that experimental X-rays attenuation images are significantly improved qualitatively and quantitatively by applying non-linearity correction and detector scattering correction. Results obtained on tomographic reconstructions from pre-processed acquisitions of the phantom are in very good agreement with expected attenuation coefficients values obtained with a multi-slice CT scanner. Thus, this paper demonstrates the efficiency of the proposed pre-processings to perform accurate quantification on radiographies and tomographies.

The purpose of this study is to characterize a newly built flat panel detector (FPD)-based cone beam CT (CBCT) prototype for dynamic imaging. A CBCT prototype has been designed and constructed by completely modifying a GE HiSpeed Advantage (HSA) CT gantry, incorporating a newly acquired large size real-time FPD (Varian PaxScan 4030CB), a new x-ray generator and a dual focal spot angiography x-ray tube that allows the full coverage of the detector. During data acquisition, the x-ray tube and the FPD can be rotated on the gantry over Nx360 degrees due to integrated slip ring technology with the rotation speed of one second/revolution. With a single scan time of up to 40 seconds , multiple sets of reconstructions can be performed for dynamic studies. The upgrade of this system has been completed. The prototype was used for a series of preliminary phantom studies: different sizes of breast phantoms, a Humanoid chest phantom and scatter correction studies. The results of the phantom studies demonstrate that good image quality can be achieved with this newly built prototype.

The new Multi-View Reconstruction (MVR) method for generating 3D vascular images was evaluated experimentally. The MVR method requires only a few digital subtraction angiographic (DSA) projections to reconstruct the 3D model of the vessel object compared to 180 or more projections for standard CBCT. Full micro-CBCT datasets of a contrast filled carotid vessel phantom were obtained using a Microangiography (MA) detector. From these datasets, a few projections were selected for use in the MVR technique. Similar projection views were also obtained using a standard x-ray image intensifier (II) system. A comparison of the 2D views of the MVRs (MA and II derived) with reference micro-CBCT data, demonstrated best agreement with the MA MVRs, especially at the curved part of the phantom. Additionally, the full 3D MVRs were compared with the full micro-CBCT 3D reconstruction resulting for the phantom with the smallest diameter (0.75 mm) vessel, in a mean centerline deviation from the micro-CBCT derived reconstructions of 29 μm for the MA MVR and 48 μm for the II MVR. The comparison implies that an MVR may be substituted for a full micro-CBCT scan for evaluating vessel segments with consequent substantial savings in patient exposure and contrast media injection yet without substantial loss in 3D image content. If a high resolution system with MA detector is used, the improved resolution could be well suited for endovascular image guided interventions where visualization of only a small field of view (FOV) is required.

Most approaches to 3D x-ray imaging geometry calibration use some well-defined calibration phantom containing point markers. The calibration aims at minimizing the so-called re-projection error, i.e., the error between the detected marker locations in the acquired projection image and the projected marker locations based on the phantom model and the current estimate of the imaging geometry. The phantoms that are being employed consist usually of spherical markers arranged in some spatial pattern. One widely used phantom type consists of spherical markers in a helical arrangement. We present a framework that establishes a good intuitive understanding of the calibration problem, and allows to evaluate the performance of different phantom designs. It is based on a linear approximation of the error propagation between parameters of the imaging geometry, a projection alignment error (which is not identical to the re-projection error), and the "backprojection misalignment", which ultimately dictates 3D image quality. This methodology enables us to characterize the statistics of the parameters describing the imaging geometry, based on simple assumptions on "measurement noise", i.e., phantom and pre-processing accuracy. We also characterize the 3D misalignment in the backprojection (which is used in the 3D reconstruction), which directly impacts 3D image quality. In a comparison of different phantom designs -using backprojection misalignment as a metric- a "candy cane" phantom was found to give superior performance. The presented approach gives many useful intuitive insights into the calibration problem and its key properties. It can also be leveraged, e.g., for an easy implementation of a fast and robust calibration algorithm.

In Computed Tomography (CT), the image quality sensitively depends on the accuracy of the X-ray projection signal, which is acquired by a two-dimensional array of pixel cells in the detector. If the signal of X-ray photons is spread out to neighboring pixels (crosstalk), a decrease of spatial resolution may result. Moreover, streak and ring artifacts may emerge. Deploying system simulations for state-of-the-art CT detector configurations, we characterize origin and appearance of these artifacts in the reconstructed CT images for different scenarios. A uniform pixel-to-pixel crosstalk results in a loss of spatial resolution only. The Modulation Transfer Function (MTF) is attenuated, without affecting the limiting resolution, which is defined as the first zero of the MTF. Additional streak and ring artifacts appear, if the pixel-to-pixel crosstalk is non-uniform. Parallel to the system simulations we developed an analytical model. The model explains resolution loss and artifact level using the first and second derivative of the X-ray profile acquired by the detector. Simulations and analytical model are in agreement to each other. We discuss the perceptibility of ring and streak artifacts within noisy images if no crosstalk correction is applied.

Multislice CT with a larger number of detector rows has recently become the mainstream. As a result, scanning with a thin slice thickness is more frequently performed. However, a large number of obvious raster-type artifacts occur when X-ray absorption in the lateral direction is extremely high, such as in the shoulder and the pelvis. There are two methods to solve this problem. In one method, X-ray output is modulated during rotation so that the exposure dose is increased in regions with high X-ray absorption and reduced in regions with low X-ray absorption. In the other method, regions that are responsible for artifacts are filter-processed using image processing to minimize artifacts. From the viewpoints of image quality and exposure dose, we have evaluated a method we have developed that combines X-ray modulation technology (X-ray Modulation) and artifact elimination processing (Boost3D). An acrylic elliptical phantom was used for evaluation. Assuming a constant image SD level, it was found that the exposure dose can be reduced by approximately 25% with the combined use of X-ray Modulation and Boost3D.

One of the most important advantages of the novel multi slice CT (MSCT) with increased number of slices is the ability to reduce the scan time. However, does the increased number of slices in the MSCT enforce us to reduce the pitch, in order to avoid windmill artifacts, hence preventing us from decreasing the scan time? In this work we address this issue along with other aspects of the windmill artifacts. We study the dependence of the windmill artifacts, their strength and frequency, on the number of slices and on the pitch. The study demonstrates, that when retaining constant bed speed while increasing the number of slices, the intensity of the windmill artifacts is reduced significantly. Images of scans performed with the same pitch, yet with various number of slices are compared. It is observed, that the intensity of the windmill artifacts is similar, independent of the number of slices. The frequency of the artifacts however, increases with the number of slices. The study concludes that updating a clinical protocol performed with a low number of slices MSCT, to a similar protocol performed with high number of slices MSCT, the same pitch can be used attaining better IQ. Scanning with the same pitch using wider coverage enables an advantageous shorter scan time in novel MSCT.

We have developed an amplitude correlated (AC) 4-dimensional cone beam CT (4D-CBCT) imaging technique on the Varian cone beam CT system. We use the Varian Real-time Positioning Monitoring (RPM) system to synchronize the recording of the respiratory motion and the 4D-CBCT imaging. The projection data of the same amplitude in respiratory motion are used to reconstruct an image of the corresponding amplitude in RPM. In the absence of hysteresis of respiratory motion, we can utilize the two CBCT projections in the same breathing cycle: one in the inspiration to expiration and one in the expiration to inspiration to improve the signal to noise ratio, reduce the aliasing due to insufficient sampling in the angular direction.

We developed and evaluated a prototype flat-panel detector based Volume CT (fpVCT) scanner. The fpVCT scanner consists of a Varian 4030CB a-Si flat-panel detector mounted in a multi slice CT-gantry (Siemens Medical Solutions). It provides a 25 cm field of view with 18 cm z-coverage at the isocenter. In addition to the standard tomographic scanning, fpVCT allows two new scan modes: (1) fluoroscopic imaging from any arbitrary rotation angle, and (2) continuous, time-resolved tomographic scanning of a dynamically changing viewing volume. Fluoroscopic imaging is feasible by modifying the standard CT gantry so that the imaging chain can be oriented along any user-selected rotation angle. Scanning with a stationary gantry, after it has been oriented, is equivalent to a conventional fluoroscopic examination. This scan mode enables combined use of high-resolution tomography and real-time fluoroscopy with a clinically usable field of view in the z direction. The second scan mode allows continuous observation of a timeevolving process such as perfusion. The gantry can be continuously rotated for up to 80 sec, with the rotation time ranging from 3 to 20 sec, to gather projection images of a dynamic process. The projection data, that provides a temporal log of the viewing volume, is then converted into multiple image stacks that capture the temporal evolution of a dynamic process. Studies using phantoms, ex vivo specimens, and live animals have confirmed that these new scanning modes are clinically usable and offer a unique view of the anatomy and physiology that heretofore has not been feasible using static CT scanning. At the current level of image quality and temporal resolution, several clinical applications such a dynamic angiography, tumor enhancement pattern and vascularity studies, organ perfusion, and interventional applications are in reach.

The physical performance of two flat panel detectors (FPD) has been evaluated using a standard x-ray beam quality set by IEC, namely RQA5. The FPDs evaluated in this study are based on an amorphous silicon photodiode array that is coupled to a thallium-doped Cesium Iodide scintillator and to a thin film transistor (TFT) array. One detector is the PaxScan 2520 that is designed for fluoro imaging, and has a small dynamic range and a large image lag. The other detector is the PaxScan 4030CB that is designed for cone beam CT, and has a large dynamic range (>16-bit), a reduced image lag and many imaging modes. Varian Medical Systems manufactured both detectors. The linearity of the FPDs was investigated by using an ionization chamber and aluminum filtration in order to obtain the beam quality. Since the FPDs are used in fluoroscopic mode, image lag of the FPD was measured in order to investigate its effect on this study, especially its effect on DQE. The spatial resolution of the FPDs was determined by obtaining the pre-sampling modulation transfer function for each detector. A sharp edge was used in accordance to IEC 62220-1. Next, the Normalized Noise Power Spectrum (NNPS) was calculated for various exposures levels at RQA5 radiation quality. Finally, the DQE of each FPD was obtained with a modified version of the international standard set by IEC 62220-1. The results show that the physical performance in DQE and MTF of the PaxScan 4030CB is superior to that of PaxScan2520.

The amount of x-ray radiation currently applied in CT practice is not utilized optimally. A portion of radiation traversing the patient is either not detected at all or is used ineffectively. The reason lies partly in the reconstruction algorithms and partly in the geometry of the CT scanners designed specifically for these algorithms. In fact, the reconstruction methods widely used in CT are intended to invert the data that correspond to ideal straight lines. However, the collection of such data is often not accurate due to likely movement of the source/detector system of the scanner in the time interval during which all the detectors are read. In this paper, a new design of the scanner geometry is proposed that is immune to the movement of the CT system and will collect all radiation traversing the patient. The proposed scanning design has a potential to reduce the patient dose by a factor of two. Furthermore, it can be used with the existing reconstruction algorithm and it is particularly suitable for OPED, a new robust reconstruction algorithm.

The purpose of this study was to investigate how tissue x-ray attenuation coefficients, and their uncertainties, vary with x-ray tube voltage in different sized patients. Anthropomorphic phantoms (newborn, 10 year old, adult) were scanned a GE LightSpeed scanner at four x-ray tube voltages. Measurements were made of tissue attenuation in the head, chest and abdomen regions, as well as the corresponding noise values. Tissue signal to noise ratios (SNR) were obtained by dividing the average attenuation coefficient by the corresponding standard deviation. Soft tissue attenuation coefficients, relative to water, showed little variation with patient location or x-ray voltage (< 0.5%), but increasing the x-ray tube voltage from 80 to 140 kV reduced bone x-ray attenuation by ~14%. All tissues except adult bone showed a reduction of noise with increasing x-ray tube voltage (kV); the noise was found to be proportional to kVⁿ and the average value of n for all tissues was -1.19 ± 0.57. In pediatric patients at a constant x-ray tube voltage, SNR values were approximately independent of the body region, but the adult abdomen soft tissue SNR values were ~40% lower than the adult head. SNR values in the newborn were more than double the corresponding SNR soft tissue values in adults. SNR values for lung and bone were generally lower than those for soft tissues. For soft tissues, increasing the x-ray tube voltage from 80 to 140 kV increased the SNR by an average of ~90%. Data in this paper can be used to help design CT imaging protocols that take into account patient size and diagnostic imaging task.

Dual-energy contrast enhanced digital mammography (DE-CEDM), which is based upon the digital subtraction of low/high-energy image pairs acquired before/after the administration of contrast agents, may provide physicians physiologic and morphologic information of breast lesions and help characterize their probability of malignancy. This paper proposes to use only one pair of post-contrast low / high-energy images to obtain digitally subtracted dual-energy contrast-enhanced images with an optimal weighting factor deduced from simulated characteristics of the imaging chain. Based upon our previous CEDM framework, quantitative characteristics of the materials and imaging components in the x-ray imaging chain, including x-ray tube (tungsten) spectrum, filters, breast tissues / lesions, contrast agents (non-ionized iodine solution), and selenium detector, were systemically modeled. Using the base-material (polyethylene-PMMA) decomposition method based on entrance low / high-energy x-ray spectra and breast thickness, the optimal weighting factor was calculated to cancel the contrast between fatty and glandular tissues while enhancing the contrast of iodized lesions. By contrast, previous work determined the optimal weighting factor through either a calibration step or through acquisition of a pre-contrast low/high-energy image pair. Computer simulations were conducted to determine weighting factors, lesions' contrast signal values, and dose levels as functions of x-ray techniques and breast thicknesses. Phantom and clinical feasibility studies were performed on a modified Selenia full field digital mammography system to verify the proposed method and computer-simulated results. The resultant conclusions from the computer simulations and phantom/clinical feasibility studies will be used in the upcoming clinical study.

This paper describes a method for single exposure contrast-enhanced dual-energy imaging of tumors utilizing a scanned multi-slit system for digital mammography. This photon counting system employs an array of silicon strip detectors in an edge-on geometry. In the multi-slit setup, the line detectors and pre-collimator slits are aligned orthogonal to the scan direction. This geometry is advantageous to dual-energy imaging, since it allows differential filtering of the x-ray beam in the pre-collimator slits. A high-energy image is constructed from those lines where the filter material has been chosen to harden the x-ray beam and the low-energy image from the lines with a filter producing softer beams. Both images are obtained in the same scan, eliminating the need to change tube voltages and anode materials and minimizing the risk of motion artifacts. The method is illustrated on a purpose-built phantom and logarithmic subtraction of the images produces images essentially free of anatomical clutter with the contrast-enhanced targets clearly visible.

Cone-beam breast computed tomography (CB Breast CT) can easily detect micro-calcifications and distinguish fat and glandular tissues from normal breast tissue. However, it may be a challenging task for CB Breast CT to distinguish benign from malignant tumors because of the subtle difference in x-ray attenuation. Due to the use of polyenergetic x-ray source, the x-ray and tissue interaction exhibits energy-dependent attenuation behavior, a phenomenon that, to date, has not been used for breast tissue characterization. We will exploit this spectral nature by equipping our CB Breast CT with dual-spectral imaging. The dual-spectral cone-beam scanning produces two spectral image datasets, from which we propose a nonlinear dual-spectral image fusion scheme to combine them into a single dataset, thereby incorporating the spectral information. In implementation, we will perform dual-spectral image fusion through a bi-variable polynomial that can be established by applying dual-spectral imaging to a reference material (with eight different thicknesses). From the fused dataset, we can reconstruct a volume, called a reference-equivalent volume or a fusion volume. By selecting the benign tissue as a reference material, we obtain a benign-equivalent volume. Likewise, we obtain a malignant-equivalent volume as well. In the pursuit of the discrimination of benign versus malignant tissues in a breast image, we perform intra-image as well as inter-image processing. The intra-image processing is an intensity transformation imposed only to a tomographic breast image itself, while the inter-image processing is exerted on two tomographic images extracted from two volumes. The nonlinear fusion scheme possesses these properties: 1) no noise magnification; 2) no feature dimensionality problem, and 3) drastic enhancement among specific features offered by nonlinear mapping. Its disadvantage lies in the possible misinterpretation resulting from nonlinear mapping.

In this paper, we aim at a polychromatic physics model of dual-energy medical x-ray imaging and present a corresponding computation method, which includes two steps: first, bd(s, theta), i.e. the parameter used for expressing the component of Compton scatter, and bp(s, theta), i.e. the parameter used for expressing the component of Photoemission Effect, are decided by solving a nonlinear equation system; then a FBP ( Filtered Back Projection ) algorithm is used to reconstruct the decomposition images from the sinograms of bp(s, theta) and bp(s, theta). It is noticed that the first step is the most time-consuming, so it is very important to find out a high-speed and effective iteration computation method. In this paper, we propose a Newton iteration method with an effective estimation strategy of initial value for quickly solving the nonlinear equation system. A CT simulation experiment was implemented to validate the effectiveness of the whole procedure.

Dual-energy X-ray imaging is an important method of medical imaging, capable of not only obtaining spatial information of imaging object but also disclosing its chemical components, and has many applications in clinic. The current computation methods of dual-energy imaging are still based on the model of mono-energy spectrum imaging with some linear calibration, while they are incapable to reflect correctly the physical characteristics of dual-energy imaging and obstruct deeper research in this field. The article presents a new medical X-ray imaging model in accordance with physics of imaging and its corresponding computational method. The computation process includes two steps: first, to compute two attenuation parameters that have clear physical meaning: equivalent electron density and attenuation parameter of photoemission; then to compute the components of high- and low-density mass through a group of simple equation with two variables. Experiments showed that such method has quite a satisfactory precision in theory, that is, the solutions of parameters under different exposure voltages and thickness of tissue for several main tissues of human body are much low in deviations, whose quotient of standard deviation divided by mean are mostly under 0.1%, and at most 0.32%. The method provides not only a new computational way for dual-energy X-ray imaging, but also a feasible analysis for its nature. In addition, the method can be used to linearly rectify data of dual-energy CT and analyze the chemical component of reconstructed object by means of parameters clear in physics.

A scintillator type Flat Panel Detector (FPD)¹ has a good noise performance especially in Fluoroscopic images because of high DQE. Almost same dose as I.I. and CCD system is accepted in clinical use. According to the clinical study, the dose in fluoroscopy will be decreased if we can reduce the line noise coming from gate line of the Thin Film Transistor (TFT). The purpose of this study is to detect and reduce this line noise from the fluoroscopic images making it possible to perform a lower dose of fluoroscopy imaging. We detected the line noise by acquiring a dark image (without exposure) and then comparing the average of the line data along to the gate line to the neighborhood lines. We have applied this method to the dark area taken by the collimator of the Lucite phantom image and detected it. The detected line will be compensated by interpolation with neighborhood lines. The FPD of our system² has a big detecting area (40cm x 30cm) and a zoom mode is selected in fluoroscopy because the doctor is watching an edge of the guide-wire and a contrast medium. The collimated area of the detector is displayed in a monitor after the zooming process and we can take a collimated dark area for detecting the line noise. As we applied this method to the dark image (1024pixels x 1024lines) including 54 lines with noise, we can improve 10% of SD. Visible line noise of chest phantom image was reduced with this method. It will help to lower the fluoroscopy dose.

Amorphous silicon photodiodes are increasingly being used as fundamental components in digital diagnostic medical imaging system including large area chest radiography, mammography and real time fluoroscopy. The intrinsic a-Si:H material (i-a-Si:H), commonly deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD), is well known to suffer from both light and bias stress induced instabilities over time that can result in an increase in dark current and a decrease in photoconductivity. In contrast, research in Hot-Wire Chemical Vapor Deposition (HWCVD) indicates that a-Si:H films grown by HWCVD can have superior physical and electronic properties to those grown by PECVD. In this research, we report on the material properties and stability of i-a-Si:H material by comparing the photoconductivity degradation of the HWCVD and PECVD films over time. Then, we discuss the p-i-n diode fabrication process and examine the leakage and photo-current degradation in the HWCVD and PECVD photodiode structures over time via bias and time stress measurements. Also, we investigate the quantum efficiency degradation over time in a-Si:H p-i-n detectors grown by PECVD.