This PDF files contrains the front matter associated with SPIE Proceedings Volume 6510, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

This paper surveys our current understanding of the statistical properties of radiological images and their effect on image quality. Attention is given to statistical descriptions needed to compute the performance of ideal or ideal-linear observers on detection and estimation tasks. The effects of measurement noise, random objects and random imaging system are analyzed by nested conditional averaging, leading to a three-term expansion of the data covariance matrix. Characteristic functionals are introduced to account for the object statistics, and it is shown how they can be used to compute the image statistics.

Cone-beam CT images of a patient with a pre-defined distribution of noise in the image and dose to the patient can be accomplished through the development of advanced compensation schemes. Such compensation schemes involve delivery of x-ray fluence patterns that vary in intensity both across a single projection image (u,v) and for different projection view angles (θ) and provide the ability to perform intensity-modulated cone-beam CT. Implementation of an intensity-modulated cone-beam CT system for task-specific imaging has potential for tremendous reductions in patient dose and x-ray scatter reaching the detector. Pursuing this advanced imaging technique requires detailed characterization of the cone-beam CT platform. Determination of appropriately modulated fluence patterns relies on knowledge of numerous properties of the imaging system, including the constraints imposed by the modulator, the magnitude of x-ray scatter under different patient sizes and modulator positions, and properties of the detector. With an estimate of the patient anatomy and knowledge of the imaging system, an iterative process can be used to determine modulated fluence patterns corresponding to an image prescribed for the specific task and patient. Delivery of such modulated fluence patterns provide a CBCT image tailored to a specific patient and imaging task offering the optimum balance between image quality and patient dose. Specifically, arbitrary regions of interest requiring high image signal-to-noise ratio can be generated through knowledgeable spatio-angular intensity modulation, allowing image quality to degrade in other regions in order to minimize x-ray scatter and imaging dose.

We have developed a method for producing simulated mammograms from high fidelity breast specimen radiographs. The method has the advantage of having access to all the truth information for the lesions. By modeling different parts of a screen-film system, we simulated the output of the system, and compared it to the real mammography images from the same samples. In this work we show how our simulation program produces realistic mammography images and also the observer study that tests how well the observers can distinguish the real and simulated images. Preliminary results from the ROC study show that the observers could not distinguish the two types of images very well.

Dual-energy imaging increases the possibility of pulmonary nodule detection by reducing the bone structure noise. The major problem of the dual-energy acquisition process with digital flat-panel detectors is the interval of time between low-energy (LE) exposure and high-energy (HE) exposure. Due to misregistration between LE and HE images, motion artifacts pollute the subtracted image. This paper presents a new acquisition approach for dual-energy imaging developed in order to reduce this inter-exposure time. The idea is to keep the tube voltage constant and to just switch a filter in front of the imaged object and thus to modulate the outgoing x-ray spectrum. The first part of this study presents how to optimize system parameters for the new acquisition protocol: source voltage, dynamic filtration before the patient, exposure time for LE and HE acquisition. The tube load is kept constant to focus the optimization study on the dose and the exposure time. A noise quality factor (NQF) and a spectral quality factor (SQF) are used as criteria for optimization. The new approach system is then compared to the state-of-the-art system with voltage switching between low and high energy. A filtering algorithm of dual energy acquisitions enabling a significant noise reduction is presented. Performance between its combination with the new acquisition protocol and the reference one are compared. For a limited noise quality factor, three times faster acquisition time is obtained using the new system. Noise reduction techniques improve the image SNR by 61% in the new system and only 32% in the reference system, without taking into account the impact of better registration on the dual-energy image quality.

Mounting evidence suggests that the superposition of anatomical clutter in a projection radiograph poses a major impediment to the detectability of subtle lung nodules. Through decomposition of projections acquired at multiple kVp, dual-energy (DE) imaging offers to dramatically improve lung nodule detectability and, in part through quantitation of nodule calcification, increase specificity in nodule characterization. The development of a high-performance DE chest imaging system is reported, with design and implementation guided by fundamental imaging performance metrics. A diagnostic chest stand (Kodak RVG 5100 digital radiography system) provided the basic platform, modified to include: (i) a filter wheel, (ii) a flat-panel detector (Trixell Pixium 4600), (iii) a computer control and monitoring system for cardiac-gated acquisition, and (iv) DE image decomposition and display. Computational and experimental studies of imaging performance guided optimization of key acquisition technique parameters, including: x-ray filtration, allocation of dose between low- and high-energy projections, and kVp selection. A system for cardiac-gated acquisition was developed, directing x-ray exposures to within the quiescent period of the heart cycle, thereby minimizing anatomical misregistration. A research protocol including 200 patients imaged following lung nodule biopsy is underway, allowing preclinical evaluation of DE imaging performance relative to conventional radiography and low-dose CT.

We have developed a dual-energy subtraction technique for contrast-enhanced breast tomosynthesis. The imaging system consists of 48 photon-counting linear detectors which are precisely aligned with the focal spot of the x-ray source. The x-ray source and the digital detectors are translated across the breast in a continuous linear motion; each linear detector collects an image at a distinct angle. A pre-collimator is positioned above the breast and defines 48 fan-shaped beams, each aligned with a detector. Low- and high-energy images are acquired in a single scan; half of the detectors capture a low-energy beam and half capture a high-energy beam, as alternating fan-beams are filtered to emphasize low and high energies. Imaging was performed with a W-target at 45 and 49 kV. Phantom experiments and theoretical modeling were conducted. Iodine images were produced with weighted logarithmic subtraction. The optimal tissue cancellation factor, w_t, was determined based on simultaneous preservation of the iodine signal and suppression of simulated anatomic background. Optimal dose allocation between low- and high-energy images was investigated. Mean glandular doses were restricted to ensure clinical relevance. Unlike other dual-energy approaches, both spectra must have the same peak energy in this system design. We have observed that w_t is mainly dependent on filter combination and varies only slightly with kV and breast thickness, thus ensuring a robust clinical implementation. Optimal performance is obtained when the dose fraction allocated to the high energy images ranges from 0.55 to 0.65. Using elemental filters, we have been able to effectively suppress the anatomic background.

Novel CdTe photon counting x-ray detectors (PCXDs) have been developed for very high count rates [1-4] suitable for x-ray micro computed tomography (μCT) scanners. It counts photons within each of J energy bins. In this study, we investigate use of the data in these energy bins for material decomposition using an image domain approach. In this method, one image is reconstructed from projection data of each energy bin; thus, we have J images from J energy bins that are associated with attenuation coefficients with a narrow energy width. We assume that the spread of energies in each bin is small and thus that the attenuation can be modeled using an effective energy for each bin. This approximation allows us to linearize the problem, thus simplify the inversion procedure. We then fit J attenuation coefficients at each location x by the energy-attenuation function [5] and obtain either (1) photoelectric and Compton scattering components or (2) 2 or 3 basis-material components. We used computer simulations to evaluate this approach generating projection data with three types of acquisition schemes: (A) five monochromatic energies; (B) five energy bins with PCXD and an 80 kVp polychromatic x-ray spectrum; and (C) two kVp with an intensity integrating detector. Total attenuation coefficients of reconstructed images and calculated effective atomic numbers were compared with data published by National Institute of Standards and Technology (NIST). We developed a new materially defined "SmileyZ" phantom to evaluate the accuracy of the material decomposition methods. Preliminary results showed that material based 3-basis functions (bone, water and iodine) with PCXD with 5 energy bins was the most promising approach for material decomposition.

The material specificity of computed tomography is quantified using an experimental benchtop imaging system and a physics-based system model. The apparatus is operated with different detector and system configurations each giving X-ray energy spectral information but with different overlap among the energy-bin weightings and noise statistics. Multislice, computed tomography sinograms are acquired using dual kVp, sequential source filters or a detector with two scintillator/photodiodes layers. Basis-material and atomic number images are created by first applying a material decomposition algorithm followed by filtered backprojection. CT imaging of phantom materials with known elemental composition and density were used for model validation. X-ray scatter levels are measured with a beam-blocking technique and the impact to material accuracy is quantified. The image noise is related to the intensity and spectral characteristics of the X-ray source. For optimal energy separation adequate image noise is required. The system must be optimized to deliver the appropriate high mA at both energies. The dual kVp method supports the opportunity to separately engineer the photon flux at low and high kvp. As a result, an optimized system can achieve superior material specificity in a system with limited acquisition time or dose. In contrast, the dual-layer and sequential acquisition modes rely on a material absorption mechanism that yields weaker energy separation and lower overall performance.

Recently developed solid-state detectors combined with high-speed ASICs that allow individual pixel pulse processing may prove useful as detectors for small animal micro-computed tomography. One appealing feature of these photon-counting x-ray detectors (PCXDs) is their ability to discriminate between photons with different energies and count them in a small number (2-5) of energy windows. The data in these energy windows may be thought of as arising from multiple simultaneous x-ray beams with individual energy spectra, and could thus potentially be used to perform material composition analysis. The goal of this paper was to investigate the potential advantages of PCXDs with multiple energy window counting capability as compared to traditional integrating detectors combined with acquisition of images using x-ray beams with 2 different kVps. For the PCXDs, we investigated 3 potential sources of crosstalk: scatter in the object and detector, limited energy resolution, and pulse piluep. Using Monte Carlo simulations, we showed that scatter in the object and detector results in relatively little crosstalk between the data in the energy windows. To study the effects of energy resolution and pulse-pileup, we performed simulations evaluating the accuracy and precision of basis decomposition using a detector with 2 or 5 energy windows and a single kVp compared to an dual kVp acquisitions with an integrating detector. We found that, for noisy data, the precision of estimating the thickness of two basis materials for a range of material compositions was better for the single kVp multiple energy window acquisitions compared to the dual kVp acquisitions with an integrating detector. The advantage of the multi-window acquisition was somewhat reduced when the energy resolution was reduced to 10 keV and when pulse pileup was included, but standard deviations of the estimated thicknesses remained better by more than a factor of 2.

Currently, the most accurate measurement of the detector point response can be performed with the pinhole method. The small size of the pinhole however, severely reduces the x-ray intensity output, requiring long exposures, something that can potentially reduce the x-ray tube life-cycle. Even though deriving the 1D Line Response Function (LRF)of the detector using the edge method is much more effcient, the measurement process introduces a convolution with a line, in addition to the common pixel sampling, effectively broadening the LRF. We propose a practical method to recover the detector point response function by removing the effects of the line and the pixel from a set of Edge Response Function (ERF) measurements. We use the imaging equation to study the effects of the edge,line and pixel measurements, and derive an analytical formula for the recovered detector point response function based on a gaussian mixture model. The method allows for limited recovery of asymmetries in the detector response function. We verify the method with pinhole and edge measurements of a digital flat panel detector. Monte Carlo simulations are also performed, using the MANTIS x-ray and optical photon and electron transport simulation package, for comparison. We show that the standard LRF underestimates the detector when compared with the recovered response. Our simulation results suggest that both hole methods for estimating the detector response have limitations in that they cannot completely capture rotational asymmetries or other morphological details smaller than the detector pixel size.

Software scoring approaches provide an attractive alternative to human evaluation of CDMAM images from digital mammography systems, particularly for annual quality control testing as recommended by the European Protocol for the Quality Control of the Physical and Technical Aspects of Mammography Screening (EPQCM). Methods for correlating CDCOM-based results with human observer performance have been proposed. A common feature of all methods is the use of a small number (at most eight) of CDMAM images to evaluate the system. This study focuses on the potential variability in the estimated system performance that is associated with these methods. Sets of 36 CDMAM images were acquired under carefully controlled conditions from three different digital mammography systems. The threshold visibility thickness (TVT) for each disk diameter was determined using previously reported post-analysis methods from the CDCOM scorings for a randomly selected group of eight images for one measurement trial. This random selection process was repeated 3000 times to estimate the variability in the resulting TVT values for each disk diameter. The results from using different post-analysis methods, different random selection strategies and different digital systems were compared. Additional variability of the 0.1 mm disk diameter was explored by comparing the results from two different image data sets acquired under the same conditions from the same system. The magnitude and the type of error estimated for experimental data was explained through modeling. The modeled results also suggest a limitation in the current phantom design for the 0.1 mm diameter disks. Through modeling, it was also found that, because of the binomial statistic nature of the CDMAM test, the true variability of the test could be underestimated by the commonly used method of random re-sampling.

We consider in this report, a tomosynthesis (TS) system utilizing a recently developed Selenium flat panel detector (Shimadzu Safire II) capable of acquiring 30 pulsed frames per second (fps) with precise geometric registration to a moving gantry. The reconstructed TS resolution achieved with this system is reported in relation to the resolution obtained with modern volumetric computed tomography (vCT) systems. Computed tomography resolution is assessed using a sphere phantom. To describe the resolution of coronal/sagittal re-formatted planes, we average the transverse and axial response to express typical vCT performance as 1.00 mm (FWHM PlSF) and 0.83 cycles/mm (10% MTF). Tomosynthesis response is assessed using a wire phantom. To describe the resolution of the coronal/sagittal TS images, we use the FWHM of the LSF and the frequency at 20% of the peak spatial frequency response to express typical TS performance as 2.41 mm (FWHM LSF) and 2.62 cycles/mm (20% Peak). Cadaver images confirm that the TS resolution is about 3 times better than that for vCT systems. On the other hand, the TS slice sensitivity of 3 mm is about 3 times larger than vCT resolution (FWHM LSF).

In many patients respiratory motion causes motion artifacts in CT images, thereby inhibiting precise treatment planning and lowering the ability to target radiation to tumors. The 4D Phantom, which includes a 3D stage and a 1D stage that each are capable of arbitrary motion and timing, was developed to serve as an end-to-end radiation therapy QA device that could be used throughout CT imaging, radiation therapy treatment planning, and radiation therapy delivery. The dynamic accuracy of the system was measured with a camera system. The positional error was found to be equally likely to occur in the positive and negative directions for each axis, and the stage was within 0.1 mm of the desired position 85% of the time. In an experiment designed to use the 4D Phantom's encoders to measure trial-to-trial precision of the system, the 4D Phantom reproduced the motion during variable bag ventilation of a transponder that had been bronchoscopically implanted in a canine lung. In this case, the encoder readout indicated that the stage was within 10 microns of the sent position 94% of the time and that the RMS error was 7 microns. Motion artifacts were clearly visible in 3D and respiratory-correlated (4D) CT scans of phantoms reproducing tissue motion. In 4D CT scans, apparent volume was found to be directly correlated to instantaneous velocity. The system is capable of reproducing individual patient-specific tissue trajectories with a high degree of accuracy and precision and will be useful for end-to-end radiation therapy QA.

Cone-beam computed tomography (CBCT) provides a real volume imaging modality, which can reconstruct a digital volume with an isotropic grid resolution. However, the imaging resolution of a CBCT system is not identical among spatial orientations, which manifests an anisotropic three-dimensional point spread function (3D PSF), or a PSF ellipsoid. According to the CBCT imaging procedure, the anisotropic factors include the x-ray cone-beam quality, focal spot size and shape, divergence projection angle, scanning orbit, and reconstruction algorithms. Since x-rays are generated by electron bombardment on a slope target anode, the x-ray source is of finite size and non-circle shape, which can neither be considered as a point nor a planar spot in strict sense. Due to non-uniform penetration depths in the anode target, the wavefronts of the emanating x-rays assume non-spheric distributions, as described by the heel effect, which becomes pronounced in a large cone-angle projection. The spatial blurring thereby is rather complicated. In this paper, we will study the anisotropy of 3D PSF under diverse imaging factors, including the heel effect, the focal spot size and shape, and the cone angle. Both computer simulation and phantom experiment on a flat-panel-detector CBCT system will be reported.

A small animal multimodality tomographer dedicated to the co-registration of fluorescence optical signal and X-rays measurements has been developed in our laboratory. The purpose of such a system is to offer the possibility to get in vivo anatomical and functional information at once. Moreover, anatomical measurements can be used as a regularization factor in order to get the reconstructions of the biodistribution of fluorochromes more accurate and to speed up the treatment. The optical system is basically composed with a CW laser (Krypton, 752 nm) for an optimal excitation of Alexa-Fluor 750 fluorochromes, and a CCD camera coupled with a combination of filters for the fluorescence detection. The animal is placed inside a transparent tube filled with an index matching fluid. In order to perform multiple views of fluorescence data acquisitions, the cylinder is fixed to a rotating stage. The excitation beam is brought to the cylinder via two mirrors mounted on translation plates allowing a vertical scan. The optical data acquisitions are performed with a high sensitivity CCD camera. The X-ray generator and the X-ray detector have been placed perpendicularly to the optical chain. A first study on phantoms was conducted to evaluate the feasibility, to test the linearity and the reproducibility, and to fix the parameters for the co-registration. These test experiments were reproduced by considering mice in the oesophagus of which thin glass tubes containing fluorochromes were inserted. Finally, the performance of the system was evaluated in vivo on mice bearing tumours in the lungs, tagged with Transferin-AlexaFluor 750.

Third-generation CT architectures are approaching fundamental limits. Spatial resolution is limited by the focal spot size and the detector cell size. Temporal resolution is limited by mechanical constraints on gantry rotation speed, and alternative geometries such as electron-beam CT and two-tube-two-detector CT come with severe tradeoffs in terms of image quality, dose-efficiency and complexity. Image noise is fundamentally linked to patient dose, and dose-efficiency is limited by finite detector efficiency and by limited spatio-temporal control over the X-ray flux. Finally, volumetric coverage is limited by detector size, scattered radiation, conebeam artifacts, Heel effect, and helical over-scan. We propose a new concept, multi-source inverse geometry CT, which allows CT to break through several of the above limitations. The proposed architecture has several advantages compared to third-generation CT: the detector is small and can have a high detection efficiency, the optical spot size is more consistent throughout the field-of-view, scatter is minimized even when eliminating the anti-scatter grid, the X-ray flux from each source can be modulated independently to achieve an optimal noise-dose tradeoff, and the geometry offers unlimited coverage without cone-beam artifacts. In this work we demonstrate the advantages of multi-source inverse geometry CT using computer simulations.

A compact multi-spectral imaging system is presented as diagnostic tool in dermatology and neurosurgery. Using an electronically tunable filter, a sensitive high resolution digital camera, 140 spectral images from 400 nm up to 720 nm are acquired in 40 s. Advanced image processing algorithms are used to enable interactive acquisition, viewing, image registration and image analysis. Experiments in the department of dermatology and neurosurgery show that multispectral imaging reveals much more detail than conventional medical photography or a surgical microscope, as images can be reprocessed to enhance the view on e.g. tumor boundaries. Using a hardware-based interactive registration algorithm, multi-spectral images can be aligned to correct for motion occurred during image acquisition or to compare acquisitions from different moments in time. The system shows to be a powerful diagnostics tool for medical imaging in the visual and near IR range.

To the best of our knowledge, for the first time, biological Three Dimensional (3-D) imaging has been achieved using an electronically controlled optical lens to accomplish no-moving parts depth section scanning in a modified commercial 3-D confocal microscope. Specifically, full 3-D views of a standard CDC blood vessel (enclosed in a glass slide) have been obtained using the modified confocal microscope operating at the red 633 nm laser wavelength.

Several studies have shown that the concentration of certain elements may be a disease indicator. We are developing a spectroscopic imaging technique, Neutron Stimulated Emission Computed Tomography (NSECT), to non-invasively measure and image elemental concentrations within the body. The region of interest is interrogated via a beam of high-energy neutrons that excite elemental nuclei through inelastic scatter. These excited nuclei then relax by emitting characteristic gamma radiation. Acquiring the gamma energy spectrum in a tomographic geometry allows reconstruction of elemental concentration images. Our previous studies have demonstrated NSECT's ability to obtain spectra and images of known elements and phantoms, as well as, initial interrogations of biological tissue. Here, we describe the results obtained from NSECT interrogation of a fixed mouse specimen. The specimen was interrogated via a 5MeV neutron beam for 9.3 hours in order to ensure reasonable counting statistics. The gamma energy spectrum was obtained using two High-Purity Germanium (HPGe) clover detectors. A background spectrum was obtained by interrogating a specimen container containing 50mL of 0.9% NaCl solution. Several elements of biological interest including ¹²C, ⁴⁰Ca, ³¹P, and ³⁹K were identified with greater then 90% confidence. This interrogation demonstrates the feasibility of NSECT interrogation of small animals. Interrogation with a commercial neutron source that provides higher neutron flux and lower energy (~2.5MeV) neutrons would reduce scanning time and eliminate background from certain elements.

We have investigated experimentally and theoretically the imaging performance of our newly constructed in-line holography x-ray phase-contrast imaging system with an ultrafast laser-based x-ray source. Projection images of nylon fibers with diameters in the 10-330 μm range were obtained using an ultrafast (100 Hz, 28 fs, 40 mJ) laser-based x-ray source with Mo and Ta targets and Be filter, and Gaussian spatial-intensity distribution (FWHMS = 5 μm). A cooled CCD camera (24 μm pitch) with a Gd₂OS₂ screen coupled via 1:1 optical taper was used (FWHMD = 50 μm). We have investigated nylon-fiber image quality vs. imaging setup geometry and x-ray spectra. The following parameters were evaluated: contrast, signal-to-noise ratio (SNR), resolution, and sampling. In addition, we performed theoretical simulation of image formation for the same objects but within a wide range of geometrical parameters. The rigorous wave-optical formalism was used for modeling of the free-space propagation of x-rays from the object plane to the detector, and the "projection approximation" was used. We found reasonable agreement between predictions of our analytical model and the experiments. We conclude that: a) Optimum magnification maximizing contrast and SNR is almost independent of the source-to-detector (R) distance and depends strongly on the diameter of the fiber. b) The corresponding maximum values of the contrast and SNR are almost linear with respect to R; the optimum magnification decreases with fiber diameter. c) The minimum diameter of fiber defines the minimum source-to-object distance R₁ if R is fixed and the object is moved.

I-ImaS (Intelligent Imaging Sensors) is a European project aiming to produce adaptive x-ray imaging systems using Monolithic Active Pixel Sensors (MAPS) to create optimal diagnostic images. Initial systems concentrate on mammography and cephalography. The on-chip intelligence available to MAPS technology will allow real-time analysis of data during image acquisition, giving the capability to build a truly adaptive imaging system with the potential to create images with maximum diagnostic information within given dose constraints. In our system, the exposure in each image region is optimized and the beam intensity is a function not only of tissue thickness and attenuation, but also of local physical and statistical parameters found in the image itself. Using a linear array of detectors with on-chip intelligence, the system will perform an on-line analysis of the image during the scan and then will optimize the X-ray intensity in order to obtain the maximum diagnostic information from the region of interest while minimizing exposure of less important, or simply less dense, regions. This paper summarizes the testing of the sensors and their electronics carried out using synchrotron radiation, x-ray sources and optical measurements. The sensors are tiled to form a 1.5D linear array. These have been characterised and appropriate correction techniques formulated to take into account misalignments between individual sensors. Full testing of the mammography and cephalography I-ImaS prototypes is now underway and the system intelligence is constantly being upgraded through iterative testing in order to obtain the optimal algorithms and settings.

Recent advances have been made in a new class of CCD-based, single-photon-counting gamma-ray detectors which offer sub-100 μm intrinsic resolutions.^1-7 These detectors show great promise in small-animal SPECT and molecular imaging and exist in a variety of cofigurations. Typically, a columnar CsI(Tl) scintillator or a radiography screen (Gd₂O₂S:Tb) is imaged onto the CCD. Gamma-ray interactions are seen as clusters of signal spread over multiple pixels. When the detector is operated in a charge-integration mode, signal spread across pixels results in spatial-resolution degradation. However, if the detector is operated in photon-counting mode, the gamma-ray interaction position can be estimated using either Anger (centroid) estimation or maximum-likelihood position estimation resulting in a substantial improvement in spatial resolution.² Due to the low-light-level nature of the scintillation process, CCD-based gamma cameras implement an amplfication stage in the CCD via electron multiplying (EMCCDs)^8-10 or via an image intensfier prior to the optical path.¹ We have applied ideas and techniques from previous systems to our high-resolution LumiSPECT detector.^{11, 12} LumiSPECT is a dual-modality optical/SPECT small-animal imaging system which was originally designed to operate in charge-integration mode. It employs a cryogenically cooled, high-quantum-efficiency, back-illuminated large-format CCD and operates in single-photon-counting mode without any intermediate amplfication process. Operating in photon-counting mode, the detector has an intrinsic spatial resolution of 64 μm compared to 134 μm in integrating mode.

This paper describes a CMOS Active Pixel Image Sensor developed for generic X-ray imaging systems using standard CMOS technology and an active pixel architecture featuring low noise and a high sensitivity. The image sensor has been manufactured in a standard 0.35 μm technology using 8" wafers. The resolution of the sensor is 3360x3348 pixels of 40x40 μm² each. The diagonal of the sensor measures little over 190 mm. The paper discusses the floor planning, stitching diagram, and the electro-optical performance of the sensor that has been developed.

Development of active matrix imagers fabricated on plastic substrates has become increasingly possible due to widespread efforts to develop the means to create inexpensive, very large area, flexible displays. In addition to benefits associated with cost, robustness, and weight, such novel x-ray imaging devices could provide significant performance improvements by virtue of substrates offering a thinner profile, lower density, and lower atomic number composition, as well as the ability to mechanically conform to non-planar geometries. One potential candidate for advantageous utilization of plastic substrates is that of a high resolution, indirect detection active matrix imager operated at mammographic x-ray energies. Such an imager, configured in an arc and operated in a back illumination geometry, could offer enhanced modulation transfer function and detective quantum efficiency by virtue of reduced optical spread in the scintillator relative to the optical sensor as well as reduction (or elimination) of oblique angles of incidence of primary x-rays. Motivated by such prospects, it is of interest to quantify the degree of performance improvement to be expected from such novel devices. In this paper, we describe a methodology under development by our collaboration to quantitatively examine the performance of indirect detection active matrix imagers incorporating plastic substrates. This methodology is based on the cascaded systems formalism with input provided by, among other things, Monte Carlo simulation of radiation and optical transport in the detector. Finally, illustrative examples of the use of this methodology are presented - although key input parameters are, as yet, insufficiently precise to allow the simulations to accurately depict the complete physical situation.

A new TFT array has been developed specifically for mercuric iodide (HgI₂) deposition. This new TFT array combined with a modified HgI₂ Physical Vapor Deposition (PVD) process provides less than 10 pA/mm² dark current at room temperature (22 °C) measured at 1 V/&mgr;m electrical field. This photoconductor (direct) imager was run at 10 fr/s framerate and gave a measured sensitivity of 19 μC/(R*cm²) using a RQA5 radiation quality x-ray beam (70kVp x-ray with 21 mm Al filtering). This sensitivity value is higher than the sensitivity reported by our group for any previous HgI₂ imagers. MTF, NPS and DQE values were also evaluated on this 13 cm x 13 cm size imager with 127 μm pixel pitch. The MTF value is higher than 40% at the Nyquist frequency (3.9 lp/mm). This is much better than the MTF of a 600 μm CsI scintillator/photodiode (indirect) imager, which is only 16% (Varian internal data) and it is similar to the MTF value of the a-Se (another photoconductor) imagers. The first frame image lag is less than 8% when the imager was run at a 10 fr/s framerate. The low dark current and some noise reduction in the detector electronics, made it possible for the DQE to be measured down to low fluoroscopic dose levels (< 4 μR/fr). The DQE(0) value is over 50% at a dose of 35 μR/fr and still about 40% at 3.76 μR/fr. The 270 μm thick PVD HgI2 layer only absorbs less than 75% of the ~51 keV mean energy X-ray photons (70 kVp RQA5 filtered beam). This means that if the thickness of the HgI₂ layer is increased to 500 μm (increasing the absorption up to over 90%) the DQE(0) should then increase to about 60- 65% (assuming everything else remains unchanged). This value is close to the 65 - 70 % DQE(0), measured for the indirect (CsI) imagers at higher doses. Such a high DQE value makes this material competitive both for fluoroscopic and for radiographic applications.

The coherent scattering form factor defines a material's small angle x-ray scattering properties. These scattering properties can provide useful medical diagnostic information if properly utilized. Measurement of the coherent scattering form factor is difficult, requiring expensive equipment and long measurement times. We show that it is possible to measure the coherent scattering form factor using standard clinical equipment through a matrix equation. The matrix elements are constructed from knowledge of the input x-ray spectrum. In the ideal case, the form factor can be extracted from this equation by inverting the matrix. For typical x-ray spectra and form factors, however, the matrix tends to be poorly conditioned, leading to large errors upon inversion. We have developed a sub-matrix method that constructs a series of smaller, well-conditioned matrices that can be accurately inverted to give the required form factor. We show through numerical simulations that the sub-matrix method can accurately measure the form factor of common tissue materials. Root mean square deviations of 0.0502 and 0.0804 were calculated for the form factors of water and fat with 90 kV spectra and 0.2 mm of tungsten filtration. Over the measurement range, the form factors vary between approximately 0.5 and 2.5. We show that the optimal spectral shape when using the sub-matrix method is one that is strongly peaked at high energies and that using an improperly chosen spectrum can result in a significant loss of accuracy. We also demonstrate that the sub-matrix method is not readily applicable for measurements of strongly ordered materials.

Conventional energy filters for x-ray imaging are based on absorbing materials which attenuate low energy photons, sometimes combined with an absorption edge, thus also discriminating towards photons of higher energies. These filters are fairly inefficient, in particular for photons of higher energies, and other methods for achieving a narrower bandwidth have been proposed. Such methods include various types of monochromators, based on for instance mosaic crystals or refractive multi-prism x-ray lenses (MPL's). Prism-array lenses (PAL's) are similar to MPL's, but are shorter, have larger apertures, and higher transmission. A PAL consists of a number of small prisms arranged in columns perpendicular to the optical axis. The column height decreases along the optical axis so that the projection of lens material is approximately linear with a Fresnel phase-plate pattern superimposed on it. The focusing effect is one dimensional, and the lens is chromatic. Hence, unwanted energies can be blocked by placing a slit in the image plane of a desired energy. We present the first experimental and theoretical results on an energy filter based on a silicon PAL. The study includes an evaluation of the spectral shaping properties of the filter as well as a quantification of the achievable increase in dose efficiency compared to standard methods. Previously, PAL's have been investigated with synchrotron radiation, but in this study a medical imaging setup, based on a regular x-ray tube, is considered.

Purpose: The first clinical facility for synchrotron radiation (SR) mammography is now operative at the SYRMEP beamline of ELETTRA, the SR facility in Trieste, Italy. The mammographic facility and the preliminary results of the clinical trial are presented in this contribution. Method and Materials: The distance between the SR source and the patient is about 30 m; the main features of the X-ray beam are: monochromaticity at ~0.2% bandwith in the energy range 8-35 keV, photon flux of about 10⁸ ph/(mm² s) and dimensions of 21 cm x 3.5 mm at the compressed breast. An innovative dosimetric system allows the on-line dose control during the examination. The images are acquired by scanning the patient, in prone position, in front of the stationary laminar beam; the average scanning time is about 10 s. The detector is a screen film system; it is at ~2 m from the breast in order to fulfil the so-called Phase Contrast (PhC) requirements. The breast thickness and glandularity defines the optimal beam energy for each examination. The patients are enrolled by radiologists, after routine examinations, on the basis of BI-RADS classification, according the research program approved by the local Ethical Committee. Results: This communication concerns the first 9 patients underwent the SR PhC mammography; the images match the quality obtained in previous in vitro studies. With reference to conventional mammography the diagnostic quality of the radiological images is better, without increasing the delivered dose to the patient.

In this paper, we present the development of dual-energy Contrast-Enhanced Digital Breast Tomosynthesis (CEDBT). A method to produce background clutter-free slices from a set of low and high-energy projections is introduced, along with a scheme for the determination of the optimal low and high-energy techniques. Our approach consists of a dual-energy recombination of the projections, with an algorithm that has proven its performance in Contrast-Enhanced Digital Mammography¹ (CEDM), followed by an iterative volume reconstruction. The aim is to eliminate the anatomical background clutter and to reconstruct slices where the gray level is proportional to the local iodine volumetric concentration. Optimization of the low and high-energy techniques is performed by minimizing the total glandular dose to reach a target iodine Signal Difference to Noise Ratio (SDNR) in the slices. In this study, we proved that this optimization could be done on the projections, by consideration of the SDNR in the projections instead of the SDNR in the slices, and verified this with phantom measurements. We also discuss some limitations of dual-energy CEDBT, due to the restricted angular range for the projection views, and to the presence of scattered radiation. Experiments on textured phantoms with iodine inserts were conducted to assess the performance of dual-energy CEDBT. Texture contrast was nearly completely removed and the iodine signal was enhanced in the slices.

We have designed and constructed a small-animal adaptive SPECT imaging system as a prototype for quantifying the potential benefit of adaptive SPECT imaging over the traditional fixed geometry approach. The optical design of the system is based on filling the detector with the object for each viewing angle, maximizing the sensitivity, and optimizing the resolution in the projection images. Additional feedback rules for determining the optimal geometry of the system can be easily added to the existing control software. Preliminary data have been taken of a phantom with a small, hot, offset lesion in a flat background in both adaptive and fixed geometry modes. Comparison of the predicted system behavior with the actual system behavior is presented along with recommendations for system improvements.

State-of-the-art tomographic imaging technique is based upon of simple serial imaging scheme. The tomographic scanners collect the projection images sequentially in the time domain, by a step-and-shoot process using a single-pixel x-ray source. The inefficient serial data collection scheme severely limits the data collection speed, which is critical for imaging of objects in rapid motion such as for diagnosis of cardiovascular diseases, CT fluoroscopy, and airport luggage inspection. Further improvement of the speed demands an increasingly high x-ray peak workload and gantry rotation speed, both of which have approached the engineering limits. Multiplexing technique, which has been widely adopted in communication devices and in certain analytical instruments, holds the promise to significantly increase the data throughput. It however, has not been applied to x-ray radiography, mainly due to limitations of the current x-ray source technology. Here we report a method for frequency multiplexing radiography (FMR) based on the frequency multiplexing principle and the carbon nanotube field emission x-ray technology. We show the feasibility of multiplexing radiography that enables simultaneous collection of multiple projection images. It has the potential to significantly increase the imaging speed for tomographic imaging without compromising the imaging quality.

This paper presents a novel framework for the systematic assessment of the impact of scattered radiation in .at-detector based cone-beam CT. While it is well known that scattered radiation causes three di.erent types of artifacts in reconstructed images (inhomogeneity artifacts such as cupping and streaks, degradation of contrast, and enhancement of noise), investigations in the literature quantify the impact of scatter mostly only in terms of inhomogeneity artifacts, giving little insight, e.g., into the visibility of low contrast lesions. Therefore, for this study a novel framework has been developed that in addition to normal reconstruction of the CT (HU) number allows for reconstruction of voxelized expectation values of three additional important characteristics of image quality: signal degradation, contrast reduction, and noise variances. The new framework has been applied to projection data obtained with voxelized Monte-Carlo simulations of clinical CT data sets of high spatial resolution. Using these data, the impact of scattered radiation was thoroughly studied for realistic and clinically relevant patient geometries of the head, thorax, and pelvis region. By means of spatially resolved reconstructions of contrast and noise propagation, the image quality of a scenario with using standard antiscatter grids could be evaluated with great detail. Results show the spatially resolved contrast degradation and the spatially resolved expected standard deviation of the noise at any position in the reconstructed object. The new framework represents a general tool for analyzing image quality in reconstructed images.

Typical methods to measure the resolution properties of x-ray detectors use slit or edge devices. However, complete models of imaging systems for system optimization require knowledge of the point-response function of the detector. In this paper, we report on the experimental methods developed for the validation of the point-response function of an indirect columnar CsI:Tl detector predicted by Monte Carlo using mantis. We describe simulation results that replicate experimental resolution measurements using edge and pinhole devices. The experimental setup consists of a high-resolution CCD camera with a 1-to-1fiber optic faceplate that allows measurements for different scintillation screens. The results of these experiments and simulations constitute a resource for the development and validation of the columnar models of phosphor screens proposed as part of previous work with mantis. We compare experimental high-resolution pinhole responses of two different CsI(Tl) screens to predictions from mantis. The simulated response matches reasonably well the measurements at normal and off-normal x-ray incidence angle when a realistic pinhole is used in the simulation geometry. Our results will be combined with results on Swank factors determined from Monte Carlo pulse-height spectra to provide a comprehensive validation of the phosphor models, therefore allowing their use for in silico system optimization.

X-ray imaging system optimization increases the benefit-to-cost ratio by reducing the radiation dose to the patient while maximizing image quality. We present a new simulation tool for the generation of realistic medical x-ray images for assessment and optimization of complete imaging systems. The Monte Carlo code simulates radiation transport physics using the subroutine package PENELOPE, which accurately simulates the transport of electrons and photons within the typical medical imaging energy range. The new code implements a novel object-oriented geometry package that allows simulations with homogeneous objects of arbitrary shapes described by triangle meshes. The flexibility of this code, which uses the industry standard PLY input-file format, allows the use of detailed anatomical models developed using computer-aided design tools applied to segmented CT and MRI data. The use of triangle meshes highly simplifies the ray-tracing algorithm without reducing the generality of the code, since most surface models can be tessellated into triangles while retaining their geometric details. Our algorithm incorporates an octree spatial data structure to sort the triangles and accelerate the simulation, reaching execution speeds comparable to the original quadric geometry model of PENELOPE. Coronary angiograms were simulated using a tessellated version of the NURBS-based Cardiac-Torso (NCAT) phantom. The phantom models 330 objects, comprised in total of 5 million triangles. The dose received by each organ and the contribution of the different scattering processes to the final image were studied in detail.

A generalized method to evaluate the noise transfer properties of the base material decomposition has been developed. We apply the method to a typical dual-energy CT scan with energy weightings and doses of a 80kV / 140kV scan. For sets {P1, P2} of dual-energy projections with P_i = 10^-4.5 ... 1, both the water and bone decomposition and the Compton and Photo Effect decomposition are analyzed. As a figure of merit we determine the noise amplification factors A₁, A₂. They are given by the ratio of the relative noise of the dual-energy projections B₁, B₂ to the relative noise of the combined projection data P. The B₁, B₂ and their variance are simulated by numerical inversion and integration. For the water and bone decomposition an average noise amplification of 3 to 5 is shown. For small contributions of one base material, the noise amplification becomes critically large. In this case the water and bone base material decomposition seems not to be usable for quantitative CT. The Compton and Photo effect decomposition are shown to be more robust in this respect. Physically, both coefficients can only reach zero simultaneously. The Compton coefficient has significantly better noise characteristics than the Photo Effect coefficient. For a partial region of the P₁, P₂ plane it shows better noise performance than the combined raw data P.

In order to obtain motion-compensated reconstructions of calcified coronary plaques in cardiac CT, the dynamic trajectory of the plaque must be known rather accurately. The purpose of this study is to evaluate whether the dynamic trajectories of a plaque extracted from reconstructions provided by a previously developed tracking algorithm can be used for obtaining motion-compensated reconstructions of this plaque. A single projection dataset of the modified FORBILD phantom containing a calcified plaque undergoing continuous periodic motion was acquired with a gantry rotation time of 0.4 s and a heart rate of 90 bpm. Three sets of phase-correlated 4D ROI images centered on the calcified plaque (labeled G1, G2, and G3) were obtained from this dataset by varying the numbers of data segments used for cardiac gating (N = 1, 2, 3) during the reconstruction steps of the tracking algorithm. Dynamic trajectories from each of these datasets were calculated from edge-based segmentations of these datasets. When compared to the true trajectory (labeled T), root-mean-square (RMS) values of position for trajectories G1, G2, and G3 were 1.473 mm, 1.166 mm, and 0.736 mm, respectively. Trajectories G1, G2, G3, and T then were used to obtain motion-compensated reconstructions MC1, MC2, MC3, and MCT, respectively, at 6.25 ms time intervals over 2 cardiac cycles. The areas (number of pixels) of the plaque then were measured at all time intervals for each set of reconstructions. When compared against areas obtained for MCT, RMS values of areas for reconstructions MC1, MC2, and MC3 were 26.888, 12.384, and 4.837, respectively. On visual inspection, MC3 also exhibited the least motion artifacts at most time intervals.

Radiation doses for 3 common types of cardiac radiological examinations where investigated: coronary angiography (CA), percutaneous coronary intervention (PCI) and pacemaker insertions (PPI). 22 cardiac imaging suites participated in the study. Radiation dose was monitored for 1804 adult patients using dose area product (DAP) meters. Operational and examination details such as cardiologist grade, patient details and examination complexity were recorded for each examination. Both intra and inter-hospital variations where demonstrated by the results. Individual patient DAP values ranged from 136-23,101cGycm², 475-41,038cGycm² and 45- 17,192cGycm² for CA, PCI and PPI respectively, with third quartile values of 4,173cGycm², 8,836cGycm² and 2,051cGycm². Screening times varied from 0.22-27.6mins, 1.8-98mins and 0.33-54.5mins for CA, PCI and PPI respectively.

In this study we develop a novel ECG-gated method of HYPR (HighlY constrained backPRojection) CT reconstruction for low-dose myocardial perfusion imaging and present its first application in a porcine model. HYPR is a method of reconstructing time-resolved images from view-undersampled projection data. Scanning and reconstruction techniques were explored using x-ray projections from a 50 sec contrast-enhanced axial scan of a 47 kg swine on a 64-slice MDCT system. Scans were generated with view undersampling factors from 2 to 10. A HYPR reconstruction algorithm was developed in which a fully-sampled composite image is generated from views collected from multiple cardiac cycles within a diastolic window. A time frame image for a heartbeat was produced by modifying the composite with projections from the cycle of interest. Heart rate variations were handled by automatically selecting cardiac window size and number of cycles per composite within defined limits. Cardiac window size averaged 35% of the R-R interval for 2x undersampling and increased to 64% R-R using 10x undersampling. The selected window size and cycles per composite was sensitive to synchrony between heart rate, gantry rate, and the view undersampling pattern. Temporal dynamics and perfusion metrics measured in conventional short-scan (FBP) images were well-reproduced in the undersampled HYPR time series. Mean transit times determined from HYPR myocardial time-density curves agreed to within 8% with the FBP results. The results indicate potential for an order of magnitude reduction in dose requirement per image in cardiac perfusion CT via undersampled scanning and ECG-gated HYPR reconstruction.

The combination of real-time fluoroscopy and 3D cardiac imaging on the same C-arm system is a promising technique that might improve therapy planning, guiding, and monitoring in the interventional suite. In principal, to reconstruct a 3D image of the beating heart at a particular cardiac phase, a complete set of X-ray projection data representing that phase is required. One approximate approach is the retrospectively ECG-gated FDK reconstruction (RG-FDK). From the acquired data set of N_s multiple C-arm sweeps, those projection images which are acquired closest in time to the desired cardiac phase are retrospectively selected. However, this approach uses only 1/ N_s of the obtained data. Our goal is to utilize data from other cardiac phases as well. In order to minimize blurring and motion artifacts, cardiac motion has to be compensated for, which can be achieved using a temporally dependent spatial 3D warping of the filtered-backprojections. In this work we investigate the computation of the 4D heart motion based on prior reconstructions of several cardiac phases using RG-FDK. A 4D motion estimation framework is presented using standard fast non-rigid registration. A smooth 4D motion vector field (MVF) represents the relative deformation compared to a reference cardiac phase. A 4D deformation regridding by adaptive supersampling allows selecting any reference phase independently of the set of phases used in the RG-FDK for a motion corrected reconstruction. Initial promising results from in vivo experiments are shown. The subjects individual 4D cardiac MVF could be computed from only three RG-FDK image volumes. In addition, all acquired projection data were motion corrected and subsequently used for image reconstruction to improve the signal-to-noise ratio compared to RG-FDK.

Two major problems with the current electrocardiogram-gated cardiac computed tomography (CT) imaging technique are a large patient radiation dose (10-15 mSv) and insufficient temporal resolution (83-165 ms). Our long-term goal is to develop new time resolved and low dose cardiac CT imaging techniques that consist of image reconstruction algorithms and estimation methods of the time-dependent motion vector field (MVF) of the heart from the acquired CT data. Toward this goal, we developed a method that estimates the 2D components of the MVF from a sequence of cardiac CT images and used it to "reconstruct" cardiac images at rapidly moving phases. First, two sharp image frames per heart beat (cycle) obtained at slow motion phases (i.e., mid-diastole and end-systole) were chosen. Nodes were coarsely placed among images; and the temporal motion of each node was modeled by B-splines. Our cost function consisted of 3 terms: mean-squared-error with the block-matching, and smoothness constraints in space and time. The time-dependent MVF was estimated by minimizing the cost function. We then warped images at slow motion phases using the estimated vector fields to "reconstruct" images at rapidly moving phase. The warping algorithm was evaluated using true time-dependent motion vector fields and images both provided by the NCAT phantom program. Preliminary results from ongoing quantitative and qualitative evaluation using the 4D NCAT phantom and patient data are encouraging. Major motion artifact is much reduced. We conclude the new image-based motion estimation technique is an important step toward the development of the new cardiac CT imaging techniques.

As digital radiography (DR) systems are being increasingly adopted for clinical applications, automatic exposure control (AEC) has remained a critically important component. Unlike traditional screen/film systems, however, there does not currently exist a widely accepted AEC calibration criterion for DR systems. This is due mainly to the signal response characteristics and wide dynamic range of a DR detector, which are inherently different from those of a screen/film system. Consequently, the AEC cutoff dose and its dependence on the kVp selection (i.e., kVp compensation) should be calibrated differently for DR systems. In this paper, we have investigated three possible schemes to set up the AEC compensation based on a constant response of the detector in terms of the signal, receptor dose, or signal-to-noise-ratio (SNR) respectively. The results for each of the setup schemes were evaluated on four different DR detectors (Gd₂O₂S, CsI(Tl), a-Se, or BaFBrI as x-ray absorption material) based on the measured signal and noise response of the detector under the ISO beam conditions (ISO 6236-1). The results showed that all three setup schemes produced similar results for clinical beams above 70 kVp. Significant differences were observed only at lower kVp (≤60) beams. In addition, schemes of constant signal and constant SNR produced similar results with the only exception for the a-Se detector at low kVp (≤60) beam. These results indicate that the choice of the kVp schemes would be important only for low kVp exams.

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 (AMFPIs) are extraordinarily expensive compared to the systems they are replacing. Thus there is a need for a low cost digital imaging system for general applications in radiology. Different approaches have been considered to make lower cost, integrated x-ray imaging devices for digital radiography, including: scanned projection x-ray, an integrated approach based on computed radiography technology and optically demagnified x-ray screen/CCD systems. These approaches suffer from either high cost or high mechanical complexity and do not have the image quality of AMFPIs. We have identified a new approach - the X-ray Light Valve (XLV). The XLV has the potential to achieve the immediate readout in an integrated system with image quality comparable to AMFPIs. The XLV concept combines three well-established and hence lowcost technologies: an amorphous selenium (a-Se) layer to convert x-rays to image charge, a liquid crystal (LC) cell as an analog display, and an optical scanner for image digitization. Here we investigate the spatial resolution possible with XLV systems. Both a-Se and LC cells have both been shown separately to have inherently very high spatial resolution. Due to the close electrostatic coupling in the XLV, it can be expected that the spatial resolution of this system will also be very high. A prototype XLV was made and a typical office scanner was used for image digitization. The Modulation Transfer Function was measured and the limiting factor was seen to be the optical scanner. However, even with this limitation the XLV system is able to meet or exceed the resolution requirements for chest radiography.

The scientific community has generally adopted use of the modulation transfer function (MTF) and detective quantum efficiency (DQE) as primary measures of performance of radiographic detectors. However, measurement of these parameters is generally restricted to experts in laboratory environments due to the required x-ray physics knowledge, specialized instrumentation and computational analyses. We have developed a prototype instrument that automates both the physical measurement and subsequent image analysis to determine the MTF, noise power spectrum (NPS) and DQE of radiographic and mammographic systems. The instrument is placed in the x-ray path directly in front of the detector. A series of images are acquired, saved in "raw" DICOM format and then used to determine the MTF (using the slanted-edge method) and NPS. The number of incident quanta is calculated from measurements of the incident exposure including corrections for air temperature and pressure and ionization chamber spectral response. The primary sources of error are backscatter from the detector and scatter generated within the instrument. These have been minimized to achieve an incident exposure measurement within 2% of a calibrated electrometer and chamber in free space. The MTF and DQE of a commercial CsI-based flat-panel detector were measured over a range of incident exposures from 20 uR to 20 mR per image. Results agreed with both our own laboratory measurements and previously published measurements performed elsewhere with a similar detector within 2% for the MTF and 5% for the DQE. A complete DQE analysis of a clinical digital flat-panel detector is completed in 30 minutes and requires no system modifications.

High-performance intraoperative imaging is essential to an ever-expanding scope of therapeutic procedures ranging from tumor surgery to interventional radiology. The need for precise visualization of bony and soft-tissue structures with minimal obstruction to the therapy setup presents challenges and opportunities in the development of novel imaging technologies specifically for image-guided procedures. Over the past ~5 years, a mobile C-arm has been modified in collaboration with Siemens Medical Solutions for 3D imaging. Based upon a Siemens PowerMobil, the device includes: a flat-panel detector (Varian PaxScan 4030CB); a motorized orbit; a system for geometric calibration; integration with real-time tracking and navigation (NDI Polaris); and a computer control system for multi-mode fluoroscopy, tomosynthesis, and cone-beam CT. Investigation of 3D imaging performance (noise-equivalent quanta), image quality (human observer studies), and image artifacts (scatter, truncation, and cone-beam artifacts) has driven the development of imaging techniques appropriate to a host of image-guided interventions. Multi-mode functionality presents a valuable spectrum of acquisition techniques: i.) fluoroscopy for real-time 2D guidance; ii.) limited-angle tomosynthesis for fast 3D imaging (e.g., ~10 sec acquisition of coronal slices containing the surgical target); and iii.) fully 3D cone-beam CT (e.g., ~30-60 sec acquisition providing bony and soft-tissue visualization across the field of view). Phantom and cadaver studies clearly indicate the potential for improved surgical performance - up to a factor of 2 increase in challenging surgical target excisions. The C-arm system is currently being deployed in patient protocols ranging from brachytherapy to chest, breast, spine, and head and neck surgery.

A CR system based on line-scanning and needle image plate technologies (CsBr), was modified by means of an improved line light source for stimulation and a high aperture line optics for light collection. For comparison, two types of needle image plates were used, i.e. standard and high resolution plates. The image quality performance was investigated in terms of modulation transfer function, detective quantum efficiency and visual impression of both anthropomorphic and technical phantom images, with respect to application for X-ray examinations of extremities. MTF and DQE at beam quality RQA 3 were clearly improved by the modified scanning components. Further improvement especially with respect to MTF at spatial frequencies beyond 2 LP/mm was obtained via the high resolution image plate, but at reduced low frequency DQE values. The visual inspection of images of a high contrast resolution pattern and an anthropomorphic hand phantom revealed a clear preference for the modified system. The use of the high resolution image plate alone resulted in images with higher sharpness but increased noise. The best performance - at constant X-ray exposure dose - was delivered by the combination of improved optics components with the adapted needle image plate.

A new high-resolution, high-sensitivity, low-noise x-ray detector based on EMCCDs has been developed. The EMCCD detector module consists of a 1kx1k, 8μm pixel EMCCD camera coupled to a CsI(Tl) scintillating phosphor via a fiber optic taper (FOT). Multiple modules can be used to provide the desired field-of-view (FOV). The detector is capable of acquisitions over 30fps. The EMCCD's variable gain of up to 2000x for the pixel signal enables high sensitivity for fluoroscopic applications. With a 3:1 FOT, the detector can operate with a 144μm effective pixel size, comparable to current flat-panel detectors. Higher resolutions of 96 and 48μm pixel size can also be achieved with various binning modes. The detector MTFs and DQEs were calculated using a linear-systems analysis. The zero frequency DQE was calculated to be 59% at 74 kVp. The DQE for the 144μm pixel size was shown to exhibit quantum-noise limited behavior down to ~0.1μR using a conservative 30x gain. At this low exposure, gains above 30x showed limited improvements in DQE suggesting such increased gains may not be necessary. For operation down to 48µm pixel sizes, the detector instrumentation noise equivalent exposure (INEE), defined as the exposure where the instrumentation noise equals the quantum-noise, was <0.1μR for a 20x gain. This new technology may provide improvements over current flat-panel detectors for applications such as fluoroscopy and angiography requiring high frame rates, resolution, dynamic range and sensitivity while maintaining essentially no lag and very low INEE. Initial images from a prototype detector are also presented.

We use a contrast-detail observer study to compare performance of a novel 3D computed mammotomography (CmT) system with a commercially developed full-field digital mammography (FFDM) system. A contrast-detail phantom comprised of uniform acrylic spheres of various diameters was developed and placed in a variety of mediums including uniform water (simulating low contrast lesions within a uniform background), water and acrylic yarn (simulating low contrast lesions with over/under-lying structure), oil only (simulating higher contrast lesions in a uniform background), and oil and acrylic yarn (simulating higher contrast lesions with over/under-lying structure). For CmT, the phantom was placed in a 14.6 cm diameter uncompressed breast phantom and projections acquired using a simple circular orbit, W-target tube, 60 kVp tube potential, 0.05 cm Ce filtration, 4 mAs per projection, and a CsI(Tl) digital x-ray detector. Reconstructions used an iterative OSTR algorithm. For FFDM, the phantom was placed in a 5.3-cm-thick compressed breast phantom. Single CC-view mammograms were acquired using a clinical W-target tube with 50 um Rh filtration, 28 kVp, photo-timed mAs per our clinical mammography operation, and a Selenium-based flat-panel detector (Mammomat Novation, Siemens). Six observers evaluated the images in terms of the number of detectable spheres. FFDM performed significantly better for the low contrast lesions in uniform water background (p<0.05). However, CmT performed significantly better for all other cases (p<0.05). Results indicate that CmT shows significant advantage in soft tissue detection over FFDM in otherwise low contrast dense breasts.

A hybrid SPECT-CT system for dedicated 3D breast imaging (mammotomography) is currently under development. Each imaging system will be placed on top of a single rotation stage and moved in unison azimuthally, with the SPECT system additionally capable of polar and radial motions. In this initial prototype, the CT system will initially be positioned at a fixed polar tilt. Using a phantom with three tungsten wires, the MTF of the CT system was measured in 3D for different CT system tilts. A phantom with uniformly arranged 0.5cm diameter acrylic spheres was suspended in air in the CT field of view, and also placed at multiple locations and orientations inside an oil-filled breast phantom to evaluate the effect of CT system tilt on lesion visibility and distortion. Projection images were collected using various simple circular orbits with fixed polar tilts ranging between ±15°, and complex 3D saddle trajectories including combined polar and azimuthal motions at maximum polar tilt angles. Reconstructions were performed using an iterative reconstruction algorithm on 4x4 binned projection images with 0.508mm3 voxels. There was minor variation in the MTF in the imaged volume for the CT system at all trajectories, potentially due to the use of an iterative reconstruction algorithm. Results from the spherical cross phantoms indicated that there was more reconstruction inaccuracy and geometric distortion in the reconstructed slices with simple circular orbits with fixed tilt in contrast to complex 3D trajectories. Line profiles further showed a cupping artifact in planes farther away from the flat plane of the x-ray cone beam placed at different tilts. However, this cupping artifact was not seen for images acquired with complex 3D trajectories. This indicated that cupping artifacts can also be caused by undersampled cone beam data. These findings generally indicate that despite insufficient sampling with the cone beam imaging geometry, it is possible to place the CT system at a stationary polar tilt with the CT tube positioned upward such that a patient can be comfortably placed above the system and allow complete sampling near the top of the pendant, uncompressed breast and chest wall. However, a complex 3D trajectory allows for more complete sampling of the entire image volume.

The purpose of this project was to design, build, and characterize the performance of a volume breast ultrasound (VBUS) scanner that images the pendant breast. VBUS scanner design includes a: 1) clinical ultrasound scanner and transducer; 2) scanning table with a hole for the pendant breast; 3) rotational gantry; 4) probe mounting assembly; 5) compressionless breast stabilization device; 6) acquisition, control, reconstruction, and display software. Performance assessment characterized a variety of parameters including: spatial resolution, uniformity, and distortion using high and low contrast test objects in both horizontal and vertical scanning modes. The VBUS scanner modules have been constructed and initial performance evaluated. Approximately 300 scans are acquired over 360 degrees in 18 seconds. Reconstruction requires 25 ms per slice. Test object images depicted hyper- and hypo-echoic masses and demonstrated good resolution, soft tissue contrast and reduced speckle compared to conventional US images. Although scanner performance is satisfactory, additional developments including reduced spacing transducer - scanned object spacing and corrections for sound velocity and aberrations will improve operation. Future work will continue system optimization.

A hybrid SPECT-CT system for dedicated 3D breast cancer imaging (mammotomography) is in development. Using complex 3D imaging acquisition trajectories, the versatile integrated system will be capable of contouring and imaging an uncompressed breast suspended in a 3D volume located below a radio-opaque patient bed, providing co-registered volumetric anatomical and functional information. This study examines tradeoffs involved in the design of the patient bed to satisfy concomitant and competing technical and ergonomic requirements specific to this imaging paradigm. The complementary source-detector arrangement of the CT system is geometrically more restrictive than that of the single detector SPECT system. Additionally, the compact dimensions and size of the CT system components (primarily the x-ray tube) are key constraints on the bed design and so the focus is concentrated there. Using computer-aided design software, several design geometry options are examined to simultaneously consider and optimize the following parameters: image magnification, imaged breast volume, azimuthal imaging span, and patient comfort. Several CT system source to image distances are examined (55-80cm), as well as axial patient tilt up to 35°. An optimal patient bed design for a completely under-bed hybrid imaging system was determined. A 60cm SID, magnification factor of ~1.5, and patient bed angled at ~15° provided the optimal dimensions. Additional bed dimensions allow the CT projection beam to nearly entirely image the chest wall, however at the cost of reduced angular sampling for CT. Acquired x-ray mammotomographic image data is used to assess the feasibility of this reduced angle acquisition approach.

As a new three-dimensional imaging technique, digital breast tomosynthesis allows the reconstruction of an arbitrary set of planes in the breast from a limited-angle series of projection images. Though several tomosynthesis algorithms have been proposed, no complete optimization and comparison of different tomosynthesis acquisition techniques for available methods has been conducted as of yet. This paper represents a methodology of noise-equivalent quanta NEQ (f) analysis to optimize and compare the efficacy of tomosynthesis algorithms and imaging acquisition techniques for digital breast tomosynthesis. It combines the modulation transfer function (MTF) of system signal performance and the noise power spectrum (NPS) of noise characteristics. It enables one to evaluate the performance of different acquisition parameters and algorithms for comparison and optimization purposes. An example of this methodology was evaluated on a selenium-based direct-conversion flat-panel Siemens Mammomat Novation prototype system. An edge method was used to measure the presampled MTF of the detector. The MTF associated with the reconstruction algorithm and specific acquisition technique was investigated by calculating the Fourier Transform of simulated impulse responses. Flat field tomosynthesis projection sequences were acquired and then reconstructed. A mean-subtracted NPS on the reconstructed plane was studied to remove fixed pattern noise. An example of the application of this methodology was illustrated in this paper using a point-by-point Back Projection correction (BP) reconstruction algorithm and an acquisition technique of 25 projections with 25 degrees total angular tube movement.

The purpose of this work was to evaluate and compare the visibility of tumors in digital mammography (DM) and breast tomosynthesis (BT) images. Images of the same women were acquired on both a DM system (Mammomat Novation, Siemens) and a BT prototype system adapted from the same type of DM system. Simulated 3D tumors (average dimension: 8.4 mm x 6.6 mm x 5 mm) were projected and added to each DM image as well as each BT projection image prior to 3D reconstruction. The same beam quality and approximately the same total absorbed dose were used for each breast image acquisition on both systems. Two simulated tumors were added to each of thirty breast scans, yielding sixty cases. A series of 4-alternative forced choice (4-AFC) human observer performance experiments were conducted in order to determine what projected tumor signal intensity in the DM images would be needed to achieve the same detectability as in the reconstructed BT images. Nine observers participated. For the BT experiment, when the tumor signal intensity on the central projection was 0.010 the mean percent of correct responses (PC) was measured to be 81.5%, which converted to a detectability index value (d') of 1.96. For the DM experiments, the same detectability was achieved at a signal intensity determined to be 0.038. Equivalent tumor detection in BT images were thus achieved at around four times less projected signal intensity than in DM images, indicating that the use of BT may lead to earlier detection of breast cancer.

In this study, we used a mathematical observer model to combine information obtained from multiple angular projections of the same breast to determine the overall detection performance of a multi-projection breast imaging system in detectability of a simulated mass. 82 subjects participated in the study and 25 angular projections of each breast were acquired. Projections from a simulated 3 mm 3-D lesion were added to the projection images. The lesion was assumed to be embedded in the compressed breast at a distance of 3 cm from the detector. Hotelling observer with Laguerre-Gauss channels (LG CHO) was applied to each image. Detectability was analyzed in terms of ROC curves and the area under ROC curves (AUC). The critical question studied is how to best integrate the individual decision variables across multiple (correlated) views. Towards that end, three different methods were investigated. Specifically, 1) ROCs from different projections were simply averaged; 2) the test statistics from different projections were averaged; and 3) a Bayesian decision fusion rule was used. Finally, AUC of the combined ROC was used as a parameter to optimize the acquisition parameters to maximize the performance of the system. It was found that the Bayesian decision fusion technique performs better than the other two techniques and likely offers the best approximation of the diagnostic process. Furthermore, if the total dose level is held constant at 1/25th of dual-view mammographic screening dose, the highest detectability performance is observed when considering only two projections spread along an angular span of 11.4°.

Digital tomosynthesis (DTS) is emerging as an advanced imaging technique that enables volumetric slice imaging with a detector typically used for projection radiography. An understanding of the interactions between DTS acquisition parameters and characteristics of the reconstructed slice images is required for optimizing the acquisition protocols of various clinical applications. This paper presents our investigation of the effects and interactions of acquisition parameters, including sweep angle, number of projections, and dose, on clinically relevant image-quality metrics. Metrics included the image characteristics of in-slice resolution, depth resolution, image noise level, and presence of ripple. Phantom experiments were performed to characterize the relationship between the acquisition parameters and image quality. Results showed that the depth resolution was mainly dependent on sweep angle. Visibility of ripple was determined by the projection density (number of projections divided by sweep angle), as well as properties of the imaged object. Image noise was primarily dependent on total dose and not significantly affected by the number of projections. These experimental and theoretical results were confirmed using anthropomorphic phantoms and also used to develop clinical acquisition protocols. Assessment of phantom and clinical images obtained with these protocols revealed that the use of acquisition protocols optimized for a given clinical exam enables rapid, low-dose, high quality DTS imaging for diverse clinical applications including abdomen, hand, shoulder, spine, and chest. We conclude that DTS acquisition parameters have a significant effect on image quality and should be tailored for the imaged anatomy and desired clinical application. Relationships developed in this work will guide the selection of acquisition protocols to improve image quality and clinical utility of DTS for a wide variety of clinical exams.

In breast tomosynthesis there are tradeoffs between resolution, noise and acquisition speed for a given glandular dose. The purpose of the present work is to investigate the dependence of tomosynthesis imaging performance on system configuration, which includes detector operational modes and image acquisition geometry. A prototype Siemens breast tomosynthesis system with maximum angular range of +/- 25 degrees was used in our investigation. The system was equipped with an amorphous selenium (a-Se) full field digital mammography detector with pixel size of 85µm. The detector can be read out with full resolution or 2x1 binning (binning in the tube travel direction), which increases the image readout rate and decreases the degradation effect of electronic noise. The total number of views can be varied from 11 to 49, and filtered back projection (FBP) method was used to reconstruct the tomosynthesis images. We investigated the effects of detector operational modes (binning) and imaging geometry (view angle and number) on temporal performance and spatial resolution of the projection images. The focal spot blur due to continuous tube travel was measured for different acquisition geometry, and its effect on in-plane presampling modulation transfer function (MTF) was compared to that due to pixel binning. A three-dimensional cascaded linear system model was developed for tomosynthesis to predict the 3D MTF, NPS and DQE. The results were compared with experimental measurements, and reasonable agreement was achieved. The understanding of the relationship between the 3D and projection image quality will lead to optimization of the x-ray spectrum, imaging geometry and reconstruction filters for digital breast tomosynthesis.

There exists a strong desire for a platform in which researchers may investigate planar tomosynthesis (i.e. all source positions reside in a single plane that is parallel to the reconstructed image planes) trajectories directly on an interventional C-arm system. In this work we describe an experimental system designed to accomplish this aim, as well as the potential of this system for testing multiple aspects of the tomosynthetic image acquisition process. The system enables one to evaluate the effect of the physical imaging parameters on the image quality, as well as the effect of the reconstruction algorithm utilized. The experimental data collection for this work is from the Innova 4100 (Flat-panel based interventional C-arm system manufactured by GE Healthcare). The system is calibrated using a phantom with known geometrical placement of multiple small metallic spheres. Initial performance was assessed with three physical phantoms and performance was assessed by varying: the reconstruction algorithm (backprojection, filtered backprojection), the half tomographic angle (15°, 25°, 35°), and the angular sampling (20,40,80 views / acquisition). Initial results demonstrate the ability to well differentiate simulated vessels separated by 1 cm, even with the modest half tomographic angle of 15° and modest sampling of 20 views/acquisition.

We developed a Flat-panel detector based Volume CT (VCT) prototype scanner with large z-coverage. In that prototype scanner a Varian 4030CB a-Si flat-panel detector was mounted in a multi slice CT-gantry (Siemens Medical Solutions) which provides a 25 cm field of view with 18 cm z-coverage at isocenter. The large volume covered in one rotation can be used for visualization of complete organs of small animals, e.g. rabbits. By implementing a mode with continuous scanning, we are able to reconstruct the complete volume at any point in time during the propagation of a contrast bolus. Multiple volumetric reconstructions over time elucidate the first pass dynamics of a bolus of contrast resulting in 4-D angiography and potentially allowing whole organ perfusion analysis. We studied to which extent pixel based permeability and blood volume calculation with a modified Patlak approach was possible. Experimental validation was performed by imaging evolution of contrast bolus in New Zealand rabbits. Despite the short circulation time of a rabbit, the temporal resolution was sufficient to visually resolve various phases of the first pass of the contrast bolus. Perfusion imaging required substantial spatial smoothing but allowed a qualitative discrimination of different types of parenchyma in brain and liver. If a true quantitative analysis is possible, requires further studies.

Spatial resolution is one of the most crucial parameters for an imaging system. The modulation transfer function (MTF) was physically measured using wire images on a prototype breast CT scanner previously¹. In this study, the effects on MTF from different components in the imaging chain, including the x-ray focal spot distribution, detector lag, and x-ray detector MTF were physically measured. The contributions of these three factors and gantry motion affecting CT system resolution were studied using computer simulation. The CT system MTF was computed and the role of each factor was studied independently. The simulated MTF results demonstrated that the x-ray focal spot size and detector MTF have an effect on the system resolution, while the scanner motion degrades only the azimuthal MTF, with greater degradation occurring further from isocenter where greater rotational velocities occur. The azimuthal MTF of this system has a cutoff frequency of 2.0 cycles/mm at the isocenter but degrades to 1.0 cycles/mm at the periphery. The radial MTF has a cutoff frequency of 2.0 cycles/mm, at both the isocenter and periphery. The comparison between the computer simulated and physically measured MTF values demonstrates reasonable accuracy in the simulation process. The results from computer simulation also suggest ways in which the spatial resolution can be improved by system modification.

The first results from an interventional C-arm based computed tomography system where a complete source trajectory was used are presented. A scan with two arcs which are joined approximately at the center of their paths (CC trajectory) is utilized here. This trajectory satisfies Tuy's sufficiency condition for a large volume, but is not well populated with PI-lines. Therefore, a non-PI-line based reconstruction method is required. The desire for high dose efficiency led to the selection of an equal weighting based method. An FBP type reconstruction algorithm which was derived for two orthogonal concentric circles was utilized for reconstruction. The concept of a virtual image object was used to relate the projections from the two acquired non-orthogonal arcs to projections of a virtual object from two orthogonal arcs. Geometrical calibration is vital when performing tomography from an interventional system, and was incorporated here with the use of a homogeneous virtual projection matrix. The results demonstrate a significant reduction in cone-beam artifacts when the complete source trajectory is utilized.

In this work we used a novel CdTe photon counting x-ray detector capable of very high count rates to perform x-ray micro-computed tomography (microCT). The detector had 2 rows of 384 square pixels each 1 mm in size. Charge signals from individual photons were integrated with a shaping time of ~60 ns and processed by an ASIC located in close proximity to the pixels. The ASIC had 5 energy thresholds with associated independent counters for each pixel. Due to the thresholding, it is possible to eliminate dark-current contributions to image noise. By subtracting counter outputs from adjacent thresholds, it is possible to obtain the number of x-ray photon counts in 5 adjacent energy windows. The detector is capable of readout times faster than 5 ms. A prototype bench-top specimen μCT scanner was assembled having distances from the tube to the object and detector of 11 cm and 82 cm, respectively. We used a conventional x-ray source to produce 80 kVp x-ray beams with tube currents up to 400 μA resulting in count rates on the order of 600 kcps per pixel at the detector. Both phantoms and a dead mouse were imaged using acquisition times of 1.8 s per view at 1° steps around the object. The count rate loss (CRL) characteristics of the detector were measured by varying the tube current and corrected for using a paralyzable model. Images were reconstructed using analytical fan-beam reconstruction. The reconstructed images showed good contrast and noise characteristics and those obtained from different energy windows demonstrated energy-dependent contrast, thus potentially allowing for material decomposition.

Using a commercial clinical CT scanner (GE lightspeed), nine CT scans were performed on a 20 cm diameter plastic pipe filled with water. The mAs was varied from 10 to 400mAs and the beam energy was varied from 80 to140kVp. For each scan three volume datasets were reconstructed using different filters. Noise power spectrum (NPS) curves were measured to examine the effect of varying kVp, mAs and reconstruction filter on the noise content. Sixteen slices from each of the reconstructed volumes were used to compute the NPS; the central 192x192 pixels of each slice were split into four overlapping regions of interest (ROI) of 128x128 pixels. A total of 64 ROI were used per scan. The magnitude squared of the 2D Fourier transform of each ROI was computed. The mean of the 64 2D results was averaged over radial frequency, yielding a 1D NPS. The overall shape of the NPS was dependent on the reconstruction filter used. The magnitude of the curves decreased with the increase of mAs or kVp. kVp, mAs, and the reconstruction filter can be adjusted to modulate the amount of noise present in resulting CT volumes, but the effect these values have on the patient must be considered. The relationship between NPS and the Noise Equivalent Quanta (NEQ) makes trends in NPS important and is the motivation for this evaluation and future research.

Digital breast tomosynthesis uses a limited number of low-dose x-ray projections to produce a three-dimensional (3D) tomographic reconstruction of the breast. The purpose of this investigation was to characterize and evaluate the effect of scatter radiation on image quality for breast tomosynthesis. Generated by a Monte Carlo simulation method, scatter point spread functions (PSF) were convolved over the field of view (FOV) to estimate the distribution of scatter for each angle of tomosynthesis projection. The results demonstrated that in the absence of scatter reduction techniques, the scatter-to-primary ratio (SPR) levels for the average breast are quite high (~0.4 at the centre of mass), and increased with increased breast thickness and with larger FOV. Associated with such levels of x-ray scatter are cupping artifacts, as well as reduced accuracy in reconstruction values. The effect of x-ray scatter on the contrast, noise, and signal-difference-to-noise ratio (SDNR) in tomosynthesis reconstruction was measured as a function of tumour size. For example, the contrast in the reconstructed central slice of a tumour-like mass (14 mm in diameter) was degraded by 30% while the inaccuracy of the voxel value was 28%, and the reduction of SDNR was 60%. We have quantified the degree to which scatter degrades the image quality over a wide range of parameters, including x-ray beam energy, breast thickness, breast diameter, and breast composition. However, even without a scatter rejection device, the contrast and SDNR in the reconstructed tomosynthesis slice is higher than that of conventional mammographic projection images acquired with a grid at an equivalent total exposure.

Recently, we proposed a scatter correction method for x-ray imaging using primary modulation. A primary modulator with spatially variant attenuating materials is inserted between the x-ray source and the object to make the scatter and part of the primary distributions strongly separate in the Fourier domain. Linear filtering and demodulation techniques suffice to extract and correct the scatter for this modified system. The method has been verified by computer simulations and preliminary experimental results on a simple object. In this work, we improve performance by using a new primary modulator with a higher modulation frequency and by refining the algorithm. The improved method is evaluated experimentally using a pelvis phantom. The imaging parameters are chosen to match the Varian Acuity CT simulator, where scatter correction has been shown to be challenging due to complicated artifact patterns. The results using our approach are compared with those without scatter correction, and with scatter estimated and corrected using a slit measurement as a pre-scan. The comparison shows that the primary modulation method greatly reduces the scatter artifacts and improves image contrast. Using only one single scan, this method achieves CT HU accuracy comparable to that obtained using a slit measurement as a pre-scan.

The large contribution of scatter to cone-beam computed tomography (CBCT) x-ray projections significantly degrades image quality, both through streaking and cupping artifacts and by loss of low contrast boundary detectability. The goal of this investigation is to compare the efficacy of three widely used scatter mitigation methods: subtractive scatter correction (SSC); anti-scatter grids (ASG); and beam modulating with bowtie filters; for improving signal-to-noise ratio (SNR), contrast, contrast-to-noise ratio (CNR) and cupping artifacts. A simple analytic model was developed to predict scatter-to-primary ratio (SPR) and CNR as a function of cylindrical phantom thickness. In addition, CBCT x-ray projections of a CatPhan QA phantom were measured, using a Varian CBCT imaging system, and computed, using an inhouse Monte Carlo photon-transport code to more realistically evaluate the impact of scatter mitigation techniques. Images formed with uncorrected sinograms acquired without ASGs and bow-tie filter show pronounced cupping artifacts and loss of contrast. Subtraction of measured scatter profiles restores image uniformity and CT number accuracy, but does not improve CNR, since the improvement in contrast almost exactly offset by the increase in relative x-ray noise. ASGs were found to modestly improve CNR (up to 20%, depending ASG primary transmission and selectivity) only in body scans, while they can reduce CNR for head phantoms where SPR is low.

Motion contamination in computed tomography projection data causes significant artifacts in the reconstructed images. If during the tomographic acquisition the object is relatively stationary during a portion of the acquisition, and then moves significantly, the projection data after the motion will be inconsistent with the projection data during the period of relative stationarity. The fan-beam data consistency condition (FDCC) provides a means to directly estimate motion contaminated projection data based on all of the projection data acquired. Thus, the FDCC may be used to combat many types of motion contamination in computed tomography. This approach to motion artifact correction is novel as none of the previous methods for artifact correction utilized a direct estimation of motion contaminated data. Additionally, this methodology depends upon only a small amount of a priori information and is not based on a motion model. Another distinguishing feature of this method is that it operates directly in the projection space, and is completely independent of the reconstruction algorithm utilized. An example of clinical relevance of coronary motion artifact reduction is presented using both simulated projection data as well as projection data acquired with a porcine model using a state-of-the-art 64 row volumetric CT scanner. Significant reduction in motion related artifacts is achieved in both the simulation case and the porcine model.

In this study, we investigate the relationship between quantum noise and spatial resolution for volumetric CT. Both theoretical analysis and experiments were performed to investigate their relationship. In theory, quantum noise can be derived from its relationship to dose, in-plane spatial resolution, recon kernel, and signal-to-noise ratio (SNR). In the experiments, by scanning a Teflon sphere phantom, the 3-D MTF was measured from the edge profile along the spherical surface. Cases of different resolutions (and noise levels) were generated by adjusting recon kernel. To reduce bias, the total photon fluxes were matched: 120kVp, 260mA, and 1sec per gantry rotation. In the end, all data sets were reconstructed using modified FDK algorithm under the same condition: FOV=10cm and slice thickness=0.625mm. Finally, we investigated the efficiency of an image-space adaptive smoothing filter as a noise reduction tool and its impact on spatial resolution. The theoretical analysis indicated that the variance of noise is proportional to at least 4th power of the spatial resolution. Our experimental results supported this conclusion by showing the relationship is 4.6th (helical) or 5th (axial) power. Results also showed that, with properly designed image-space smoothing filters, it is feasible to reduce quantum noise (and the power relationship to a lower order) with smaller loss of spatial resolution.

The raw data generated by a computed tomography (CT) machine are not readily usable for reconstruction. This is the result of various system non-idealities, and although the deterministic nature of corruption effects like crosstalk and afterglow permits removal through deconvolution, there is the drawback that deconvolution increases noise. Methods that perform raw data correction combined with noise suppression are commonly termed sinogram restoration methods. The need for sinogram restoration arises, for example, when photon counts are low and non-statistical reconstruction algorithms like filtered backprojection are used. Many modern CT machines offer a so-called dual focal spot (DFS) mode, which serves the goal of increased radial sampling by switching the focal spot between two positions on the anode plate during the scan. Although the focal spot mode does not play a role with respect to how the data are affected by the above mentioned corruption effects, it needs to be taken into account, if regularized sinogram restoration is to be applied to the data. This work points out the subtle difference in processing that sinogram restoration for DFS requires, how it is correctly employed within the penalized maximum likelihood sinogram restoration algorithm, and what impact that has on image quality.

Emerging photon-counting detectors with energy discrimination ability for x-ray CT perform binning according to the energy of the incoming photons. Multiple output channels with different energy thresholds can be obtained in one irradiation. The energy dependency of attenuation coefficients can be described by a linear combination of basis functions, e.g., Compton scatter and photo-electric effect; their individual contributions can be differentiated by using the multiple energy channels hence material characterization is made possible. Conventional analytic approach is a two-step process. First decompose in the projection domain to obtain the sinograms corresponding to the coefficients of the basis functions, then apply FBP to obtain the individual material components. This two-step process may have poor quality and quantitative accuracy due to the lower counts in the separate energy channels and approximation errors propagated to the image domain from projection domain decomposition. In this work we modeled the energy dependency of linear attenuation coefficients in our problem formulation and applied the optimality transfer principle to derive a Poisson-likelihood based algorithm for material decomposition from multiple energy channels. Our algorithm reconstructs the coefficients of the basis functions directly therefore the separate non-linear estimation step in the projection domain as in conventional approaches is avoided. We performed simulations to study the accuracy and noise properties of our method. We synthesized the linear attenuation coefficients at a reference energy and compared with standard attenuation values provided by NIST. We also synthesized the attenuation maps at different effective energy bin centers corresponding to the different energy channels and compared the synthesized images with reconstructions from standard fan-beam FBP methods. Preliminary simulations showed that our reconstructed images have much better noise properties.

In tomosynthesis, cone-beam projection data are acquired from a few of view angles, which are not sufficient for an exact reconstruction of an image object using state-of-the-art image reconstruction algorithms. In the case of parallel-beam projections, the well-known projection-slice theorem may be utilized to transform the parallel-beam projections into the Fourier space of an image object. Due to the limited range of view angles, the available projection data can only populate a portion of Fourier space. Moreover, the angular sampling rate of the populated portion of the Fourier space may not satisfy the Nyquist criterion. Thus, reconstructed images using direct Fourier inversion contain severe streaking and distortion artifacts. In this paper, we present a novel image reconstruction method via minimizing the total variation (TV) of the reconstructed image for limited view angle X-ray computed tomography. Specifically, the missing data points in Fourier space, due to either the limited range or undersampling of view angles, are iteratively filled using the following two constraint conditions: (1) the total variation of the reconstructed image is minimized and (2) reconstructed image maintains fidelity to the sampled data in the Fourier space. Using analytical phantoms, numerical simulations were conducted to validate the new image reconstruction method. Images are compared with two other image reconstruction methods in terms of image artifact level and noise properties. Numerical results demonstrated that the new image reconstruction algorithm is superior to direct Fourier inversion reconstruction algorithm and the projection onto convex sets (POCS) image reconstruction algorithm.

In a 3rd generation CT system, a single source projects the entire field of view (FOV) onto a large detector opposite the source. In multi-source CT imaging, a multitude of sources sequentially project a part of the FOV on a much smaller detector. These sources may be distributed in both the trans-axial and axial directions in order to jointly cover the entire FOV. Scan data from multiple sources in the axial direction provide complementary information, which is not available in a conventional single-source CT system. In this work, an analytical 3D cone-beam reconstruction algorithm for multi-source CT is proposed. This approach has three distinctive features. First, multi-source data are re-binned transaxially to multiple offset third-generation datasets. Second, data points in sinograms from multiple source sets are either accepted or rejected for contribution to the backprojection of a given voxel. Third, instead of using a ramp filter, a Hilbert transform is combined with a parallel derivative to form the filtering mechanism. Phantom simulations are performed using a multi-source CT geometry and compared to conventional 3rd generation CT geometry. We show that multi-source CT can extend the axial scan coverage to 120mm without cone-beam artifacts, while a third-generation geometry results in compromised image quality at 60mm of axial coverage. Moreover, given that the cone-angle in the proposed geometry is limited to 7 degrees, there are no degrading effects such as the Heel effect and scattered radiation, unlike in a third-generation geometry with comparable coverage. An additional benefit is the uniform flux profile resulting in uniform image noise throughout the FOV and a uniform dose absorption profile.

In radiotherapy, the respiratory motion of the patient in treatment position is determined from gated cone-beam CT images. This method consists in selecting projections according to a respiratory signal for the reconstruction of a CT image of each respiratory state. This selection can be based on the amplitude or the phase of the signal. The number of selected projections also depends on the width of the gating window. The present study compares different reconstructions using a dynamic digital phantom of the thorax and a known respiratory signal. We applied both regular and irregular motions to this phantom and reconstructed the exhale state using different widths. We evaluated these reconstructions with the signal-to-noise ratio, the contrast-to-noise ratio and a blur criterion. In the case of a regular motion, there was no difference between the amplitude and the phase. The signal quality was high, even for the smallest width, and the blur increased with width. In the case of an irregular motion, the difference was noticeable. Amplitude-based reconstructions suffered from severe artifacts with the smallest width because there were respiratory cycles for which no projection was selected. This drawback is overcome by increasing the width of the gating window. Phase-based reconstructions also allowed to avoid artifacts, whatever the width. But the blur was higher, even for the smallest width applied. These results suggest that the gating process must be adjusted in order to select at least one projection per respiratory cycle. Phase gating is a robust way to achieve this goal when respiration is irregular. Amplitude gating may be more effective in terms of blur, but the width must be carefully chosen to avoid severe artifacts. Finally, we observed the potential of dynamic reconstruction by using a motion model to deform different gated CT images toward a common reference and compute the weighted mean. The resulting CT image suffered less from artifacts than each gated CT image separately even if artifacts were still visible.

Accurate prediction of reconstructed noise in computed tomography (CT) images is important for purposes of system design, optimization and evaluation. A large body of work describes noise prediction methods for CT, the vast majority of which assume stationarity of both noise and signal processes. Consequently, these methods are usually applied to and evaluated using simple phantoms, and only a portion of the image is scrutinized. In this work, we derive a practical method for reconstructing CT noise variance maps for arbitrary objects. Photon Poisson noise and system electronic noise are considered. The final formula has the same structure as that of the filtered backprojection (FBP) formula, but with different weighting factors and convolution kernels. The algorithm is verified using computer simulations of the Shepp-Logan phantom, and a good match is found between the predicted noise map from one single noisy scan and the measured noise using 128 noisy scans. As compared to other proposed noise models, our complementary work provides a method of noise prediction by simple adaptation of FBP reconstruction algorithms. The result is a tool that can be useful for system optimization and evaluation tasks as well as the design of reconstruction filters.

Atlases are normalized representations of anatomy that can provide a standard coordinate system for in vivo imaging studies. For Optical Bioluminescence Tomography (OBT) in small animals, the animal's surface topography can be reconstructed from structured light measurements, but internal anatomy is unavailable unless additional CT or MR images are acquired. We present a novel method for estimating the internal organ structure of a mouse by warping a labeled 3D volumetric mouse atlas with the constraint that the surfaces of the two should match. Surface-constrained harmonic maps used for this bijective warping are computed by minimizing the covariant harmonic energy. We demonstrate the application of this warping scheme in OBT, where scattering and absorption coefficients of tissue are functions of the internal anatomy and hence, better estimates of the organ structures can lead to a more accurate forward model resulting in improved source localization. We first estimated the subject's internal geometry using the atlas-based warping scheme. Then the mouse was tessellated and optical properties were assigned based on the estimated organ structure. Bioluminescent sources were simulated, an optical forward model was computed using a finite-element solver, and multispectral data were simulated. We evaluate the accuracy of the forward model computed using the warped atlas against that assuming a homogeneous mouse model. This is done by comparing each model against a 'true' optical forward model where the anatomy of the mouse is assumed known. We also evaluate the impact of anatomical alignment on bioluminescence source localization.

In this contribution, a novel adaptive finite element based source localization method is proposed and developed, which adopts the spectrum characteristic of bioluminescent source. In this method, multiple sets of surface measured data corresponding to the discretized spectral bands reduce the ill-posedness of BLT. Then, the adaptive mesh refinement using a posteriori error estimation helps not only to avoid the dimension disaster arisen from the multi-spectral measured data, but also to improve the BLT reconstruction. In addition, a posteriori permissible source region selection further reduces the ill-posedness of BLT and enhances the reconstruction stability and speed. Finally, the simulation verification of the proposed method is performed with the Monte Carlo (MC) based synthetic data. The accurate localization of one and two bioluminescent sources in different depths shows the effectiveness and potential of this algorithm for BLT.

Three-dimensional (3D) reconstruction of the coronary arteries offers great advantages in the diagnosis and treatment of cardiovascular diseases, compared to two-dimensional X-ray angiograms. Besides improved roadmapping, quantitative analysis of vessel lesions is possible. To perform 3D reconstruction, rotational projection data of the selectively contrast agent enhanced coronary arteries are acquired with simultaneous ECG recording. For the reconstruction of one cardiac phase, the corresponding projections are selected from the rotational sequence by nearest-neighbor ECG gating. This typically provides only 5-10 projections per cardiac phase. The severe angular undersampling leads to an ill-posed reconstruction problem. In this contribution, an iterative reconstruction method is presented which employs regularizations especially suited for the given reconstruction problem. The coronary arteries cover only a small fraction of the reconstruction volume. Therefore, we formulate the reconstruction problem as a minimization of the L₁-norm of the reconstructed image, which results in a spatially sparse object. Two additional regularization terms are introduced: a 3D vesselness prior, which is reconstructed from vesselness-filtered projection data, and a Gibbs smoothing prior. The regularizations favor the reconstruction of the desired object, while taking care not to over-constrain the reconstruction by too detailed a-priori assumptions. Simulated projection data of a coronary artery software phantom are used to evaluate the performance of the method. Human data of clinical cases are presented to show the method's potential for clinical application.

The total variation (TV) minimization algorithm for image reconstruction in few-view computed tomography is applied to image reconstruction in digital breast tomosynthesis. In the TV minimization algorithm, there is a parameter that regulates how close the estimated data should be to the actual projection data. The effect of this parameter on the reconstructed images is investigated. In addition, realistic noise is added to the simulated projection data.

A framework for image reconstruction from planar tomosynthesis trajectories (i.e. all source positions reside in a single plane) is presented. The parallel beam geometry is a convenient starting point in deriving reconstruction algorithms, both analytic and iterative, as the relation between frequency space and image space is well known. We present a method for utilizing parallel beam reconstruction algorithms in an internally consistent manner. The concept of a virtual image object is utilized. This virtual image object has the property that cone-beam projections through the real object are directly related to parallel-beam projections of the virtual object. The virtual object may then be reconstructed using any algorithm derived for parallel beam projections. Finally, an affine transform may be applied to the virtual image object in order to generate the reconstruction result. In the implementation described here the backprojection operation is performed such that the real image object is reconstructed without introducing a rebinning in image space. Image reconstruction comparisons are given for a standard filtered backprojection (FBP) type algorithm where parallel projections were assumed in the algorithm derivation. Numerical simulations were performed for a C-arm type geometry and parallel beam FBP reconstructions using the virtual object are compared with the standard backprojection algorithm. Finally, a comparison was made between the new parallel beam reconstruction and the standard approximation where the cone-beams are assumed to be approximately parallel beams and a cone-beam backprojection is employed. A reduction in streaking artifacts was observed using the new algorithm compared with the standard approximation.

Sub-pixel compounding is a technique that synthesizes the information of an image sequence to form a betterresolved and speckle reduced image. To avoid extra data acquisition time and patient exposure, reuse of the existing data is highly desired. In elasticity imaging, a set of images with slight changes due to deformation is produced, which provides an ideal input for the sub-pixel compounding process. In this paper, a brief review of the resolution enhancement techniques in ultrasound imaging will be provided, and then, a diffusion-regularized, least square approach is presented for sub-pixel compounding image reconstruction. Based on the results, we suggest that (1) B-mode images from elastic imaging are suitable data for sub-pixel compounding and a speckle noise reduced higher-resolution image is a co-product of elasticity imaging; (2) for breast diagnosis, resolution improvement is of strong interest since better depiction of the interior and exterior structures of a tumor provides important detection and diagnostic information; (3) a similar approach could be extended to elasticity imaging with other modalities.

In recent years, there has been a desire to reduce CT radiation dose to children because of their susceptibility and prolonged risk for cancer induction. Concerns arise, however, as to the impact of dose reduction on image quality and thus potentially on diagnostic accuracy. To study the dose and image quality relationship, we are developing a simulation code to calculate organ dose in pediatric CT patients. To benchmark this code, a cylindrical phantom was built to represent a pediatric torso, which allows measurements of dose distributions from its center to its periphery. Dose distributions for axial CT scans were measured on a 64-slice multidetector CT (MDCT) scanner (GE Healthcare, Chalfont St. Giles, UK). The same measurements were simulated using a Monte Carlo code (PENELOPE, Universitat de Barcelona) with the applicable CT geometry including bowtie filter. The deviations between simulated and measured dose values were generally within 5%. To our knowledge, this work is one of the first attempts to compare measured radial dose distributions on a cylindrical phantom with Monte Carlo simulated results. It provides a simple and effective method for benchmarking organ dose simulation codes and demonstrates the potential of Monte Carlo simulation for investigating the relationship between dose and image quality for pediatric CT patients.

We introduced and evaluated the techniques LightSpeed VCT uses to reduce X-ray dose and image noise in cardiac helical CT applications. These techniques include the use of much improved VCT data acquisition system (VDAS) with reduced electronic noise; cardiac bowtie that redistributes X-rays to have more signals for heart and much less flux to the peripheries; adaptive post-processing filters to reduce cardiac image noise; and ECG modulated tube currents to concentrate X-ray dose for desired cardiac phases. Phantom and patient scans were used to evaluate the dose saving and noise reduction potentials of these techniques. The results demonstrated that the improved VDAS reduced image noise 15-20% for cardiac imaging. With same scan technique, the use of cardiac bowtie reduced about 10% dose in terms of CTDIw measurement and clinical evaluation demonstrated additional 7% image noise reduction and equivalent image quality with cardiac bowtie vs. regular body bowtie. The adaptive filter generated 15-20% noise reduction while maintaining image resolution and artery sharpness. Finally, the use of ECG modulated mA method provided up to 50% dose reduction based on CTDIw measurements, but the saving potentials depended on the heart rate and cardiac phase selection. For heart rate of 60bpm and ±15% cardiac phase margin, the average dose reduction could be 30%. Since these dose and noise reduction methods are inclusive and can be combined to produce even greater dose/noise reduction. It is reasonable to believe that with VCT we maybe able to acquire cardiac helical CT images with only 30-40% of the dose of older generation 16-slice CT scanners with similar noise level and same slice thickness.

Pregnant women with shortness of breath are increasingly referred for CT Angiography to rule out Pulmonary Embolism (PE). While this exam is typically focused on the lungs, extending scan boundaries and overscan can add to the irradiated volume and have implications on fetal dose. The purpose of this work was to estimate radiation dose to the fetus when various levels of overscan were encountered. Two voxelized models of pregnant patients derived from actual patient anatomy were created based on image data. The models represent an early (< 7 weeks) and late term pregnancy (36 weeks). A previously validated Monte Carlo model of an MDCT scanner was used that takes into account physical details of the scanner. Simulated helical scans used 120 kVp, 4x5 mm beam collimation, pitch 1, and varying beam-off locations (edge of the irradiated volume) were used to represent different protocols plus overscan. Normalized dose (mGy/100mAs) was calculated for each fetus. For the early term and the late term pregnancy models, fetal dose estimates for a standard thoracic PE exam were estimated to be 0.05 and 0.3 mGy/100mAs, respectively, increasing to 9 mGy/100mAs when the beam-off location was extended to encompass the fetus. When performing PE exams to rule out PE in pregnant patients, the beam-off location may have a large effect on fetal dose, especially for late term pregnancies. Careful consideration of ending location of the x-ray beam - and not the end of image data - could result in significant reduction in radiation dose to the fetus.

Digital tomosynthesis is an imaging technique that reconstructs tomographic planes in an object from a set of projection images taken over a fixed angle¹. Preliminary results show that this technique increases the detectability of lung nodules². Current settings acquire images with approximately the same exposure as a screen-film lateral. However, due to the increased detectability of lung nodules from the removal of overlying structures, patient dose may be reduced while still maintaining increased sensitivity and specificity over conventional chest radiographs. This study describes a simulation method that provides realistic reduced dose images by adding noise to digital chest tomosynthesis images in order to simulate lower exposure settings for the purpose of dose optimization. Tomosynthesis projections of human subjects were taken at dose levels which were specified based on either patient thickness or a photo-timed digital chest radiograph acquired prior to tomosynthesis acquisition. For the purposes of this study, subtle nodules of varying size were simulated in the image for demonstration purposes before the noise simulation in order to have a known truth for nodule location and to evaluate the effect of additive noise on tumor detection. Noise was subsequently added in order to simulate 3/4, 1/2, and 1/4 of the original exposure in each projection. The projections were then processed with the MITS algorithm to produce slice images. This method will be applied to a study of dose reduction in the future using human subject cases.

Cone-Beam Computed Tomography (CBCT) enables three-dimensional imaging with isotropic resolution. X-ray scatter estimation is a big challenge for quantitative CBCT imaging: even in the presence of anti-scatter grid, the scatter level is significantly higher on cone beam systems compared to collimated fan beam systems. The effects of this scattered radiation include cupping artifacts, streaks, and quantification inaccuracies. In this paper, a scatter management process for tomographic projections, without supplementary on-line acquisition, is presented. The scattered radiation is corrected using a method based on scatter calibration through off-line acquisitions. This is combined with on-line analytical transformation based on physical equations, to perform an estimation adapted to the object observed. This approach has been previously applied to a system without anti-scatter grid. The focus of this paper is to show how to combine this approach with an anti-scatter grid. First, the interest of the grid is evaluated in terms of noise to signal ratio and scatter rejection. Then, the method of scatter correction is evaluated by testing it on an anthropomorphic phantom of thorax. The reconstructed volume of the phantom is compared to that obtained with a strongly collimated conventional multi-slice CT scanner. The new method provides results that closely agree with the conventional CT scanner, eliminating cupping artifacts and significantly improving quantification.

We present preliminary investigations that examine the feasibility of incorporating volumetric images generated using digital tomosynthesis into brachytherapy treatment planning. The Integrated Brachytherapy Unit (IBU) at our facility consists of an L-arm, C-arm isocentric motion system with an x-ray tube and fluoroscopic imager attached. Clinically, this unit is used to generate oblique, anterior-posterior, and lateral images for simple treatment planning and dose prescriptions. Oncologists would strongly prefer to have volumetric data to better determine three dimensional dose distributions (dose-volume histograms) to the target area and organs at risk. Moving the patient back and forth to CT causes undo stress on the patient, allows extensive motion of organs and treatment applicators, and adds additional time to patient treatment. We propose to use the IBU imaging system with digital tomosynthesis to generate volumetric patient data, which can be used for improving treatment planning and overall reducing treatment time. Initial image data sets will be acquired over a limited arc of a human-like phantom composed of real bones and tissue equivalent material. A brachytherapy applicator will be incorporated into one of the phantoms for visualization purposes. Digital tomosynthesis will be used to generate a volumetric image of this phantom setup. This volumetric image set will be visually inspected to determine the feasibility of future incorporation of these types of images into brachytherapy treatment planning. We conclude that initial images using the tomosynthesis reconstruction technique show much promise and bode well for future work.

We present a new simulation environment for X-ray computed tomography, called CatSim. CatSim provides a research platform for GE researchers and collaborators to explore new reconstruction algorithms, CT architectures, and X-ray source or detector technologies. The main requirements for this simulator are accurate physics modeling, low computation times, and geometrical flexibility. CatSim allows simulating complex analytic phantoms, such as the FORBILD phantoms, including boxes, ellipsoids, elliptical cylinders, cones, and cut planes. CatSim incorporates polychromaticity, realistic quantum and electronic noise models, finite focal spot size and shape, finite detector cell size, detector cross-talk, detector lag or afterglow, bowtie filtration, finite detector efficiency, non-linear partial volume, scatter (variance-reduced Monte Carlo), and absorbed dose. We present an overview of CatSim along with a number of validation experiments.

In this study, we propose a novel approach to dual-energy contrast-enhanced digital mammography, with the development of a dual-energy recombination algorithm based on an image chain model and the determination of the associated optimal low and high-energy techniques. Our method produces clutter-free iodine-equivalent images and includes thickness correction near the breast border. After the algorithm description, the optimal low and high-energy acquisition techniques are determined to obtain a compromise between image quality and glandular dose. The low and high-energy techniques were chosen to minimize the glandular dose for a target Signal Difference to Noise Ratio (SDNR) in the dual-energy recombined image. The theoretical derivation of the iodine SDNR in the recombined image allowed the prediction of the optimal low and high-energy techniques. Depending on the breast thickness and glandular percentage, the optimal low-energy kVp and mAs ranged from 24kVp (Mo/Mo or Mo/Rh) to 35kVp (Rh/Rh), and from 60 to 90mAs respectively, and the high-energy kVp and mAs ranged from 40kVp to 47kVp (Mo/Cu), and from 80mAs to 290mAs. We proved the better performance of our algorithm compared to the classic weighted logarithmic subtraction method in terms of patient dose and also in terms of texture cancelation, through the use of artificial textured images. Values of iodine contrast measured on phantom were close to the expected iodine thickness. Good correlation was found between the measured and theoretical iodine SDNR in the dual-energy images, which validates our theoretical optimization of the acquisition techniques.

Aim of this work is to compare the performances of a Xoran Technologies i-CAT Cone Beam CT for dental applications with those of a standard total body multislice CT (Toshiba Aquilion 64 multislice) used for dental examinations. Image quality and doses to patients have been compared for the three main i-CAT protocols, the Toshiba standard protocol and a Toshiba modified protocol. Images of two phantoms have been acquired: a standard CT quality control phantom and an Alderson Rando^® anthropomorphic phantom. Image noise, Signal to Noise Ratio (SNR), Contrast to Noise Ratio (CNR) and geometric accuracy have been considered. Clinical image quality was assessed. Effective dose and doses to main head and neck organs were evaluated by means of thermo-luminescent dosimeters (TLD-100) placed in the anthropomorphic phantom. A Quality Index (QI), defined as the ratio of squared CNR to effective dose, has been evaluated. The evaluated effective doses range from 0.06 mSv (i-CAT 10 s protocol) to 2.37 mSv (Toshiba standard protocol). The Toshiba modified protocol (halved tube current, higher pitch value) imparts lower effective dose (0.99 mSv). The conventional CT device provides lower image noise and better SNR, but clinical effectiveness similar to that of dedicated dental CT (comparable CNR and clinical judgment). Consequently, QI values are much higher for this second CT scanner. No geometric distortion has been observed with both devices. As a conclusion, dental volumetric CT supplies adequate image quality to clinical purposes, at doses that are really lower than those imparted by a conventional CT device.

Scattering in CT is a major process that may result in severe image artifacts. In order to suppress this scattering, most CT scanners are equipped with post collimation anti-scatter grid along the fan beam direction. In the longitudinal direction (z-direction) no hardware solution has been implemented since the scattering is negligible in narrow coverage CT scanners. As the coverage becomes wider in recent MSCT scanners the scattering level in the z-direction increases significantly. This scattering increase in z-direction, results in image artifacts appearing as dark shadows along highly attenuating directions. In the present work we measure the scattering level in the z-direction for a wide coverage scanner, using various phantoms. Based on the results, a semi-empirical model for scattering correction in MSCT is presented and validated. The proposed model is based on a subtraction of a low frequency offset. This offset is proportional to the scattering, corresponding to the detector that has the lowest signal at each rotation angle. To validate the model, we first calculate this low frequency offset directly from the scatter measurements and apply it to the data acquired with the wide coverage. We then use a semi-empirical function to estimate the scattering fraction from the raw data, and use it to replace the directly measured scatter values in the correction scheme. Applying the proposed semi-empirical scatter correction model to the data acquired with the wide coverage, the scattering signal is significantly decreased. The images reconstructed from the corrected data exhibit a clear reduction of the artifact level.

In multi-slice CT (MSCT), as the coverage becomes wider, the scattering contribution along the longitudinal direction (z) to the detectors' signal increases. The scattering results in image artifacts, appearing as dark shadows between highly attenuating objects. In this work we measure the scattering level systematically, using phantoms of various sizes, shapes, and materials. We study the dependencies and their effect on the scattering amount. We derive an empirical function for the scattering fraction, based on the maximal attenuation at each rotation angle. The function contains a single constant (SC). The variation of SC as a function of the different phantoms is analyzed, showing a clear dependence on the minimal water equivalent axis of each phantom. The strong dependence of the scattering fraction on the maximal attenuation along each view is shown. This phenomenon can be correlated to a single scatter process along the z axis in the presence of an anti-scatter grid along the direction of the detectors. The dependence of SC on the minimal axis indicates an additional significant scatter process. The results validate that the scattering level estimation can be achieved using the derived function, with a minimal variation in the solution parameters. Hence, enabling the introduction of this scatter estimation into an MSCT scattering correction scheme.

Anti-scatter grids are well established in the field of X-ray projection imaging. In general these grids consist of a large number of parallel lead lamellae separated by X-ray-transparent material. This regular structure defines the characteristic grid frequency. Modern X-ray imaging systems apply digital receptors, i.e. image intensifiers coupled to a CCD camera or solid state flat-panel detector. Combining a digital detector and an anti-scatter grid may lead to Moire artifacts. This results from sampling an analog X-ray image with signal components higher than half the sampling frequency. Especially in high dose DSA images (Digital Subtraction Angiography) these irritating artifacts may be visible to the user. In this paper we present a concept for minimizing these grid artifacts: Signal propagation in the detector is modeled by three steps, scintillator MTF, aperture MTF, and sampling. Since the scintillator MTF is irrelevant for the grid optimization process, we focus on aperture MTF and sampling. From the given geometry of the detector elements the corresponding 2D Fourier transform is calculated. An evaluation for typical grid frequencies, i.e. arcs around the origin of the 2D Fourier transform, results in profiles exhibiting pronounced minima. From the respective angle values for these minima, grid orientation can be optimized for minimum Moire disturbances. Simulation results for typical detector pixel geometries and for grid frequencies used in practice are validated by measurement for two different anti-scatter grids on a Siemens angiographic system with a digital flat-panel detector.

The underlying mechanism in projection radiography as well as in computed tomography (CT) is the accumulative attenuation of a pencil x-ray beam along a straight line. However, when a portion of photons is deviated from their original path by scattering, it is not valid to assume that these photons are the survival photons along the lines connecting the x-ray source and the individual locations where they are detected. Since these photons do not carry the correct spatial information, the final image is contaminated. Researchers are seeking techniques to reduce scattering, and hence, improve image quality, by scatter compensation. Previously, we presented a post-acquisition scatter compensation technique based on an underlying statistical model. We used the Poisson noise model, which assumed that the signals in the detector individually followed the Poisson process. Since most x-ray detectors are energy integrating rather than photon counting, the Poisson noise model can be improved by taking this property into account. In this study, we developed a Gaussian noise model by the matching-of-the-first-two-moments method. The Maximum Likelihood Estimator of the scatter-free image was derived via the expectation maximization (EM) technique. The maximum a posteriori estimate was also calculated. The Gaussian noise model was preliminarily evaluated on a full-field digital mammography system.

With high-speed multislice helical CT, the time needed to select the optimal cardiac phase accounts for a large percentage of the coronary CT angiography examination time because the scan time is short. To reduce the phase selection time, we have developed an automatic cardiac phase selection algorithm and implemented it in the Aquilion 64 scanner. This algorithm calculates the absolute sum of the differences between two raw data sets for subsequent cardiac phases (e.g., 4% and 0%) and generates a velocity curve representing the magnitude of cardiac motion velocity for the entire heart volume. Normally, the velocity curve has two local minimum slow-motion phases corresponding to end-systole and mid-diastole. By applying these local minimum phases in reconstruction, stationary cardiac images can be reconstructed automatically. In this report, the algorithm for generating the velocity curve and the processing time for selecting the optimal cardiac phase are discussed. The accuracy of this method is compared with that of the conventional manual method. In the manual method, a sample plane containing all four cardiac chambers was selected, reconstruction was performed for all phases at 2% intervals, and images were visually evaluated. Optimal phase selection required about 5 min/exam. With automatic phase selection, optimal phase selection required only about 1 min/exam, and the cardiac phases were close to those selected using the manual method. Automatic phase selection substantially reduces the time needed to select the optimal phase and increases patient throughput. Moreover, the influence of operator skill in selecting the optimal phase is minimized.

Multi-detector row computed tomography (MDCT) allows high-edge-response imaging in z-direction by thinner slice compared with the conventional single-detector row CT (SDCT). The edge response in x- and y-direction is, however, basically depending on the reconstruction function or the image processing. In this work, a method of enhancing the edge response in x- and y-direction, without special image processing, is proposed. In this method, projection data of a patient is acquired by high-spatial-frequency sampling, and an edge-enhanced image is reconstructed with same matrix size as a conventional image by averaging the projection data. Edge-enhancement effect in this method employs nonlinearlity of the projection data, and special image processing is not required. In order to verify this proposed method, a large water phantom that consists of five resinous rods and a small one that the similitude rate is 1/2, and is topologically the same as the former large phantom, were scanned, and high-spatial-frequency sampling was simulated. After that, reconstructed images were obtained by averaging the high-spatial-frequency sampled data and edge gradients for some rods were obtained and estimated. As a result, although image noise increased slightly, edge gradients are improved 25 to 97 % without special image processing.

The purpose of this study is to determine a stable sampling rate not to be affected by sampling shift for reducing radiation exposure with time sampling and interpolation in cerebral perfusion CT examination. Original images were obtained every 1 second for 40 time series from 3 patients, respectively. Time sampling was performed with sampling intervals (SI) from 2 to 10 seconds. Sampling shift was applied from +1 to SI-1 for each sampling rate. For each patient, 30 tissue concentration time-course data were collected, and arterial input curves were fitted by gamma-variate function. The sinc function was introduced for interpolation. Deconvolution analysis based on SVD was performed for quantifying perfusion parameters. The perfusion values through time-varying sampling and interpolation were statistically compared with the original perfusion values. The mean CBF values with increase of sampling interval and shift magnitude from the collected data had a wider fluctuation pattern centering around the original mean CBF. The mean CBV values had a similar tendency to the mean CBF values, but a relatively narrower deviation. The mean MTT values were fluctuated reversely to the trend of the mean CBF values. The stable sampling interval for quantifying perfusion parameters with lower radiation exposure was statistically acceptable up to 4 seconds. These results indicate that sampling shift limits sampling rate for acquiring acceptable perfusion values. This study will help in selecting more reasonable sampling rate for low-radiation-dose CT examination.

We have analyzed 144 ECG wave-forms that were taken during cardiac CT exams to determine in retrospect the optimized timing for updating the gantry rotation-time. A score was defined, according to the number of heart beats during X-ray on, which fulfill the temporal resolution (tR)condition, tR<100mSec. The temporal resolution calculation was based on dual-cycle π/2 sector segmentation, where the data required for any image is collected during two heart cycle. The results yield a significant improvement of the tR score with the rotation-time update method relative to using a fixed minimal rotation-time of the gantry. The analysis suggest that full heart scan with better than 100mSec temporal resolution per slice can routinely be achieved in 128 slices MSCT scanner by performing gantry rotation-time -update after patient starts its breath hold. At these conditions the required breath-hold time is expected to be less than 15 seconds.

We describe a novel system named TRVCT (Translating and Rotating Volume Computed Tomography), developed for computed tomography image from large object with simple method and low price. Tomogram images can be acquired when the object is translating and rotating simultaneously with vertical linear array detector. This method is different from the normal X-ray CT completely. We used fan-beam X-ray, and the direction of the detector and rotating axis are in parallel. Because a hundred or thousand tomograms with Z-axis from just one scanning, it has excellent Z-axis resolution and has an advantage that can improve the resolution in X-Y plane with changing translating speed and frequency of data acquisition. There is no ring artifact that is generated frequently in the third generation CT scanner. So, we can have high resolution tomograms from this TRVCT system. The TRVCT can be used to acquire images for large object like tire, engine, or whole car, and it can remove the scattering from X-ray for high resolution images.

We present the results of dose and image quality performance evaluation of a novel, prospective ECG-gated Coronary CT Angiography acquisition mode (SnapShot Pulse, LightSpeed VCT-XT scanner, GE Healthcare, Waukesha, WI), and compare it to conventional retrospective ECG gated helical acquisition in clinical and phantom studies. Image quality phantoms were used to measure noise, slice sensitivity profile, in-plane resolution, low contrast detectability and dose, using the two acquisition modes. Clinical image quality and diagnostic confidence were evaluated in a study of 31 patients scanned with the two acquisition modes. Radiation dose reduction in clinical practice was evaluated by tracking 120 consecutive patients scanned with the prospectively gated scan mode. In the phantom measurements, the prospectively gated mode resulted in equivalent or better image quality measures at dose reductions of up to 89% compared to non-ECG modulated conventional helical scans. In the clinical study, image quality was rated excellent by expert radiologist reviewing the cases, with pathology being identical using the two acquisition modes. The average dose to patients in the clinical practice study was 5.6 mSv, representing 50% reduction compared to a similar patient population scanned with the conventional helical mode.