Biomagnetic imaging is the estimation and display of current density distributions calculated from measurements of the magnetic fields they generate. The magnetic fields measurements alone, no matter how dense and extensive, are not sufficient for uniquely reconstructing the current densities. In this paper we concentrate on quantifying the limitations and on introducing constraints into the reconstruction techniques. We apply the technique of characteristic bodies as transplanted from the microwave and radar theory. This technique allows for simple calculations when introducing a particular class of constraints, i.e., limiting sources to surfaces. In particular, we look at surface current and surface monopole distributions that generate magnetic fields corresponding to the measured values. We also show how magnetic fields known on arbitrary closed surfaces may be projected to other surfaces and then the characteristic body technique used to calculate source distributions. In addition we consider differential and support constraints.

We describe here an implementation of a pulse sequence in which one of the normally fixed scan parameters is varied through the scan. The image contrast that is normally obtained with long sequence repetition time TR, and consequently long scanning times, can be achieved with relatively shorter scan times by systematically varying TR during the data acquisition. By employing the longest TR only while collecting the data for the lowest spatial frequencies and using the shortest available TR elsewhere a flexible trade-off between image features (contrast and resolution) and scan time is possible. Examples of 2-D and 3-D FT MR images at low and middle field strength are presented with sequence details and some comparison scans of similar scan times.

We describe the implementation of a field-cycling electromagnet in a whole-body clinical MRI system. A pulsed magnet is driven on during dead times of the NMR sequence and serves to increase the equilibrium magnetization induced in the patient. Signal transmission and reception occurs only during the period when the electromagnet is turned off and does not otherwise influence the data collection. Applications of the technology include image quality enhancement for low field MRI and for in vivo localized relaxation rate measurements.

The biomagnetic inverse problem is severely underdetermined. Even with large arrays (100 or more elements) of detectors we cannot measure the entire external biomagnetic field; instead we measure the magnetic field at a finite number of points on some surface. In order to determine the Nyquist sampling interval we must consider a large number of factors including the size of the source, the distance from the source to the pick-up coil, the size of the pick-up coil, the order of the gradiometer, and the baseline of the gradiometer. In this paper, we discuss the effects of the pick-up coil size and of the axial gradiometer on the Nyquist sampling interval. Our goal is to determine sampling rates for specific sources measured with circular pickup coils of various diameters. As an example of the effects of undersampling the magnetic field, we present reconstructed current densities from over- and undersampled data.

Inverse scattering is used to solve the general problem of finding an applied rf field to produce a desired selective excitation profile for MRI. The formalism is used to calculate the exact, closed form solutions to the waveforms which produce a collection of slice profiles, the limit of which is a rectangular section. These solutions are effective at producing waveforms for excitation, refocusing, or inversion pulses. The incorporation of bound states in the inverse scattering problem allow for the calculation of excitation pulses with phase control.

The chest PA examination is one of the single most performed studies in radiology today. It can provide a wealth of information in a single examination. As in many other areas of radiology there is a conflict between high contrast, which enables subtle structures to be visualized, and wide latitude, which allows all areas of interest in the chest to be displayed in a single image. In order to optimize the design of receptor systems it is useful to establish and understand the latitude required for thoracic imaging. We have measured the distributions of x-ray transmittance within the lungs, heart, and abdomen for a population of 868 out-patients. The measurements were made with a resolution element approximately 2 X 2 cm, at a single x-ray beam quality, and with a low-scatter slot-beam geometry. Under these conditions, the required receptor latitude for capturing each area of interest in the thorax is derived as a function of body habitus. To capture all three regions the required receptor latitude for the PA examination varies from 11:1 to 81:1 with increasing patient size. The implications of these results for thoracic image-receptor design is discussed.

While the design of commercial film screen systems often arrives at optimal or near optimal signal-to-noise performance, the process for achieving these optima is often arduous and expensive. Moreover, while the signal-to-noise ratio is optimized, it is often difficult to predict the optimum signal level for a given class of diagnoses. We have devised a digital system which, by performing calculations on digitally stored `baseline' phantom images recorded by a low speed x ray exposure on a wide dynamic range x-ray film, allows us to optimize system performance and to specify the design of film/screen systems for different classes of diagnostic images before screens and films are actually coated. We describe the application of this model to the development of an optimized system for chest radiology.

We have designed and built a dual-energy x-ray absorptiometry (DXA) instrument for the measurement of bone density in-vivo using a high-purity germanium detector array. This system uses a fan beam geometry and a strip of 24 2 mm wide detectors to function as a scanned projection quantitative imaging system. The use of photon-counting detectors with a k-edge filtered x-ray spectrum avoids spectral overlap in the dual-energy data, and optimizes dose efficiency. The multielement detector is energy-sensitive and capable of high counting rates (106 counts per second per element). Dual-energy data is acquired in a single short exposure, enabling high-speed scanning. We have examined the temporal stability of the system, and developed strategies for controlling against thermally induced drift. Counting rate nonlinearity due to dead-time losses was modelled, and a correction for this phenomenon was implemented. We measured scattered radiation and investigated scatter removal schemes. The spatial resolution of the system was evaluated, and the entrance skin exposure to the patient was measured using thermoluminescent dosimeters (TLDs). A noise reduction algorithm was incorporated into the scanner which exploits the correlation of the pixel noise in the basis material images, derived from dual-energy data, to improve signal-to-noise without compromising edge sharpness or quantitative accuracy. This instrument shows promise as a research tool for the investigation on new scanning methodologies in bone-mineral densitometry.

Since the design and fabrication of the first pixelated, two-dimensional, hydrogenated amorphous silicon image sensor arrays at Xerox, PARC, in 1988, a variety of milestones have been achieved including the first demonstration of high quality radiographic images of low- contrast, anatomical detail. Current array configurations and design rules offer the prospect of 100 micrometers pixel pitches over 30 by 30 cm2 areas in the next few years. Beyond this, present attempts to extend the size of the substrates to 100 cm on the diagonal by 1996 coupled with the possibility of three-dimensional thin-film electronics could eventually result in a revolution in many forms of x-ray imaging. Such arrays will present challenges in the design of the fast, analog, and digital electronic readout systems required to precisely match the characteristics of the arrays to those of the imaging needs. For such arrays, one of the most important parameters is the dynamic range. Early results are reported for the measured limits on this quantity as obtained through measurements from individual sensors and FETs as well as an improved lower limit as obtained by direct measurements of array pixels.

Solid state x-ray detectors produce high resolution medical images when scanning the patient with a narrow x-ray beam. The x-ray detection is achieved by a silicon photodiode array coated by a scintillating material. Signal is multiplexed and converted into a 12-bit coded digital signal. This technology provides several key advantages for imaging systems: very wide image dynamic range, high resolution, good linearity, no geometrical distortion. The output signal is digital and can be directly acquired by digital processing systems. Major performance in medical imaging applications is given. They fit the requirements of medical systems in several applications fields. The application in panoramic dental x-ray imaging is briefly described.

Dual-energy subtraction imaging techniques allow the tissue and bone structures in the patient to be imaged separately, thus removing some obscurity resulted from the overlapping of the two structures. Furthermore, they provide the potential for the tissue or bone contents to be quantified for diagnostic use. Thus, capabilities for dual-energy subtraction imaging are often incorporated with new digital radiography techniques. There are three different schemes for implementing dual-energy subtraction imaging techniques. Among them, dual-kVp and sandwich detector approaches are two most often used schemes. A third scheme is the single kVp-dual filter approach which allows a more flexible control of the spectra while avoiding kVp switching. It is suitable for digital radiography techniques using two linear detector arrays. In this paper, the signal-to-noise properties of these three schemes is computed for various combinations of kVp, filters and patient thicknesses (tissue and bone). Based on the signal-to-noise analysis, they are compared to each other for the efficiency of x ray usage, dose efficiency, and accuracy for background subtraction and thickness measurement.

2K (2048 X 2500) or 1K (1024 X 1250) digitized chest film images can be generated by either direct digitization or converting a 4K (4096 X 5000) digitized film image by pixel averaging. In this paper, these two methods are compared for their implication on the resolution properties of the resulting images. A film image of the lead bar resolution pattern was used as the source of all digital images. Signal profiles of the bar pattern were studied to compare the pixel averaging and direct digitization methods. Based on this comparison, it was found that pixel averaging, when used with proper filtering, can be used to simulate direct digitization using larger (210/420) apertures and result in similar square wave response. Pixel averaging, however, can result in better square wave response when 2 to 1 or 4 to 1 straight averaging is used or a sharper kernel is used in pre-filtering.

A large area self-scanned solid-state detector is being developed for digital radiology. It consists of an x-ray sensitive flat-panel employing amorphous selenium ((alpha) )-Se) as the x- ray transducer and active matrix integrated circuit for readout. In principle such detectors could be used for all the currently applied radiological modalities -- radiography, photofluorography, and fluoroscopy. Layers of (alpha) )-Se up to 500 micrometers thick are readout with an array of thin film field effect transistors. The whole structure is integrated onto a glass plate. For all practical purposes the resolution of the system is dictated by the pixel size and readout could be in real-time (i.e., 30 frames/sec).

The imaging characteristics of two chest radiographic equipment, Advanced Multiple Beam Equalization Radiography (AMBER) and Konica Direct Digitizer [using a storage phosphor (SP) plate] systems have been compared. The variables affecting image quality and the computer display/reading systems used are detailed. Utilizing specially designed wedge, geometric, and anthropomorphic phantoms, studies were conducted on: exposure and energy response of detectors; nodule detectability; different exposure techniques; various look- up tables (LUTs), gray scale displays and laser printers. Methods for scatter estimation and reduction were investigated. It is concluded that AMBER with screen-film and equalization techniques provides better nodule detectability than SP plates. However, SP plates have other advantages such as flexibility in the selection of exposure techniques, image processing features, and excellent sensitivity when combined with optimum reader operating modes. The equalization feature of AMBER provides better nodule detectability under the denser regions of the chest. Results of diagnostic accuracy are demonstrated with nodule detectability plots and analysis of images obtained with phantoms.

This paper addresses three problems in storage phosphor imaging: natural fading of latent image signals, proper erasure of the exposed plates, and re-scanning for a second readout. Signal, and signal-to-noise ratios were measured as a function of time, erasure power/time, or number of pre-scans to study these problems. The latent image signals were found to decay very rapidly during the first several minutes and stabilize after several hours. The fading effect results in a variable signal gain (signal per unit exposure) which may affect the system calibration and quantitative use of the image data. Complete erasure of the latent image signals is necessary to ensure that no residual image signals are present when the plate is exposed again. It was found that plates used in high exposure applications (GI, therapeutic imaging) may require an excessively long erasure time to prepare them for use in low exposure applications (chest imaging). Although the latent image is partially erased during the readout process, it may sometimes become necessary to re-scan the plate for a second or third readout. It was found that because a large number of energy traps are generated for each x-ray photon, a significant portion of the x-ray information remains intact for reuse after the first or second scans. Measurement of the signals and signal-to-noise ratios are presented to demonstrate and discuss the aforementioned problems or effects.

Proper scatter correction is vital to qualitative contrast measurement in digital subtraction angiography (DSA) and dual-energy subtraction processing in digital chest radiography using storage phosphor plates. Such correction requires an accurate estimation of the scatter distribution in the image field. In this paper, a novel method, referred to as the primary- modulation-demodulation (PMD) method, is introduced. With the PMD method, the primary x ray distribution is modulated and demodulated with two filters placed on the tube and detector sides of the patient. The modulation-demodulation process, while leaving the overall primary signal distribution unchanged, results in a reduction of scatter signals in selected regions in the image. This signal drop can be measured and used to estimate and construct the scatter distribution for use in image correction. Because the PMD method allows both primary and scatter signals to be acquired simultaneously, it is ideal for use in non-scanning DSA or digital storage phosphor imaging for dual-energy subtraction imaging and/or quantitative contrast measurement. In this paper, the principle and implementation of the PMD method is described. Examples of scatter measurement using this method are shown and compared to those obtained with the beam stop method.

One difficulty in radiation therapy is ensuring the correct placement of the radiation field so that the radiation is delivered to the diseased regions while healthy tissues are spared. Currently, field placement is assessed by producing a transmission radiograph (portal image) using the high-energy treatment beam. Often the quality of these images is poor. One factor which influences image quality is the size of the x-ray source found in medical linear accelerators. These sources can be large and thus reduce the spatial resolution of the portal images, thereby reducing our ability to detect bony anatomical landmarks. The high energy of the x rays generated in these accelerators make it impossible to measure the x-ray source using standard techniques (e.g., pin-hole camera or star-patterns). We have developed a reconstruction technique which allows quantitative measurement of these sources. Using this technique, we have investigated and compared the size and shape of x-ray sources on a total of seven accelerators. These include: (1) two Varian Clinac 2100cs, (2) two Atomic Energy of Canada Ltd. (AECL) Therac 6s, (3) a Varian Clinac 600c, (4) an AECL Therac 25, and (5) a Therac 20. The comparisons also include monitoring the size and shape of the sources over a two year period. For each of these measured sources the source MTF has been calculated at typical magnifications. It has been found that (1) the size and shape of the source spot varies greatly between accelerators of different design; (2) for accelerators of the same design, however, the source spots were similar; and (3) the spatial resolution of currently available on-line verification imaging systems is only marginally reduced by the size and shape these source spots.

One problem in radiation therapy is ensuring accurate positioning of the patient so that the prescribed dose is delivered to the diseased regions while healthy tissues are spared. Positioning is usually assessed by exposing film to the high-energy treatment beam. Unfortunately, these films exhibit poor image quality (primarily due to low subject contrast) and the development delays make film impractical to check patient positioning routinely. Therefore, we have been developing a digital video-based imaging system to replace film. The system consists of a copper plate/fluorescent screen detector, a 45 degree(s) mirror, and a TV camera equipped with a large aperture lens. We have determined the signal and noise transfer properties of the imaging system by measuring its MTF(f) and NPS(f) and used these valued to estimate the optimal magnification for the imaging system. We have found that the optimal magnification is 2.3 - 2.5 when optimizing signal transfer (spatial resolution) alone; however, the optimal magnification is only 1.5 - 2.0 if SNR transfer is considered.

A GE 8800 CT system has been modified into a prototype of a volume tomographic angiography imager. The system consists of an x-ray tube and an image intensifier coupled to a TV camera. The source and detector can be rotated over 360 degrees. To explore the imaging performance of the system for reconstructing a three-dimensional (3-D) vascular structure, a set of nonsubtraction projections of a vascular phantom, acquired over 25 projection angles, were digitized. These data were reconstructed using an iterative algorithm specially designed for 3-D vascular structures. The results indicate that the system can offer adequate signal-to-noise ratio (SNR) for direct 3-D vascular reconstruction when only a few projections are used without subtraction procedure, assuming intra-arterial injection of contrast.

We interface a Toshiba LX-40A radiation therapy simulator with a custom designed data acquisition system and a 386-PC. The LX-40A includes a 14 inch (35.6 cm) x-ray image intensifier and a 1 inch saticon camera. Two-dimensional cone beam projections of an anthropomorphic chest/lung phantom are collected. The Feldkamp algorithm reconstructs the volume data set into a volume image of the phantom. We discuss several volume computed tomography issues including spatial distortions in the x-ray image intensifier, scatter, and veiling glare. We compare the volume computed tomography results with two-dimensional CT imaging of the same phantom scanned on the equipment.

A conventional gamma camera is used for the external imaging of bremsstrahlung generated from pure beta-emitters such as phosphorous-32 (32P). Tomographic images of a cylindrical phantom filled with water and containing four cylindrical sources of varying diameter are recorded using two collimators with symmetrical aperture configuration but different bore-lengths. The resolution of the system is comparable to single photon emitters for both collimaters; FWHM approximately 1.8 cm and FWTM approximately 2.9 cm. An effective linear attenuation coefficient of 0.14 cm-1 for 32P, calculated from isolated spherical sources in water, is used with the post-reconstruction Chang algorithm to correct the tomographic images. The use of a broad energy window and the symmetric apertures of the collimators yields an approximately radially symmetric, shift invariant, and stationary point-spread-function with distance from the collimator face as required for the use of image restoration filters. A new filter is proposed which shares the advantages of both neural network for deconvolution and advanced nonlinear filtering for noise removal and edge enhancement. The new filter compares favorably with the Wiener for image restoration and improves the conditions for quantitative measurements with the gamma camera. In addition, its application for image restoration does not require the knowledge of the object and noise power spectra and the serious problems (ring effects and noise overriding) associated with the inverse operation encountered in the Wiener filter are avoided.

When photon scattering around a point source in a scatter medium is considered in isolation, the distribution of the scattering sites is radially symmetric and exponentially decreasing in frequency. Discontinuities in the scattering medium cause a disruption in this radial symmetry. However, when the limited intrinsic energy resolution of the gamma camera, the energy acceptance window and the collimator response are included in the consideration, the distribution of the scattering sites of photons accepted by the gamma camera is complex. Monte Carlo techniques (EGS4) have been used to determine the spatial distribution of the scatter sites. The results of the simulation demonstrate a prominent peak in scattering sites in front of the source with a rapid decrease in all other directions. The contribution of completely backscattered photons to a point spread function is virtually negligible. Also, a clear discontinuity in the scatter site distribution is observed at the boundary of two scatter media. The first order scatter spatial distribution has also been modelled using analytical techniques. The analytical results are in good agreement with the Monte Carlo simulation. Direct experimental verification of the results of the two simulation techniques is not possible, however, there is agreement between secondary results with experimental data, hence the analysis is valid by implication. The knowledge of the scattering site distribution is anticipated to assist in a more accurate method of image reconstruction in SPECT imaging.

Computed radiography (CR) utilizing imaging plates (IPs), which consist of a photostimulable BaFX:Eu2+(X equals Cl,Br) phosphor, has found wide clinical use. In the present paper, a photostimulable SrFBr:Eu2+ (SFB) phosphor, as well as an IP employing this phosphor (SFB-IP), are experimentally prepared. A test apparatus for reading the SFB-IP is constructed, and the imaging characteristics are evaluated. The x ray imaging characteristics of the SFB-IP are different from those of the BFB-IP (the IP which consists of BaFBr:Eu2+). The difference is due to their x-ray energy absorption coefficients. Under the mammographic exposure conditions, the measurements of noise equivalent quanta as well as the visual evaluation of phantom images reveal that the SFB-IP and BFB-IP have the same quality level of imaging in spite of the PSL intensity of SFB being lower than that of BFB. However, for practical use, many problems are left on the SFB-IP and its reading apparatus.

Real-time 3-D ultrasonic imaging is not practical using conventional steered focused beam techniques. However, defocused transmitted pulses, combined with an image reconstruction approach related to computed tomography, make high quality real-time 3-D ultrasonic imaging feasible. `Three-dimensional Real Time Ultrasonic Imaging using Ellipsoidal Backprojection' (F. Anderson, SPIE's Medical Imaging V, 1991) contained point images produced with 32 receivers and 16 transmitters (32 X 16). This paper shows images produced by much larger arrays (e.g., 256 X 256), and resulting lower sidelobe levels.

The most important process for pattern analysis in medical thermography is the accurate comparison of temperature distribution over various parts of the subject's body surface, particularly the comparison of an abnormal region with its symmetrical normal region. Conventional infrared thermography, however, displays the temperature distribution over a curved body surface as a plane image viewed along the optical axis of the thermocamera, accordingly plane-scanning thermography has several crucial problems in its medical application. The authors' cooperative effort has led to the development of the following new thermographic imaging techniques, capable of simultaneous display of thermograms over various aspects of the target body in a CRT frame. (1) Triple-aspect thermography allows simultaneous display of the thermograms on three aspects of the subject by reflecting the infrared emission from the bilateral surface of the subject into the thermocamera. (2) Panoramic thermography allows the capture of a continuously developed thermogram even over the whole body surface with the thermocamera repeatedly scanning along the rotary axis of the rotating subject. (3) Multi-aspect thermography allows simultaneous display of multiple thermograms on various selected aspects of the subject by taking the real-time thermograms of the rotating subject at every selected angle of rotation as circumstances require, employing a high response thermocamera and a multi-image processor. These new enhanced thermography techniques provide various unique characteristics, compensate for the weak points of conventional plane-scanning thermography, and enable accurate analysis and error-free diagnosis for the clinician working in the field of medical thermography.

In the search for better ftuoroscopic imaging, the TV camera tube performance has become more critical. Traditional camera tube test methods, which expose tubes to extreme conditions, have only marginal correlation to typical fluoroscopic usage. This paper compares these traditional measurements with test measurements made on a modern fluoroscopic digital system for two different types of TV camera tubes. The results obtained show how the x-ray system influences the importance of the TV camera tube parameters and which tube features are most relevant.

X-ray mammography is one of the most demanding radiological techniques, simultaneously requiring excellent image quality and low dose to the breast. In current mammographic practice, both image quality and dose are found to vary over a wide range of values. Previous attempts to define the optimum operating parameters for mammography systems have been limited due to the lack of realistic attenuation coefficients and absorbed dose data. These data are now available, and have been incorporated into an energy transport model which describes the image acquisition process. The model includes measured x-ray spectra and considers beam filtration, breast thickness and composition, lesion size and composition, scatter, grid transmission, and the production and propagation of light in a phosphor-based image receptor. The applied kilovoltage for molybdenum and tungsten target x-ray sources with various spectral filters and average breast composition (50% adipose, 50% fibroglandular) has been optimized with respect to signal-to-noise ratio and absorbed dose and was found to vary between 19 and 29 kVp as breast thickness increased from 4 to 8 cm. Preliminary results for various breast compositions and lesions, and experimental verification of the model are presented. The model may be extended to include either mammographic film or new detector designs for digital mammography.