The development and initial characterization of an active matrix, flat-panel imager (AMFPI) incorporating a newly designed, indirect-detection array is reported. The array has a 127 micrometers pitch, a 1536 X 1920 pixel format, and incorporates a pixel design comprising a discrete a-Si:H photodiode coupled to an a-Si:H thin-film transistor. The array represents an aggressive redesign of a previously reported array having the same pitch and format. In particular, this new array was designed with the dual goals of maximizing the optical fill factor so as to enhance sensitivity as well as minimizing the data line capacitance so as to reduce additive electronic noise. Although constrained by the sue of discrete photodiodes, the new design nevertheless successfully achieves a fill factor of approximately 56 percent along with a data line capacitance of approximately 50 pF which are significant improvements over the previous design. In this paper, considerations in the design of such arrays are reviewed and performance results of the AMFPI, based on initial empirical results and theoretical considerations, are presented. Finally, possible trends in the future development of indirect and direct detection AMFPIs are described.

This paper introduces a new high-resolution amorphous silicon (a-Si) image sensor specifically configured to demonstrate medical x-ray imaging capabilities. This new imager has an active pixel area of 243.84 mm X 195.072 mm, a pixel size of 127 micrometers X 127 micrometers , and an array size of 1536 data lines by 1920 gate lines, with over 2.95 million pixels. We have introduced a number of improvements both to the a-Si array itself and to the surrounding electronics to obtain a light sensitive area of 57 percent and a dataline capacitance of approximately 55 pF, which together provide enhanced signal/noise ratio. We have incorporated charge-sensitive amplifiers and 12 bit A/D converters, enabling us to achieve a sufficiently large dynamic range for medical imaging applications. We report on imager performance, including Signal/Noise, Modulation Transfer Function, and summarize other imaging characteristics. We compare these characteristics as a function of different x-ray phosphors and discuss avenues for future improvement.

Digital x-ray imaging detectors generally consist of an array of discrete detector elements. These devices may have regions between the elements which are insensitive to the input signal, but are often necessary due to fabrication or operational requirements. These insensitive regions can be quantified in terms of the detector 'fill factor', defined as the active area expressed as a fraction of the physical detector area.It is conventional wisdom that the fill factor be as close to unity as possible, but this can be difficult or expensive to implement. The actual loss of image quality due to a non-unity fill factor has never been quantified in detail. In this paper, the spatial-frequency dependent detective quantum efficiency (DQE) is determined for a digital detector with a non-unity fill factor. For clarity, it is assumed that each element is independent with unity quantum efficiency and no additive noise, but these assumptions can easily be removed. It is shown that a decreased fill factor increases the noise pass-band of the detector which increases noise aliasing causing a decrease of the DQE. If the interacting quanta are uncorrelated, the DQE is always degraded. The degradation is generally greatest at frequencies approaching the sampling cut-off frequency. This result applies to digital devices which detect x-rays directly, such as selenium-based active-matrix arrays. If the incident quanta are partially correlated, the DQE is degraded less. This can occur when x-rays are detected indirectly, such as detectors which make use of conversion screens. An expression is developed which allows for a simple check that can be made to determine whether the fill factor degrades the DQE significantly for a specific design.

The trend toward highly conformal radiation fields in the treatment of cancer has increased the need for accurate verification of field placement.Conventionally, the placement has ben verified on a weekly basis using a film radiograph, or more recently electronic portal imaging devices (EPIDs). Fluoroscopic EPIDs consisting of a phosphor screen, one or more mirrors, lens, and camera provide reasonable performance but suffer from the poor collection efficiency and bulky nature of their optical components. Large area a-Si:H arrays provide an ideal replacement for the optical subsystem of fluoroscopic EPIDs. The purpose of this work is to (1) characterize the performance and logistics of a prototype a-Si:H array and (2) determine the metal plate/phosphor screen combination which will maximize the system's detective quantum efficiency (DQE) for megavoltage imaging. The prototype imager is based on a 10 X 10 cm² a-Si:H array consisting of 128 X 128 sensor elements coupled to a Gd₂O₂S:Tb screen and 0.5 mm Al plate. The charge signal collected in each diode is digitized to 16 bits at frame rates of up to 25 frames/sec. Dark current and readout noise were determined under controlled conditions. The optical collection efficiency of the photodiodes and the escape fraction of optical quanta from the screen were estimated from the literature. X-ray quantum absorption efficiency, number of optical quanta from the screen were estimated from the literature. X-ray quantum absorption efficiency, number of optical quanta produced, and the Poisson excess associated with energy absorption and conversion in the plate/screen system were calculated using EGS4 Monte Carlo simulations. Using these results, the imaging system performance is estimated using the approach of Cunningham et al for both a 6 MV x-ray spectrum and a ⁶⁰Co spectrum. For both the 6 MV and ⁶⁰Co beams, the predicted system gain was approximately 60 percent of the measured value. For a 6 MV beam, the system DQE (f equals 0) was calculated to be 1.45 percent at clinical doses. Reducing the does had little effect on the DQE (f equals 0), indicating that the dark current and readout noise are not significant factors at f equals 0.

This paper describes a dual-mode, flat panel imaging system capable of both fluoroscopy and radiography. Two generations of large area sensing technology are described. The general system architecture incorporates both the high sensitivity and data throughput required for fluoroscopy with the large signal capacity, spatial resolution and form factor necessary for radiography.

We describe a prototype multi-mode CCD-based detector for digital mammography. Each module incorporates a CCD bonded to a demagnifying fiberoptic taper. The x-ray-to-light converter is formed by depositing Gd₂O₂S:Eu onto the large end of the taper. The modules are held together in an array in order to cover a large area. We have constructed a 2-module prototype to evaluate the performance of this design and to develop software and hardware methods for the final multi-module detector. The 2-module detector forms an imaging area of 10 X 20 cm. with a 30-50 micrometers gap between the modules. Each module incorporates a 3.5:1 demagnifying fiberoptic taper and a 2048 X 2048, 14 micrometers -pixel CCD, resulting in an effective pixel size of 50 micrometers . For each 20keV x-ray photon incident on the phosphor converter approximately 7 electrons are generated in the CCD. For a 1 second exposure, the image noise is approximately 15 e/pixel and the dynamic range of the detector is approximately 20,000. The detector MTF is comparable to hat of a MinR screen/film system and the detector is able to resolve all features except the smallest spec group in an ACR phantom.

We have presented the principle of an x-ray detector based upon a screen coupled to an array of multiple CCD sensors. We now focus on the characterization of the image quality: resolution (MTF) and noise behavior in the overlap area. Simple low F lenses likely show distortion which means that not all imaged pixels have the same magnification. This may affect resolution. In the overlap area the image is reconstructed by interpolation between two sensors. Interpolation affects the noise properties so care must be taken in order to avoid that the noise characterization of the reconstructed image mosaic becomes spatially non uniform.We present an analysis of the influence of lens distortion and interpolation in the overlap area on the image mosaic. The image processing appears not to diminish the image quality provided the processing parameters are set correctly. We therefore present a robust extraction algorithm. In order to evaluate in real-time the image quality of the proposed detector system, we are building a 2 by 2 lens-CCD sensor system as a lab prototype. The main interest is on MTF and quantum noise properties. The hardware is designed such that also the lens distortion can be compensated. This enables relative cheap optical components with low F and a short building length. We have obtained and will present radiographic exposures of static phantoms.

A large area x-ray sensitive vidicon is an alternative to the x-ray image intensifier and television camera combination. The proposed x-ray vidicon utilizes an amorphous selenium photoconductive layer which has a higher intrinsic resolution in comparison to the input phosphor of an XRII. This higher resolution could benefit diagnostic cardiac angiography as well as interventional cardiac procedures which now frequency utilize XRII/TV zoom modes to achieve higher resolution. Signal, noise, resolution and lag of an x-ray vidicon have been analyzed theoretically and indicate a medically practical device is possible. The use of a large potential to bias the a-Se photoconductor presents a problem with respect to instability of the a-Se surface potential and excessive dark current. The incorporation of a suppressor mesh into the vidicon has been shown to provide stable vidicon operation while experiments involving a-Se blocking contacts have lead to the development of an a-Se layer with low dark current.

Large-area x-ray sensitive vidicons have the potential to be superior to conventional x-ray image intensifiers for medical fluoroscopy. To build a large-area x-ray vidicon, an electron lens system is necessary to provide perpendicular beam landing. In this paper, we presented the results of the lens design for this purpose. In this paper, we discus in detail the method used in our design.

Progress on the development of a semiconductor-based, direct-detection, flat-panel digital radiographic imaging device will be discussed. The device consists of a 500 micrometers thick amorphous selenium sensor coupled to an amorphous silicon thin-film-transistor (TFT) readout matrix. This detector has an active imaging area of 14 inches X 17 inches, 3072 X 2560 pixels with dimensions 139 micrometers X 139 micrometers and a geometrical fill factor of 86 percent. Charges generated primarily as a consequence of photoelectric interaction between the incoming x-rays and Se are integrated on storage capacitors that are located at each pixel. The high electric field applied across the Se minimizes the lateral spreading of the signal resulting in a significantly higher spatial resolution when compared to conventional film/screen systems used for general radiography. The sensor array is read out one pixel line at a time by manipulating the source and gate lines of the TFT matrix. Data are digitized to 14 bits. This paper will discuss the statistical photon counting analysis performed on an early prototype device. Measurements will include modulation transfer function, detector quantum efficiency, linearity, and noise analysis. Image analysis will include small contrast object visibility studies using a Faxil x-ray test object T016. Advantages of this flat-panel electronic sensor over conventional systems are discussed.

A large area, flat panel solid state detector is being investigated for both digital radiography and fluoroscopy. The detector employs amorphous selenium (a-Se) to detect x- rays. The charge image formed on the surface of the a-Se is read out in situ using an active matrix array. A theoretical analysis of the spatial frequency dependent detective quantum efficiency (DQE) is performed. Because of the very high intrinsic resolution of a-Se, the detector is inherently undersampled and aliasing is always present. An interpretation of DQE(f) for the undersampled a-Se detector will be given. The analysis shows that the main factors, besides the quantum efficiency of the a-Se layer, affecting DQE(f) are: (1) aliasing; (2) gain fluctuation noise of a- Se, i.e., the Swank factor of a-Se; (3) electronic noise which prevents quantum noise limited operation at low exposure levels such as those used in fluoroscopy and (4) temporal response which causes a reduction in noise by averaging. The validity of the theoretical model was confirmed experimentally using our prototype detector with the Swank factor being established using pulse height spectroscopy. The model was then applied to three important x-ray imaging applications: mammography, chest radiography and fluoroscopy. The results show that the most important strategy for maximizing DQE(f) is to increase the pixel fill factor which can be unity using specialized techniques Methods for reducing aliasing in the detector will be described.

A variety of problems in radiological imaging can be formulated in terms of point processes, which are random processes where every sample function is a sum of delta functions. Under certain postulates, especially one relating to statistical independence of the points,t he first- and second-order statistics of the process are well known. This paper treats correlated point processes where the postulates are not satisfied. The main kinds of correlation considered result from randomness in the radiation source and image amplification. Expressions are given for the mean, the autocorrelation and autocovariance functions and, in the stationary approximation, the power spectral density.

By use of a simple model for the DQE of a CCD imaging array, the input/output imaging characteristics can be expressed in a manner which enables absolute comparisons to be made with any other image-acquisition technology. As an example a model comparison is made here between CCD-pixel-arrays and photographic-grain-arrays.In addition to DQE, it is demonstrated that other imaging parameters can be evaluated and compared in this way, including image noise, dynamic range, and output signal-to-noise ratio, and that these parameters can be related back to the respective detector mechanisms. Thus the roles played by the absolute pixel and grain dimensions and their quantum efficiencies can be identified and compared. Results of these comparisons confirm not only the obvious advantage of a multi-level mode of quantum-detection,but also present a basis for understanding the role of the CCD level-spacing function in order to fully exploit this advantage.

By consideration of the statistical interaction between exposure quanta and the mechanisms of image detection, the signal-to-noise limitations of a variety of image acquisition technologies are now well understood. However in spite of the growing fields of application for CCD imaging- arrays and the obvious advantages of their multi-level mode of quantum detection, only limited and largely empirical approaches have been made to quantify these advantages on an absolute basis. Here an extension is made of a previous model for noise-free sequential photon-counting to the more general case involving both count-noise and arbitrary separation functions between count levels. This allows a basic model to be developed for the DQE associated with devices which approximate to the CCD mode of operation, and conclusions to be made concerning the roles of the separation-function and count-noise in defining the departure from the ideal photon counter.

The scintillator performance characteristics of five different scintillating fiber optic screens and two conventional Gd₂O₂S:Tb screens were measured and compared. The measurements that were made included the angular dependence of light emission relative to the normal, the modulation transfer function, and the absolute effective conversion efficiency. The relative error in the light output calculation results from the Lambertian source assumption is discussed.

Nuclear acoustic resonance (NAR), like nuclear magnetic resonance (NMR), can be used as a spectroscopic imaging tool to detect and characterize soft tissue densities and differences on the atomic scale. Whereas NMR uses electromagnetic radiation to induce energy level transitions, NAR uses acoustic radiation. The frequency of this radiation is typically 1 to 100 MHz; NAR imaging therefore uses ultrasonic energy to induce transitions among the nuclear spin energy levels. By means of piezoelectric transducers, polarized acoustic waves are generated and propagated within a specimen. If these perturbations are in resonance with the specimen's nuclear spin system, then the acoustic waves will periodically modulate an internal magnetic dipole or electric quadrupole interaction as acoustic energy is absorbed. The measurement of this acoustic energy absorption is analogous to the computation of the spin-lattice relaxation time, T₁, caused by the release of radiofrequency energy into the surrounding lattice of an excited nucleus and used in magnetic resonance imaging. Accordingly, NAR imaging combines the tools of ultrasound with the techniques of MRI to yield a new and potentially valuable medical imaging modality. The purpose of this paper is to discuss the essential physics of NAR, and to suggest how NAR signals can be processed for medical imaging.

It has been demonstrated both theoretically and experimentally that mammographic x-ray imaging with monochromatic beams could help to improve subject contrast and to reduce the dose delivered to the patient. To this aim, quasi-monochromatic x-rays have been produced in the mammographic energy range by making use of a conventional W- anode, Be-window x-ray tube and a monochromator optical system based on an array of mosaic crystals. The mosaic crystals are highly oriented pyrolytic graphite which provide a gain in flux as compared to perfect crystals because of their higher integrated reflectivity. The monochromator optical system consists of an array of three crystals which has been assembled so as to produce in the image plane an irradiation field obtained with adjacent reflected beams. The field size reflected by each crystal on the image plane is limited by the desired energy resolution along the horizontal direction and by the crystal size along the perpendicular one. The energy spread of the reflected beams is about 10 percent. The characteristics of the system in terms of energy resolution and fluence rate are reported. Radiographs of test phantoms imaged with quasi-monochromatic beams in the energy range of 18-21 keV have been obtained with a conventional screen/film combination. To remove the spatial non-uniformities of the entire irradiation field a correction procedure has been applied.Large field quasi- monochromatic x-ray beams with the same flux of a standard Mo-anode tube with an anode current of about 600 mA.

A prototype scanning-beam digital x-ray (SBDX) system for cardiac fluoroscopy has been constructed. The unique geometry and absence of detected x-ray scatter in the SBDX image promises to provide image quality equivalent to a conventional image-intensifier-based fluoroscopic system at substantially reduced x-ray exposure to patient and staff. In order to measure the SBDX exposure advantage, a contrast- detail study was performed comparing SBDX and a conventional cardiac fluoroscopic system. Low-contrast deductibility as a function of the phantom entrance exposure was determined. The expected SBDX exposure advantage was 3.0 to 3.4, for low-contrast objects ranging in diameter from 2 to 10 mm. This exposure advantage is applicable to the AP projection through an average-size cardiac patient. Based on these results, calculations show that angulated views and larger patients will experience significantly greater exposure reductions. In addition, the results also indicate that SBDX system design modifications can provide a greater exposure reduction from that measured with this prototype.

Quantitation of nuclear medicine data is a major goal in medical imaging. It implies that photon attenuation, scatter and depth dependent spatial resolution be corrected for. Realistic, anthropomorphic numerical phantoms are needed to understand how these phenomena degrade nuclear medicine images, and to validate correction methods. We developed a Monte Carlo simulator which simulates photon transport in an anthropomorphic phantom. The main feature of our phantom consists in estimating the attenuation coefficient for the three main types of physical interaction from CT data and tissue nature in each voxel. The simulated data obtained with this approach show how accurate in terms of geometry and attenuation coefficient, a phantom must be defined to properly simulate scintigraphic acquisitions. It highlights the important of bone tissues in the formation of scatter as well as the influence of patient's morphology in attenuation phenomena.

We review the general methodology and discuss practical issues of using mathematical model observers for task-based assessment of image quality in detection tasks with possible signal and background uncertainty. Various aspects of selecting a task, model observer and method of evaluation are discussed. The results of this work re a number of practical guidelines for conducting image quality studies with model observers.

The potential of a research prototype Kinestatic Charge Detector and data acquisition system for megavoltage portal imaging is discussed. Monte Carlo modeling of, and experimental results for, the line-spread function, modulation transfer function, energy efficiency and quantum detection efficiency are given and compared with those of portal film detectors. The first phantom images from the small-field system are compared with images of the same phantoms taken with commercial portal film systems. Future directions are discussed.

Many clinical applications demand CT scanners to provide 15 to 20 line pairs per centimeter resolution. For a conventional third generation CT scanner, this demand present a special challenge because of the inherent sampling limitations. If special care is not taken, aliasing artifacts could result. These artifacts typically appear as fine streaks irradiating from high density objects.One of the methods used to achieve aliasing-free CT images is to increase the sampling density by x-ray focal spot wobbling. This is achieved by first acquiring a projection with focal spot at one position. The gantry is then rotated to a position so that the detector cells straddle the cell positions at the previous view.At the same time, the focal spot is deflected back so that it overlaps the previous focal spot position. The two set of projections form an interlaced projection with much higher sampling density. Since the detector cells form an arc concentric to the x-ray focal spot, the interlaced samples are not uniformly spaced. Detailed analysis indicates that the position displacement increases as a function of the detector angle, which results in less effective aliasing artifact reduction for objects located away from the iso-center. To overcome this shortcoming,w e propose a new detector geometry. Computer simulations have shown that further aliasing artifact reduction can be achieved for off-centered objects. In addition, an improvement in in-plane resolution can be realized, due to an increase in the magnification factor and a reduction in the effective detector cell cross-section. We further show that a closed form solution for the tomographic reconstruction is possible when certain constraints are met.

We present an approximate algorithm for image reconstruction in spiral cone-beam CT at small cone-angles. The first step of the algorithm is a rebinning from fanbeam to parallel beam projections, that is performed independently for each detector row. The result of this rebinning procedure is a set of parallel views, where all rays are tilted against the z-axis by their cone angle and the rays within each view have different z-positions. After that, as with most of the approximate algorithms, the basic idea is to let each oblique ray contribute to the image with a weight that depends on its distance to the image plane. Because the distance of a ray to the image plane changes along the ray, so does its weight. This is the one important difference compared to standard 2D reconstruction procedures. However, for small cone angles, the variation of the weight along a ray is rather smooth, so that we can synthesize it as a Fourier series using only few Fourier coefficients. In standard 2D image reconstruction, we can build the 2D Fourier spectrum of the image on a polar grid from the Fourier transforms of the views, where each view contributes to a radial line only, because the weights are constant along the rays. In our algorithm, the weights change along the rays. This variation is modeled as a Fourier series with 2N_(mu ) + 1 coefficients. Hence, the views contribute not only to one but to 2N_(mu ) + 1 lines in the 2D Fourier space of the image. For a given pitch, the Fourier coefficients that take into account the variations of the weights are precalculated before the reconstruction and stored in a table. From the generated 2D image spectrum, we calculate the final image using a gridding technique to convert the sampling grid to cartesian and finally doing a 2D IFFT. We have evaluated our method using simulated test data for various test cases assuming sampling conditions and image quality requirements typical to medical CT. These experiments have shown that the method is applicable for detector arrays with up to at least 32 rows.

The use of a C-arm radiographic system for 3D reconstruction of opacified vasculature presents several computational and engineering challenges. Factors that may lead to inconsistency at the projection data set and subsequent reconstruction errors include image noise, variations in vessel opacification during the acquisition, and inaccurate determination of the imaging geometry. We have utilized simulations to study the effect of these factors on 3D reconstruction with algebraic reconstruction technique (ART) in order to identify possible artifacts and loss of image quality in the 3D image. Corrective measures designed to counter artifacts such as smoothing, averaging, and use of constraints with ART have been developed and validated. These studies have made it possible to identify the causes of artifacts in preliminary in vivo applications, and to estimate the tolerance for imperfections in data acquisition. Moreover, these works have established modifications to the reconstruction procedure for reducing image artifacts and improving image quality.

An image intensifier-based rotational volume tomographic digital angiography (VTDA) imaging system has been constructed. The system consists of an x-ray tube and an image intensifier that are separately mounted on a gantry. This system uses an image intensifier coupled to a charge coupled device camera as a 2D detector so that a set of 2D projection can be acquired for a direct 3D reconstruction. This paper presents the results of the system evaluation and preliminary animal studies using the volume tomographic system. The results indicate that the system has isotropic resolution of 0.5-1.0 1p/mm in the x, y and z directions, and the contrast resolution of the current system is 150 Hounsfield Unit for a 2mm vessel at 806 mR exposure. The results from the animal studies demonstrate that VTDA only needs a single injection of contrast and volume scanning to provide adequate image contrast and resolution for quantitative assessment of vascular anatomy.

Digital angiography remains valuable in a diagnostic environment where anatomical definition is important. However it is not sufficiently reliable in circumstances where precise geometry is important. The geometric distortion of digital angiography is mainly caused by the inherent, non-uniform electromagnetic field of the image intensifier board. In this paper we quantify this distortion so as to evaluate its clinical relevance. We utilize image fusion with fiducial markers and image exploration with cursor projection in this investigation. Fiducial markers on the images were normalized first and then used as baselines to scale and orient the corresponding images so they could be accurately registered, superimposed, and subtracted. Image exploration with cursor projection allowed for easy identification of the same point on corresponding images, which provided quantitative evaluation of geometric differences between digital and analogue angiography images. Based on our study, we concluded: (1) compared with MRI geometric distortion of as much as 2 mm, digital angiography is clinically appropriate for stereotactic application; (2) with an improved test object, the results would be more accurate.

High speed magnetic resonance imaging is frequently accomplished using one of a family of echo planar imaging (EPI) methods. Many of these methods move through k-space in a decidedly non-linear fashion. Typically, the non- rectilinear data sets are interpolated onto a rectilinear grid and then reconstructed using an inverse FFT. We present a method of reconstructing non-rectilinear EPI MRI data sets utilizing an optoelectronic implementation of the 2D discrete Fourier transform (DFT) bypassing the need for interpolation or regridding. Each point in k-space is represented as a fringe pattern and is written onto a charge coupled device photosensing array. The transforms of each point are initially summed on the photodetector and finally digitally summed to form the complex image. Up to 64K arbitrarily spaced complex points can be transformed into a 256 X 256 complex output matrix in as little as 50 msec. Reconstruction of blipped sinusoidal data using a DFT results in image quality similar to traditional methods whereas preliminary results of DFT reconstruction of spiral k-space trajectory data sets shows improved resolution. We also examine methods of determining the true k-space trajectory and the affect on reconstruction artifacts.

Screen-film and digital mammography typically utilize grids to remove scatter and a phosphor screen to convert the x- rays to light. We propose a square pore microchannel plate (SPMCP) hybrid grid-detector system that offers improvement in scatter rejection and quantum efficiency with acceptable resolution. SPMCP's are effective mammographic grids. Packing the exit side of the SPMCP with phosphor creates the hybrid system. The SPMCP confines the lateral spread of the emitted light and allows for a high efficiency phosphor layer. The hybrid system was investigated using Monte Carlo simulation of diffusive transport of optical photons. The size of the SPMCP pores, reflective coatings, scattering length within the phosphor, and phosphor thickness, were varied to maximize light output while maintaining resolution. The light output of the hybrid grid-detector systems is dependent upon the reflective properties of the SPMCP pores. WIth absorptive walls, the output increases with phosphor thickness, then decreases as absorption dominates. With reflective walls, the output is increased by 50 percent over the output of a conventional mammographic screen. Pore size and photon scattering length have minimal effect on light output. The MTF of a SPMCP detector is primarily dependent upon the size of the pores.

Thallium activated CsI scintillation screen (CsI:Tl) has advantages over rare-earth phosphor screens when used in a scanning slot x-ray detector for digital mammography. The scintillation decay time for CsI:Tl is only approximately 1 microsecond(s) which eliminates the afterglow effect associated with the use of rare-earth phosphor. The CsI needles serve to limit the spread of the scintillation light which permits the use of a relatively thick CsI:Tl screen to improve detector x-ray interaction efficiency without sacrificing resolution performance. A prototype scanning slot detector was made of a CsI:Tl screen and was optically coupled to two CCDs by plastic optical fiber image guides. The screen was composed of prismatic CsI crystals with a needle size of approximately 5 micrometers . Image guides used in the detector had an input to output ratio of 1:1, and were made of 10 micrometers diameter plastic optical fibers. Each CCD, operated in the time-delayed integration mode, was an 1100 X 330 pixel array with pixel size of 24 micrometers . A full scale scanning slot detector will contain eight rather than two such modules. The x-ray interaction efficiency of the slot x-ray detector was calculated to be approximately 84 percent at 20 keV for the CsI:Tl screen, and was approximately 20 percent greater than that of a 31.7 mg/cm² thick Gd₂O₂S:Tb phosphor screen. Limiting spatial resolution was investigated by taking the images of a 1 degree star resolution test pattern as a function of detector scan speed. Limiting spatial resolution was approximately 15 1p/mm at a scan speed of 4 cm/s and improved only slightly to approximately 16 1p/mm as the scanning speed decreased to 1 cm/s.

Film digitalization is the process of mapping the optical densities of a radiographic film into a digital matrix. In this work, a film digitizer based on charge-coupled device was evaluated and optimized for digital mammography applications. The characteristics of the digital output were determined for various spatial resolutions and dynamic ranges. Furthermore, the reproducibility of the system was tested as needed for computer assisted diagnosis (CAD) applications. Practical and relevant to the application quality control procedures were established for the system that will allow early troubleshooting and close monitoring of image quality for consistent performance. Overall, the characteristics of the scanner matched the properties of the tested screen/films and its performance generally met digital mammography and CAD requirements.

Computer analysis of mammography phantom images (CAMPI) is a method for objective and precise measurements of phantom image quality in mammography. This investigation applied CAMPI methodology to the Fischer Mammotest Stereotactic Digital Biopsy machine. Images of an American College of Radiology phantom centered on the largest two microcalcification groups were obtained on this machine under a variety of x-ray conditions. Analyses of the images revealed that the precise behavior of the CAMPI measures could be understood from basic imaging physics principles. We conclude that CAMPI is sensitive to subtle image quality changes and can perform accurate evaluations of images, especially of directly acquired digital images.

The basic imaging characteristics of a new mammographic screen-film combination have been experimentally determined. These characteristics include absolutely calibrated sensitometry, modulation transfer functions, noise power spectra, noise equivalent quanta, and detective quantum efficiency. Specific image quality advantages of this new system are discussed, including a substantial reduction in film noise, and comparisons are made with an established mammography system.

Dual-screen CR imaging technique can be used to improve x- ray absorption and image signal-to-noise ratio (SNR). Previous research has shown that it is possible to optimize the image superimposition process for best SNR in the superimposed image. Optimal weighting factors and SNRs have ben theoretically derived and related to the SNRs in the front and back images. The relationship has been experimentally verified. Practical implementation of the technique, however, involves non-uniform image signals and SNRs. To understand how the optimal weighting factors can be determined and how they vary spatially in non-uniform images, theoretical, numerical and experimental studies have been conducted. The results indicate that spatial variation of optimal weighting factors is minimal. An empirical method can be used to determine the optimal weighting factors over regions with minimal signal non-uniformity. The resulting weighting factors can be applied to superimposition of the entire front and back images for best SNR.

The purpose of this study was to critically assess and compare the intrinsic resolution, noise, and signal-to-noise transfer characteristics of two modern digital photostimulable phosphor (PSP) radiographic systems. Two commercial PSP systems were evaluated by identical methodologies. Measurements were made at three beam qualities using standard-resolution and high-resolution screens. The presampled modulation transfer functions (MTF) of the systems were measured using an edge method. The noise power spectra (NPS) were determined by 2D Fourier analysis of uniformly exposed radiographs. The frequency-dependent detective quantum efficiencies (DQE) were obtained from the MTF and NPS measures and the input signal-to-noise ratio, determined by a computational model for x-ray spectrum which was verified against exposure and half-value-layer measurements. The physical performance of the systems were very similar; a DQE of 0.24 and a spatial frequency of 2.5 cycles/mm at 0.2 MTF were estimated for both systems at 115 kVp using a pixel size of 0.1 mm and standard-resolution screens. The HD screen provided improved MTF with an adverse effect on noise as compared to the GP-25 screen using the same pixel size and beam quality.

Nevoscope is a transillumination imaging device which was designed to obtain an estimate of the 3D shape and volume of an embedded skin lesion using light in the visible spectrum. Light which is introduced into the skin surrounding the lesion, undergoes multiple scattering and re-emerges as back-scattered radiation through the lesion and surrounding skin. Since this light illuminates the lesion from within the skin structure, the images of the lesion as viewed at various angles are two dimensional optical projections of the skin lesion. Tomographic reconstruction techniques are used to reconstruct these lesions. Previous work assumed parallel ray geometry and used ART for reconstruction. In this paper we present an improved reconstruction technique based on the image formation principle at the surface of plane mirrors which are positioned inclined from the vertical on the Nevoscope.Phantoms are created to validate the volumes of the reconstructed lesions. A qualitative comparison of the volumes depict a better representation of the volumes using the new improved algorithm.

We propose a cone-beam computed tomography (CT) system that has a large field of view and high image quality, and that can be applied to the chest imaging. This system uses a 16- inch x-ray image intensifier and a television camera. To enlarge the field of view, an ellipsoid-scan sequence is used, which requires the subject to be moved parallel to the transaxial plane during scanning. The contrast resolution is increased by applying a television camera with high sensitivity and non-linear efficiency. We tested this system using a stationary apparatus, and obtained an isotropic 3D image of a chest phantom which had 0.64 mm voxels and covered both lungs. This system attained better spatial resolution for coronal images and its contrast resolution in the transaxial images was only 3 to 7 times larger than conventional spiral-scan CT at the same x-ray dose. This system is promising for ling cancer screening, precise diagnosis and surgery.

The purpose of this work is to evaluate the imaging and dose performance of an x-ray imaging system optimized for mammography. The x-ray system design was developed by the University of Southern California and the Center for Devices and Radiological Health using multiparameter optimization techniques. The prototype was built by Fischer Imaging and is now under evaluation at the National Institutes of Health. While previous reports have concentrated on demonstrating the does reduction potential of the system, for this study the x-ray spectrum was modified to maximize imaging performance. Measurements were made to assess the level of imaging performance achieved and to determine the increase in dose. Contrast-detail analysis along with qualitative evaluation of images of conventional mammography phantoms were used to assess imaging performance. Both imaging performance and the dose delivered by the system were compared to those of a conventional mammography system. Because of the current interest in digital mammography, the performance of the optimized system with a storage phosphor plate image receptor, sometimes referred to as computed radiography (CR), was also studied. The optimized system provided significantly better imaging performance than the conventional system with both film-screen and CR detectors. The dose was increased to a level comparable to the average value for conventional systems using grids.

In order to improve image quality of a high definition real- time I.I-DR system, we have developed an evaluation system using a 4M-CCD camera modifying a conventional system using a pick-up tube camera. An analysis of the conventional system shows that the image quality of total system is expected to be improved by changing camera. Basing on the analysis, we select a FFT type 4M-CCD camera for our purpose. From comparative evaluations of the 4M-CD DR system and the conventional system, the following results are obtained: (1) the image quality of high definition real-time DR is effectively improved by changing a conventional pick- up tube camera to a 4M-CCD camera; (2) the image quality of improved system is almost equal to that of S-F system, in case that the FOV of I.I is in 9 inch mode, which is most usual in GI-tract examination.

In 3D positron emission tomography (PET), all the coincidence events can be collected to increase the sensitivity of signal detection. However, the sensitivity increase results in the enlargement of scatter fraction which degrades image quality. For improving the accuracy of PET images, an effective scatter correction technique is necessary. In this paper, Monte Carlo simulations were done according to the system configuration of the animal PET design at the Institute of Nuclear Energy Research. From the simulation data we could understand what the scatter effect of our planned system will be. The convolution-subtraction method was chosen to correct for the scatter. A new approach to determine the scatter kernel function which could do better job on scatter correction will be presented.

In a previous paper an x-ray medical imaging system was described that used a liquid nitrogen cooled slow-scan CCD TV camera coupled to a Gd₂O₃(Eu) transparent ceramic scintillator plate with a high speed macro lens. This imaging system, which has a high spatial resolution and high x-ray quantum efficiency, suffers in the normal diagnostic x-ray energy range from added noise due to the secondary light photon quantum sink. For each x-ray photon absorbed, less than one electron is produced in a CCD pixel. However in the x-ray energy range used in radiation therapy the light output per absorbed x-ray photon is much higher, making the transparent scintillator technique more practical. Also the dose applied in radiation therapy is high anyway, allowing the use of higher dose to give better image quality. The 16-bit resolution of the cooled CCD allows very accurate x-ray absorption data to be acquired compared to the 8-bit CCDs used in commercially available portal imagers. Images have been acquired of human bones using the Gd₂O₃(Eu) screen and a 3 mm thick CsI(Tl) crystal.

This paper discusses the x-ray detection and imaging characteristics of anew semiconductor material, lead iodide, when prepared in form of a vapor deposited film for use in digital imaging. Lead iodide is a wide bandgap semiconductor and provides direct conversion of x-ray energy into electrical charges. This provides higher signal amplitude than conventional systems using scintillation or phosphor screens since only about 5 eV is required to form a charge pair in lead iodide as opposed to more than 30 eV in case of phosphors to produce optical photons. FUrthermore due to very little lateral diffusion of charge pairs, high spatial resolution can be obtained with such direct conversion films. Finally, due to low dark current in these films, the electronic noise in the films is also very low. In this paper we discuss the lead iodide film preparation procedure, its electronic properties such as resistivity and charge transport, its signal amplitude, and its x-ray imaging performance.

Scatter might significantly alter the diagnosis in mammography. For example, the sharpness of a mass edge is an important indicator of malignancy. Unfortunately scatter can blur the edges and so make the diagnosis harder. Sharpness enhancement, based on image processing might wrongly sharpen a smooth edge and so alter the diagnosis. We present here a scatter correction method based on the physical equations. The main problem of this approach lies on the non-knowledge of the three-dimensional structure of the object. In order to be able to calculate the map of scattered intensity using the physical laws, we propose an approximation of this structure derived from the primary x-ray flux map representative of its projection. This approximation takes into account the physical parameters relative to x-ray interactions in breast tissues, as well as geometric parameters characterizing the acquisition procedure. This approximation allows us to build an equivalent object for x- ray photons scattering, and the physical laws can be applied to this equivalent object. The validity of this approximation has been evaluated on simulated objects. Finally to remove the scattering flux, we propose an inversion scheme to build the primary flux from the observed flux. The obtained primary flux map is then representative of the projection of the 3D structures of the breast. To validate our scatter correction model, we have developed a direct simulation model taking into account the true 3D model is then compared to the result from the approximated model. The performances of our scatter correction approach will be validated on phantoms representative of the structure and the components of the breast.

Tungsten (W) target x-rays tubes are being studied for use in digital mammography to improve x-ray flux, reduce noise and increase tube heat capacity. A parametric model was developed for digital mammography to evaluate optimization of x-ray spectra for a particular sensor. The model computes spectra and mean glandular doses (MGD) for combinations of W target, beam filters, kVp, breast type and thickness. Two figures of merit were defined: (signal/noise)²/MGD and spectral quantum efficiency; these were computed as a means to approach optimization of object contrast. The model is derived from a combination of classic equations, XCOM from NBS, and published data. X-ray spectra were calculated and measured for filters of Al, Sn, Rh, Mo and Ag on a Eureka tube. (Signal/noise)²/MGD was measured for a filtered W target tube and a digital camera employing CsI scintillator optically coupled to a CCD for which the detective quantum efficiency (DQE) was known. A 3-mm thick acrylic disk was imaged on thickness of 3-8 cm of acrylic and the results were compared to the predictions of the model. The relative error between predicted and measured spectra was +/- 2 percent from 24 to 34 kVp. Calculated MGD as a function of breast thickness, half-value layer and beam filter compares very well to published data. Best performance was found for the following combinations: Mo filter with 30 mm breast, Ag filter with 45 mm, Sn filter for 60 mm, and Al filter for 75 mm thick breast. The parametric model agrees well with measurement and provides a means to explore optimum combinations of kVp and beam filter. For a particular detector, this data may be used with the DQE to estimate total system signal-to-noise ratio for a particular imaging task.

We have developed a new, active detector for dual energy computed radiography. The active detector consists of a computer controlled x-ray generator that produces two, fast voltage switched x-ray pulses, with the high voltage pulse first. An image acquisition unit contains two storage phosphor screens arranged in a 'sandwich'. The screen nearest the x-ray tube is in an optical chamber coupled to high intensity flashlamps. These lamps produce a short duration light pulse during the inter-exposure period that erases the high voltage x-ray data from the front screen but not from the back screen. The active detector is thus able to acquire the high and low voltage images fast enough to minimize patient motion artifacts with no mechanical motion of the storage phosphor screens. The voltage switched x-ray spectra are much better conditioned for dual energy image processing than the effective spectra produced by conventional, single exposure 'sandwich' detectors. The processed dual energy images with the active detector have much lower noise for the same patient dose as single exposure detectors. We experimentally characterized the performance of the active detector and show initial human subject images. We measured the following parameters: (1) speed of tow exposure image acquisition, and (2) repeatability of x-ray spectra and flux. We characterized the imaging performance by making images of a chest phantom with our detector and a conventional sandwich detector using the same incident exposure. Finally, we show a chest image of a human subject.

An algorithm has been developed to correct the image distortion introduced by the image intensifier - CCD camera chain in our volume tomographic angiography imaging system. First the distorted image is partitioned into small quadrilateral regions, then each partition is mapped individually to the corrected image by its corresponding bilinear functions. The parameters of these bilinear functions can be acquired by a special calibration procedure and stored as a lookup table. The maximum errors before and after the distortion correction were measured to be 5.96 mm and 0.03 mm. The method is further applied to the 3D image reconstruction of a simulated vascular phantom and live animal studies.

In a previous study, an exact and computationally efficient cone beam reconstruction algorithm for the circular-and-line orbit was proposed and its excellent numerical performance was demonstrated.In this paper, we define the scan field-of- view (FOV) of the system as a cylinder ended with a cone at each end, whose axes coincide with the axis of rotation. We demonstrate that an exact regional reconstruction of the longitudinally-unbounded object can be achieved with this system within the scan FOV. This and the previous studies combined defines a simple and practical cone beam tomographic system and an exact and computational efficient reconstruction for general imaging applications.

The goal of this work is to demonstrate the feasibility of 3D imaging of coronary stents using fluoroscopy. This technique could potentially provide an inexpensive and non- invasive alternative to stent inspection by intravascular ultrasound. The major difficulty to e overcome is real or apparent motion of the stent between successive views. We solve this problem by tracking a feature point ont he stent prior to performing the 3D reconstruction. We shifted the images to eliminate this apparent motion, then reconstructed the 3D stent image using iterative backprojection. The stent cross-sectional images are successfully reconstructed with spatial resolution of approximately 0.4 mm. This successful reconstruction of a coronary stent in vitro demonstrates the feasibility of 3D imaging of coronary stents using fluoroscopy.

The detected signal and noise contributions were measured and related to the radiation exposure and tube current tube setting. Furthermore, the detector contrast has been experimentally determined. The experimental results indicate that Cd_1-xZn_xTe detectors have high detector contrast resolution. Therefore, they appear to be very attractive for x-ray digital imaging applications.

The goal of this study is to optimize the gas composition medium and strip geometry of a small-field of view gas- microstrip detector for medical imaging applications, such as x-ray digital imaging, computed tomography, quantitative autoradiography, including other nuclear medicine applications. The gas multiplication factor as well as the electrical parameters of the microstrip substrate have been studied. The results of this study indicate that an adequate gas multiplication process can be achieved with a Xe filled gas detector operating up to 10 atm.

The purpose of the study is to optimize the input and the output parameters of a dual energy CdZnTe semiconductor detector for chest radiography. The optimal detector parameters were obtained by maximizing the figure of merit, defined as the ratio between the square of the signal-to- noise ratio and the absorbed dose, for chest radiography.

The purpose of this paper is to present the engineering principles of a cost-effective and efficient positron emission tomography (PET) detector operating on novel hybrid principles. The novelty of the proposed technology, which can be considered a low-cost alternative to the photomultiplier tube, consists of a BaF₂ crystal coupled to coupled to an ultra low-pressure noble-gas filled tube, which operates under photoionization of excited states, in the prebreakdown regime. The design detector principles, the first Townsend coefficient, and the gas multiplication factor of the hybrid microstrip detector have been studied. The results of this study indicate that an adequate gas multiplication process can be achieved with a Xe filled gas detector operating at low gas pressures. Also, low-pressure gas-filled detectors have the advantages of being themselves insensitive to radiation, ensuring that only the light from the scintillator is detected in a high-energy radiation environment.

The use of a gas microstrip detector in dual-energy radiography for certain clinical applications is explored. Optimal conditions of this technology for digital chest radiography are presented. These optimal conditions were obtained via computer simulations. The gas microstrip detector shows promise for achieving high spatial resolution, high internal gain, low noise and through the use of dual-energy techniques, high contrast resolution.

The purpose of this study is to measure the electrical parameters of the Cd_1-1Zn_xTe detectors, with the aim of characterizing the optimal detector performance parameters for digital radiographic applications.

We present an optical measuring system based on laser structured light suitable for its diary use in orthodontics clinics that fit four main requirements: (1) to avoid use of stone models, (2) to automatically discriminate geometric points belonging to teeth and gum, (3) to automatically calculate diagnostic parameters used by orthodontists, (4) to make use of low cost and easy to use technology for future commercial use. Proposed technique is based in the use of hydrocolloids mould used by orthodontists for stone model obtention. These mould of the inside of patient's mouth are composed of very fluent materials like alginate or hydrocolloids that reveal fine details of dental anatomy. Alginate mould are both very easy to obtain and very low costly. Once captured, alginate moulds are digitized by mean of a newly developed and patented 3D dental scanner. Developed scanner is based in the optical triangulation method based in the projection of a laser line on the alginate mould surface. Line deformation gives uncalibrated shape information. Relative linear movements of the mould with respect to the sensor head gives more sections thus obtaining a full 3D uncalibrated dentition model. Developed device makes use of redundant CCD in the sensor head and servocontrolled linear axis for mould movement. Last step is calibration to get a real and precise X, Y, Z image. All the process is done automatically. The scanner has been specially adapted for 3D dental anatomy capturing in order to fulfill specific requirements such as: scanning time, accuracy, security and correct acquisition of 'hidden points' in alginate mould. Measurement realized on phantoms with known geometry quite similar to dental anatomy present errors less than 0,1 mm. Scanning of global dental anatomy is 2 minutes, and generation of 3D graphics of dental cast takes approximately 30 seconds in a Pentium-based PC.

The performance of a new CdTe based x-ray detector devoted to digital radiography are presented. The detectors consist of a 6 cm² CdTe 2D-array connected to CMOS readout circuit by indium bumps. The final image has 400 X 600 pixels with a 50 micron pitch. This solid-state detector presents the advantages of direct conversion, i.e. high stopping power with high spatial resolution and a significantly higher signal than commercially available scintillator/photodetector systems. The experimental results show excellent linearity, spatial resolution and detective quantum efficiency. The MTF was measured by the angled-slit method: 20 to 30 percent at 10 1p/mm depending on the incident x-ray energy. The measured DQE is about 0.8 at 40 KeV and 100 (mu) Gray dose. Our simulation shows that these experimental results do not reach the theoretical limit. Further improvements are in progress. The first industrial application will be dental radiography due to the small size and the excellent performances. We also tested the detector with x-rays form 20 KeV to 1.25 MeV. Of course the CdTe thickness should then be adapted to the incident x-ray energy.

Radiation therapy portal images have traditionally exhibited poor discrimination of areas of interest, due to low subject contrast of anatomical parts being imaged at megavoltage energies, and the contrast capabilities of the image receptors. As a result of this low contrast, positioning of the radiation beam and placement of shielding blocks can be difficult. A novel, high-contrast cassette/film/screen system has ben developed and clinically evaluated for portal imaging. This system features a copper front screen, a gadolinium oxysulfide, terbium activated intensifying screens and a slow speed film with inherently high contrast. Very high film contrast is achieved by narrow grain size distribution and metal ion doping of the silver halide microcrystals. This high-contrast film is exposed by light from the intensifying screen, further increasing contrast. Sensitometric data indicates this new system to have 3.5X greater contrast than conventional portal localization imaging systems at comparable monitor units. Initial clinical evaluation indicates this new system to yield significantly superior images showing clearer definition of structures and was much easier to read and interpret.

The matching pursuit (MP) algorithm developed by S. Mallat and Z. Zhang is applied to magnetic resonance (MR) imaging. Since matching pursuit is a greedy algorithm to find waveforms which are the best match for an object-signal, the signal can be decomposed with a few iterations. In this paper, we propose an application of the MP algorithm to the MR imaging to reduce imaging time. Inner products of residual signals and selected waveforms in the MP algorithm are derived from the MR signals by excitation of RF pulses which are Fourier transforms of selected waveforms. Results from computer simulations demonstrate that the imaging time is reduced by using the MP algorithm and further a progressive reconstruction can be achieved.