The x-ray flat panel detector (FPD) is a key component of the next generation x-ray imaging systems which promote digitization of x-ray images. By developing FPD applicable to both fluoroscopy and radiography, it is expected that x-ray diagnostic systems will change dramatically not only in terms of performance, but also in shape and form. The purpose of this research is to develop a selenium-based FPD applicable to both radiography and fluoroscopy. This report presents the results of the work on new technology which can be applied to create clinically-useful view size detector. The prototype detector adopts 1000 micron thick selenium as the photoconductor, 23 cm square of field of view, 150 X 150 micron pixel pitch, 1536 X 1536 pixel number, and is capable of capturing images at up to 30 frames/second. The features of this prototype detector are: simple-structure selenium suitable for real-time readout, and a high voltage protection structure for the TFT array which acts at x-ray overexposure. To realize large field of view, the field uniformity performance of the selenium and TFT array has been improved, and noise from the TFT array has been minimized. In this paper, new physical performance related to MTF, input- and-output characteristics, image lag, blooming, etc., are discussed.

This paper presents results of an image quality evaluation performed on a prototype DirectRay system (Sterling Diagnostic Imaging, Direct Radiography Corp.) operating in a UK radiology department. Physical imaging characteristics measured included characteristic (sensitometric) response, square-wave response function, Wiener spectrum and detective quantum efficiency (DQE). In addition, subjective image quality was assessed using a calibrated threshold contrast detail detectability (TCDD) test object. Results produced by the DirectRay system were compared with those achieved by a modern photo-stimulable computed radiography (CR) system. DirectRay and CR systems exhibit similarly wide exposure dynamic ranges, but in practice exploit this property in differing ways. Measured values of DQE(0) for the DirectRay system were found to be higher compared to those obtained for CR, for the majority of the exposure range examined. These results were confirmed subjectively by TCDD test-object analysis. No evidence of a 'memory artifact' phenomenon was found.

The x-ray imaging performance is reported using polycrystalline lead iodide as a thick semiconductor detector on an active matrix flat panel array. We have developed a test image sensor with 100 micron pixel size in a 512 X 512 format, using amorphous silicon TFTs for matrix addressing. The new 14 bit electronic system allows radiographic and fluoroscopic x-ray imaging. PbI₂ has larger x-ray absorption and higher charge generation efficiency than selenium, and has the potential for higher sensitivity imaging. The films are deposited by vacuum sublimation and have been grown thicker than 100 micrometer. Measurements of the carrier transport and charge collection, together with modeling studies show how the x-ray sensitivity depends on the material properties. Imaging measurements find excellent spatial resolution and confirm models of the x-ray sensitivity. Both radiographic and fluoroscopic imaging are demonstrated. While good overall imaging is obtained, the dark leakage current and image lag need further improvement.

A preliminary investigation on the temperature stability of selenium alloys has been carried out to determine their performance limitations in terms of properties relevant to x- ray imaging applications. The selenium properties tested as a function of temperature were the rate of crystallization, dark current, x-ray sensitivity and the residual signal. Results from crystal growth data show that the lifetime of conventional selenium can be up to 30 years of operation at 40 degrees Celsius but only about 3 years at 50 degrees Celsius. However, selenium alloys produced with a new process exhibited improved thermal stability by a factor of more than 2, with expected lifetime of up to 7 years at 50 degrees Celsius. Long term temperature cycling at 50 degrees Celsius show that the dark current remained below 50 pA/cm² for the test period of about 100 operational days but was projected to remain below 100 pA/cm² for up to 10 operational years. Testing under operational conditions showed a slight but reversible increase in the dark current occurred at 50 and 60 degrees Celsius, accompanied by a corresponding reversible decrease in the x-ray sensitivity in this temperature range. The loss in the x-ray sensitivity however, could be recovered by suitably increasing the operating electric field.

A new technique called Variable-Resolution X-ray (VRX) detection that dramatically increases the spatial resolution in computed tomography (CT) and digital radiography (DR) is presented. The technique is based on a principle called 'projective compression' that allows the resolution element of a CT detector to scale with the subject or field size. For very large (40 - 50 cm) field sizes, resolution exceeding 2 cy/mm is possible and for very small fields, microscopy is attainable with resolution exceeding 100 cy/mm. Several effects that could limit the performance of VRX detectors are considered. Experimental measurements on a 16-channel, CdWO₄ scintillator + photodiode test array yield a limiting MTF of 64 cy/mm (8(mu) ) in the highest-resolution configuration reported. Preliminary CT images have been made of small anatomical specimens and small animals using a storage phosphor screen in the VRX mode. Measured detector resolution of the CT projection data exceeds 20 cy/mm (less than 25 (mu) ); however, the final, reconstructed CT images produced thus far exhibit 10 cy/mm (50 (mu) ) resolution because of non-flatness of the storage phosphor plates, focal spot effects and the use of a rudimentary CT reconstruction algorithm. A 576-channel solid-state detector is being fabricated that is expected to achieve CT image resolution in excess of that of the 26-channel test array.

Large area, flat panel solid state detectors are being investigated for digital radiography and fluoroscopy. These detectors employ an x-ray imaging layer of either photoconductor ('direct' conversion method) or phosphor ('indirect' conversion method) to detect x-rays. In both cases the image formed at the surface of the layer is read out in situ using an active matrix array. Depending upon the resolution of the layer compared to the pixel size, undersampling of the image and hence aliasing may occur. Aliasing is always present regardless of the pixel size in direct detectors based on amorphous selenium because of its high intrinsic resolution. Aliasing gives rise to increased noise which results in reduction of detective quantum efficiency DQE at high spatial frequencies. The aliasing can be reduced or even eliminated by blurring prior to pixel sampling (e.g., by scattering in a phosphor layer). However, blurring, which may be quantified by the spatial frequency f dependent modulation transfer function MTF(f), also has a deleterious effect: the imaging system becomes much more susceptible to noise for example that arising in the charge amplifiers or secondary quantum statistics. Note that in principle, the system MTF can be corrected to any desired values in a digital system thus MTF has no predictive value for the quality of an imaging system, rather it is the DQE(f) which determines the overall signal to noise ratio independently of the MTF enhancement chosen. Nevertheless, determining the ideal level of presampling blurring (i.e., the Presampling Modulation Transfer function) is not straightforward. A problem caused by blurring is that the degree of blurring often depends on the depth of absorption of the x-ray in the imaging layer. In such cases (as pointed out by Lubberts) additional noise is transferred to the image. The predictions of a Lubberts model will be compared with published measurements of DQE for both direct and indirect detectors. A preliminary conclusion, is that blurring by CsI phosphor layers is non-ideal and leads to a significant loss of DQE at high spatial frequencies while no such loss is occurring in (alpha) -Se layers due to the equal MTF (in this case MTF(f) approximately equals 1) at all depths. Thus the only method which appears practical to cause blurring so as to avoid noise aliasing while avoiding the Lubberts depth dependent effect is to have a perfect MTF in the imaging layer and then blur before sampling. Such an approach has been proposed for the direct method based on the use of a partially conducting layer. Theoretical estimates of the final DQE(f) to be expected using this variation of the direct conversion method are produced.

The objective of this paper is to analyze quantitatively and systematically the major electronic noise source and provide design guidelines to improve signal to noise ratio in large area flat panel x-ray imaging systems. A transmission line model combined with a thin-film transistor model and transfer functions of charge-amplifier and correlated-double sampling is employed to simulate the electronic noises arising from the external amplifiers, data lines, gate lines and pixels. Simulation results using simple discrete RC models are presented for comparison. The noise analysis method and noise formula presented will provide guidelines to achieve the goal of optimization in imaging performance and quantum noise limited operation of the detector.

Offset and gain corrections are indispensable to exploit images from large image sensors, because of the pixel to pixel variation in dark current and sensitivity. However, an inappropriate correction may be detrimental to the signal to noise ratio of the raw image. This is especially critical in X-ray imaging, where the quantum noise is filtered by the detector spatial response. The noise power spectrum (NPS) in the corrected image is a combination of the initial noise spectrum in the raw image (quantum noise and electronic noise) with the noise in the offset and gain images. The dark image noise power just adds up to the noise power in the current image. The noise in the gain image alters the noise of the current image in a more intricate way. This is illustrated by simulations and experimental measurements. The conditions to safeguard the signal to noise ratio in the current image are detailed: There is an optimal number of dark and 'white' images to be averaged in order to keep their electronic and quantum noise negligible compared to that of the current image. Real conditions often force trade-offs between the desirable large number of offset/gain images to be averaged and the time effectively assigned to such acquisitions. Furthermore, the residual noise spectrum in the gain images is dependent on dose, uniformity of irradiation, temperature and detector spatial response. In the appropriate conditions, the intrinsic signal to noise ratio of an image can be preserved by offset and gain correction. Nevertheless, at high dose, the gain correction unavoidably introduces some high frequency proportional noise which degrades the DQE.

We seek to optimize a SPECT brain-imaging system for the task of detecting a small tumor located at random in the brain. To do so, we have created a computer model. The model includes three-dimensional, digital brain phantoms which can be quickly modified to simulate multiple patients. The phantoms are then projected geometrically through multiple pinholes. Our figure of merit is the Hotelling trace, a measure of detectability by the ideal linear observer. The Hotelling trace allows us to quantitatively measure a system's ability to perform a specific task. Because the Hotelling trace requires a large number of samples, we reduce the dimensionality of our images using Laguerre-Gauss functions as channels. To illustrate our method, we compare a system built from small high-resolution cameras to one utilizing larger, low-resolution cameras.

We show how high-resolution parallel-projection images can be obtained with a synthetic collimator, a device that consists of a multiple-pinhole aperture with detector arrays in several locations behind it. Simulation studies indicate that this system is superior to a parallel-hole collimator system for parallel-projection imaging. We also show how three- dimensional reconstructions can be created with the synthetic collimator and present evidence from simulations that lesion detection will be better in these reconstructions than in either of the parallel-projection imaging systems.

The objective imaging characteristics of a new high-speed mammographic screen-film combination is analyzed in detail. By combining a large reduction in film granularity with substantial improvements in screen technology, high-speed mammography can now be re-optimized to yield improvements in imaging system performance. Optimal use of the materials for reducing patient motion and for magnification techniques is potentially more effective than previous high-speed systems.

Since the video monitor is widely believed to be the weak link in the imaging chain, it is critical, to include it in the total image quality evaluation. Yet, most physical measurements of mammographic image quality are presently limited to making measurements on the digital matrix, not the displayed image. A method is described to quantitatively measure image quality of mammographic monitors using ACR phantom-based test patterns. The image of the test pattern is digitized using a charge coupled device (CCD) camera, and the resulting image file is analyzed by an existing phantom analysis method (Computer Analysis of Mammography Phantom Images, CAMPI). The new method is called CCD-CAMPI and it yields the Signal-to-Noise-Ratio (SNR) for an arbitrary target shape (e.g., speck, mass or fiber). In this work we show the feasibility of this idea for speck targets. Also performed were physical image quality characterization of the monitor (so-called Fourier measures) and analysis by another template matching method due to Tapiovaara and Wagner (TW) which is closely related to CAMPI. The methods were applied to a MegaScan monitor. Test patterns containing a complete speck group superposed on a noiseless background were displayed on the monitor and a series of CCD images were acquired. These images were subjected to CCD-CAMPI and TW analyses. It was found that the SNR values for the CCD-CAMPI method tracked those of the TW method, although the latter measurements were considerably less precise. The TW SNR measure was also about 25% larger than the CCD-CAMPI determination. These differences could be understood from the manner in which the two methods evaluate the noise. Overall accuracy of the CAMPI SNR determination was 4.1% for single images when expressed as a coefficient of variance. While the SNR measures are predictable from the Fourier measures the number of images and effort required is prohibitive and it is not suited to Quality Control (QC). Unlike the Fourier measures and the TW method, CCD-CAMPI is capable of yielding speck SNR on a single image. This is based on preliminary work and more complete testing is underway. Based on the early promising results, we expect that the CCD-CAMI method can be adapted to routine image QC of monitors using inexpensive equipment.

The detection characteristics of digital x-ray and film-screen mammography systems are different and thus current film-screen techniques are not ideal for digital mammography. Therefore optimum technical parameters required for digital mammography are likely to be different compared with film-screen mammography. The goal of this study is to evaluate the optimum technical parameters for full-field digital mammography by experimental and computer simulation methods. A General Electric Full Field Digital Mammography (FFDM) prototype unit using Cesium Iodide (CsI) on an amorphous Silicon photodiode array was used for the experimental measurements. Using breast equivalent phantoms, images were acquired for a set of x-ray target-filters for a range of peak kilovoltage, varying breast composition and thickness, with and without an anti-scatter grid. The signal-to-noise ratio (SNR) and figure-of-merit (FOM) were determined for simulated calcification and mass targets, independently by the two methods. The results for noise, contrast, SNR and FOM were compared and agree within 5% and 6% respectively. Combined results are presented for the case of 50% glandular - 50% adipose tissue breast composition using the grid and for the calcification target. Based on the FOM approach, preliminary results suggest that a Rhodium target-filter combination will be beneficial for higher breast thickness and for denser breasts.

The detection characteristics of digital x-ray and film-screen mammography systems are different and thus current film-screen techniques are not ideal for digital mammography. Therefore optimum technical parameters required for digital mammography are likely to be different compared with film-screen mammography. The goal of this study is to evaluate the optimum technical parameters for full-field digital mammography by experimental and computer simulation methods. A General Electric Full Field Digital Mammography (FFDM) prototype unit using Cesium Iodide (CsI) on an amorphous Silicon photodiode array was used for the experimental measurements. Using breast equivalent phantoms, images were acquired for a set of x-ray target-filters for a range of peak kilovoltage, varying breast composition and thickness, with and without an anti-scatter grid. The signal-to-noise ratio (SNR) and figure-of-merit (FOM) were determined for simulated calcification and mass targets, independently by the two methods. The results for noise, contrast, SNR and FOM were compared and agree within 5% and 6% respectively. Combined results are presented for the case of 50% glandular - 50% adipose tissue breast composition using the grid and for the calcification target. Based on the FOM approach, preliminary results suggest that a Rhodium target-filter combination will be beneficial for higher breast thickness and for denser breasts.

Monte Carlo calculations were performed to generate the point spread functions of x-ray photons absorbed in a CsI:Tl x-ray detector at the x-ray energies normally used in mammography (i.e., 20 keV to 50 keV). The corresponding modulation transfer functions [MTF(f)] for the CsI:Tl screen were also computed, taking into account the optical spread of light within the CsI:Tl crystals. The computed MTF(f)s were dominated by scintillation light lateral dispersion within the CsI:Tl screen. For the photon energy range encountered in digital mammography, the MTF(f) was a minimum at an x-ray photon energy just above the k-edge of Iodine (33 keV). Noise propagation theory for a cascaded imaging system was subsequently used to derive a theoretical expression for the detector DQE(f), including the dependence of DQE(f) on the spatial distribution of x-ray photon energy deposition. Detector performance was investigated as a function of x-ray exposure, CCD electronic noise, coupling efficiency of the fiber optical coupler, and the CCD quantum efficiency. Although most of the x-rays are absorbed via the photoelectric effect, the deposited x-ray energy spread within the CsI:Tl screen from the emission of characteristic x-rays can have a marked effect on detector performance, and the DQE(f) was found to decrease rapidly with photon energy just above the Iodine K-edge. X-ray exposure levels to the detector should be greater than or equal to 5 mR with a CCD electronic noise of approximately 20 electrons rms to ensure that DQE(f) performance is not significantly degraded at the spatial frequencies important in digital mammography (i.e., 0 to 10 lp/mm). Light dispersion within the CsI:Tl crystals was the major factor degrading imaging system DQE(f) at higher spatial frequencies. Optical coupling efficiency and CCD quantum efficiency are important system design parameters, which need to be maintained at a relatively high value. An optical coupling efficiency of approximately 0.7, and a CCD quantum efficiency of approximately 0.4, would still permit system DQE values greater than 60% at a spatial frequency of 5 lp/mm.

An x-ray camera for imaging with a spatial resolution in the micrometer and sub-micrometer range has been developed. The camera consists of a scintillator, light microscopy optics and a cooled charged-coupled device (CCD). A transparent scintillator converts the x-ray field into a visible light image which is projected onto the CCD by the light optics. A resolution of 0.8 micrometer fwhm was achieved using 12-keV x rays and a 5-micrometer thick commercially available Y₃Al₅O₁₂:Ce (YAG:Ce) scintillator. The detective quantum efficiency (DQE) of the camera is mainly limited by the low absorption of x rays in the thin layer of the scintillator. To increase the absorption, Lu₃Al₅O₁₂ (LAG) scintillators have been grown by liquid phase epitaxy (LPE). The characteristics of LAG activated with Eu or Tb have been investigated, in particular spectral emission, efficiency of x-ray to light conversion, and time response. Three-dimensional computed x-ray microtomography (3D-CMT) images of mouse bone samples have been recorded with this camera using a 5-micrometer thick YAG:Ce screen. The 3D-CMT system uses a parallel monochromatic x-ray beam extracted from the synchrotron radiation. A series of 2D x-ray projection images at different angles were recorded, and processed numerically to yield the 3D image of the bone structure with a voxel size of 1.8 micrometer. Image features smaller than 3 micrometer are clearly visible in the reconstructed tomogram. The quality of the images allows the analysis of the trabecular bone structure that is important for the understanding of the mechanisms of osteoporosis.

This paper presents a calibration and correction method for detector cell gain variations. To provide variable slice thickness capability in multislice volumetric scanners, while minimizing costs, it is necessary to combine the signals from several detector cells. The process of combining the output of several detector cells with non-uniform gains can introduce numerical errors when the impinging x-ray signal varies over the range of the combined cells. These scan dependent numerical errors can lead to artifacts in the reconstructed images, particularly when the numerical errors vary from channel-to-channel. A projection data correction algorithm has been developed to subtract the associated numerical errors. For effectiveness and data flow reasons, the algorithm works on a slice-by-slice basis. An initial error vector is calculated by applying a high-pass filter to the projection data. The essence of the algorithm is to correlate that initial error vector, with a calibration vector obtained by applying the same high-pass filter to various z-combinations of the cell gains. The solution to the least-square problem gives the coefficients of a polynomial expansion of the signal z-slope and curvature. From this information, and given the cell gains, the final error vector is calculated and subtracted from the projection data.

Recent development of large area flat panel solid state detector arrays indicates that flat panel image sensors have some common potential advantages: compactness, absence of geometric distortion and veiling glare with the benefits of high resolution, high DQE, high frame rate and high dynamic range, small image lag (less than 1%) and excellent linearity (approximately 1%). The advantages of the new flat-panel detector make it a promising candidate for cone beam volume CT imaging. The purpose of this study is to characterize an amorphous silicon thin film transistor (STFT) flat panel detector-based imaging system for cone beam volume CT applications. A prototype STFT detector-based cone beam volume CT imaging system has been designed and constructed based on the modification of a GE 8800 CT scanner. This system is evaluated using two phantoms: a low contrast phantom, and a GE CT quality assurance phantom. The results indicate that the flat panel detector-based volume cone beam can achieve a better spatial resolution than that of a standard fan beam CT and a low contrast resolution approaching that of a standard fan beam CT. The results also suggest that to use a flat panel detector for cone beam Volume CT, the design and construction of the detector should be optimized for tomographic acquisition and reconstruction.

To characterize the performance of a cone-beam computed tomography (CBCT) imaging system based upon an indirect- detection, amorphous silicon flat-panel imager (FPI). Tomographic images obtained using the FPI are presented, and the signal and noise characteristics of reconstructed images are quantified. Specifically, the spatial uniformity, CT linearity, contrast performance, noise characteristics, spatial resolution, and soft-tissue visualization are examined. Finally, the performance of the FPI-based CT system is discussed in relation to existing clinical technologies. A table-top measurements system was constructed to allow investigation of FPI performance in CBCT within a precisely controlled and reproducible geometry. The FPI incorporates a 512 X 512 active matrix array of a-Si:H thin-film transistors and photodiodes in combination with an overlying (133 mg/cm² Gd₂O₂S:Tb) phosphor. The commercially available prototype FPI has a pixel pitch of 400 micrometer, a fill factor of approximately 80%, can be read at a maximum frame rate of 5 fps, and provides 16 bit digitization. Mounted upon an optical bench are the x-ray tube (in a rigid support frame), the object to be imaged (upon a precision rotation/translation table), and the FPI (mounted upon a precision translation table). The entire setup is directed under computer control, and volumetric imaging is accomplished by rotating the object incrementally over 360 degrees, delivering a radiographic x-ray pulse (e.g., 100 - 130 kVp, approximately 0.1 - 10 mAs), and acquiring a projection image at each increment. Prior to reconstruction, dark and flood- field corrections are applied to account for stationary nonuniformities in detector response and dark current. Tomographic images are reconstructed from the projections using the Feldkamp filtered back-projection algorithm for CBCT. The linearity of the CBCT system was compared to that of a commercial scanner (Philips SR-7000) using materials ranging in CT number from approximately 900 to 1100. The contrast sensitivity of the CBCT system and the conventional scanner was compared using these same materials. Images of a uniform water bath were acquired for characterization of the response uniformity and the dependence of noise on exposure. The spatial frequency response characteristics of the system were measured using a steel wire, from which the point spread function and modulation transfer function were determined. Finally, the soft-tissue contrast and spatial resolution of the CBCT system was demonstrated in volumetric images of a euthanized rat. The image quality was compared to images of the same subject acquired with an equivalent technique on the commercial scanner. A table-top CBCT scanner based upon an a- Si:H FPI has been constructed, and a system for CBCT image acquisition, processing, and reconstruction has been implemented. This system is capable of producing high-quality volumetric images. Reconstructions were generated from 300 radiographs (100 kVp; 1 mAs per projection) obtained at 1.2 degree increments through 360 degrees. Image acquisition and reconstruction required approximately 30 min and approximately 2 h 20 min (250 MHz UltraSparc), respectively. The system has demonstrated signal and noise performance comparable to that of commercial CT scanners. The imaging performance of the prototype supports the hypothesis that FPIs can be employed in computed tomography applications.

In this paper, we report our investigation using a CsI(Tl) transparent scintillator x-ray detector together with the multi-level scheme algebraic reconstruction technique (MLS- ART) for megavoltage computed tomography (CT) reconstructions. The reconstructed CT images may be useful for positional verification in radiotherapy. The CsI(Tl) imaging system consists of a scintillator screen coupled to a liquid- nitrogen-cooled slow-scan CCD-TV camera. This system provides better contrast resolution than the standard electronic portal imaging system (EPID), which is especially useful given the low number of projections we are aiming at. The geometry of the imaging system has also been optimized to achieve high spatial resolution (1 lp/mm) in spite of the thickness of the screen. We present the images reconstructed using a pediatric head phantom using a total of 99 projections, and a combined phantom using 50 projections. Image reconstruction was carried out using the MLS-ART technique. We also present the CT images obtained using the back projection technique for comparison purposes.

Ultrasonography, a widely used imaging modality for the diagnosis and staging of many diseases, is an important cost- effective technique, however, technical improvements are necessary to realize its full potential. Two-dimensional viewing of 3D anatomy, using conventional ultrasonography, limits our ability to quantify and visualize most diseases, causing, in part, the reported variability in diagnosis and ultrasound guided therapy and surgery. This occurs because conventional ultrasound images are 2D, yet the anatomy is 3D; hence the diagnostician must integrate multiple images in his mind. This practice is inefficient, and may lead to operator variability and incorrect diagnoses. In addition, the 2D ultrasound image represents a single thin plane at some arbitrary angle in the body. It is difficult to localize and reproduce the image plane subsequently, making conventional ultrasonography unsatisfactory for follow-up studies and for monitoring therapy. Our efforts have focused on overcoming these deficiencies by developing 3D ultrasound imaging techniques that can acquire B-mode, color Doppler and power Doppler images. An inexpensive desktop computer is used to reconstruct the information in 3D, and then is also used for interactive viewing of the 3D images. We have used 3D ultrasound images for the diagnosis of prostate cancer, carotid disease, breast cancer and liver disease and for applications in obstetrics and gynecology. In addition, we have also used 3D ultrasonography for image-guided minimally invasive therapeutic applications of the prostate such as cryotherapy and brachytherapy.

Elasticity imaging is an emerging diagnostic modality whose development now is largely empirical. This paper outlines a framework for designing ultrasonic bioelasticity imaging systems based on a maximum-likelihood estimator of tissue motion. A principal goal for image formation is to maximize waveform coherence, which is to correctly match Fourier coefficients of the echo data recorded before deformation to those recorded after deformation. A crosstalk matrix is developed for strain imaging to obtain new insights into the physics of elasticity imaging, in particular resolution, and a simple figure of merit for evaluating system designs.

Ultrasound is an attractive modality for adjunctive characterization of certain breast lesions, but it is not considered specific for cancer and it is not recommended for screening. An imaging technique remarkably different from pulse-echo ultrasound, termed Optical Sonography^TM (Advanced Diagnostics, Inc.), uses the through-transmission signal. The method was applied to breast examinations in 41 asymptomatic and symptomatic women ranging in age from 18 to 83 years to evaluate this imaging modality for detection and characterization of breast disease and normal tissue. This approach uses coherent sound and coherent light to produce real-time, large field-of-view images with pronounced edge definition in soft tissues of the body. The system patient interface was modified to improve coupling to the breast and bring the chest wall to within 3 cm of the sound beam. System resolution (full width half maximum of the line-spread function) was 0.5 mm for a swept-frequency beam centered at 2.7 MHz. Resolution degrades slightly in the periphery of the very large 15.2-cm field of view. Dynamic range of the reconstructed 'raw' images (no post processing) was 3000:1. Included in the study population were women with dense parenchyma, palpable ductal carcinoma in situ with negative mammography, superficial and deep fibroadenomas, and calcifications. Successful breast imaging was performed in 40 of 41 women. These images were then compared with images generated using conventional X-ray mammography and pulse-echo ultrasound. Margins of lesions and internal textures were particularly well defined and provided substantial contrast to fatty and dense parenchyma. In two malignant lesions, Optical Sonography^TM appeared to approximate more closely tumor extent compared to mammography than pulse-echo sonography. These preliminary studies indicate the method has unique potential for detecting, differentiating, and guiding the biopsy of breast lesions using real-time acoustical holography.

An advanced Scanning-Beam Digital X-ray (SBDX) system for cardiac angiography has been constructed. The 15-kW source operates at 70 - 120 kVp and has an electron beam that is electromagnetically scanned across a 23-cm X 23-cm transmission target. The target is directly liquid cooled for continuous full-power operation and is located behind a focused source collimator. The collimator is a rectangular grid of 100 X 100 apertures whose axes are aligned with the center of the detector array. X-ray beam divergence through the collimator apertures is matched to the 5.4-cm X 5.4 cm detector, which is 150 cm from the source. The detector is a 48 X 48 element CdZnTe direct-conversion photon-counting detector. A narrow x-ray beam scans the full field of view at up to 30 frames per second. A custom digital processor simultaneously reconstructs sixteen 1,000² pixel tomographic images in real time. The slices are spaced 1.2 cm apart and cover the entire cardiac anatomy. The small detector area and large patient-detector distance result in negligible detected x-ray scatter. Image signal-to-noise ratio is calculated to be equal to conventional fluoroscopic systems at only 12% of the patient exposure and 25% of the staff exposure. Exposure reduction is achieved by elimination of detected scatter, elimination of the anti-scatter grid, increased detector DQE, and increased patient entrance area.

This investigation defines and tests a simple, nonlinear, task-specific method for rapid tomosynthetic reconstruction of radiographic images designed to allow an increase in specificity at the expense of sensitivity. Representative lumpectomy specimens containing cancer from human breasts were radiographed with a digital mammographic machine. Resulting projective data were processed to yield a series of tomosynthetic slices distributed throughout the breast. Five board-certified radiologists compared tomographic displays of these tissues processed both linearly (control) and nonlinearly (test) and ranked them in terms of their perceived interpretability. In another task, a different set of nine observers estimated the relative depths of six holes bored in a solid Lucite block as perceived when observed in three dimensions as a tomosynthesized series of test and control slices. All participants preferred the nonlinearly generated tomosynthetic mammograms to those produced conventionally, with or without subsequent deblurring by means of iterative deconvolution. The result was similar (p less than 0.015) when the hole-depth experiment was performed objectively. We therefore conclude for certain tasks that are unduly compromised by tomosynthetic blurring, the nonlinear tomosynthetic reconstruction method described here may improve diagnostic performance with a negligible increase in cost or complexity.

The paper investigates the variability of the geometrical deformation in digital X-ray image intensifier images as a function of acquisition parameters and time. We have increased our understanding of the deformation patterns and of how they are influenced by normal clinical use. Our goal is to reduce the frequency of calibration and the complexity of the protocol for diagnostic and interventional procedures so that geometrical distortion correction can become part of daily clinical practice.

Routine mammographic screening has proven to be the only reliable diagnostic method for early detection of breast cancers. The introduction of digital mammographic imaging to clinical practice should provide significant improvement in image quality and lead to better detection sensitivity. One limitation of some recently developed indirect digital imagers is a small fill factor in the photodetector. A small fill factor reduces the amount of light collected from individual x-ray interactions in the screen and can lead to an overall reduction in the number of x-ray quanta detected. At the SPIE meeting last year, we suggested using a focused microlens array between the florescent screen and photodetector as a method for improving x-ray utilization in indirect digital mammographic imaging systems. This year we have conducted a Monte Carlo simulation of a uniform field x-ray source and a digital imager. We have used this simulation tool to compare the proposed imager, which employed microlens focusing, with more standard imager configurations. The evaluation included light collection efficiency and x-ray utilization performance measure. The results indicate that including microlens focusing between the scintillation screen and the photodetector can improve light collection and increase the number of x-ray quanta detected.

This paper proposes a realistic joint energy-spatial description of the various phenomena encountered in low energy gamma imagery: Compton diffusion, photoelectric absorption, collimator and detector behavior, shot noise affecting the images. This physical model takes into account the full energy dimension of the scattered photons, and can construct a series of images indexed in energy. It is based on the last generation of SPECT (single photon emission computed tomography) medical gamma cameras which operate in list mode (both the coordinates and the energy of the (gamma) photons are measured), in the energy range [50 - 500 keV]. In the model proposed, two hypotheses are made: (1) only the first order scattering is considered, (2) the diffusing medium is homogeneous. This model is computationally light and could lead to the development of new enhancing techniques using the energy dimension.

Following our previous report concerning the development of a 127 micrometer resolution, 7.4 million pixel, 30 X 40 cm² active area, flat panel amorphous Silicon (a-Si) x-ray image sensor, this paper describes enhancements in image sensor performance in the areas of image lag, linearity, sensitivity, and electronic noise. New process improvements in fabricating a-Si thin film transistor (TFT)/photodiode arrays have reduced first-frame image lag to less than 2%, and uniformity in photoresponse to less than 5% over the entire 30 X 40 cm² active area. Detailed analysis of image lag vs. time and x-ray dose will be discussed. An improved charge amplifier has been introduced to suppress image cross-talk artifacts caused by charge amplifier saturation, and system linearity has been optimized to eliminate banding effects among charge amplifiers. Preliminary sensitivity improvements through the deposition of CsI(Tl) directly on the arrays are reported, as well as overall imaging characteristics of this improved image sensor.

A novel solid state detector has been developed for digital radiography. It is based on an assembly of CCD chips operated in TDI mode. The X-ray detection is achieved by a CsI scintillator layer which features a needle structure. It combines an excellent resolution with a high absorption efficiency. The detector is able to operate in 2 modes: a standard resolution mode and a high resolution mode. In the standard mode, the detector features a 440 mm long sensitive area and a 162 micrometer pixel size. The total number of pixels along detector direction is 2720. Along the scan direction, the detector features a 10.8 mm wide sensitive area. A TDI process is performed in the chip itself over 67 X- ray sensitive elements. A 440 mm X 440 mm sized image can be acquired in typically 1.2 second. The high resolution mode allows producing images with an 81 micrometer pixel size. The detection dynamic range is 10,000:1. The main performance characteristics of the detector are presented, such as DQE, MTF, dynamic range.

Advanced technical investigations, including DQE measurements and threshold contrast detail-detectability experiments, have been performed in order to demonstrate the superior image quality of an experimental flat dynamic X-ray image detector (FDXD) system. The dose efficiencies throughout a range of dose levels used in fluoroscopic and radiographic applications have been measured and are presented. Together with the results of a range of clinical patient examinations, the results of the technical investigations fully confirm earlier expectations in terms of increased image quality and improved dose efficiency with respect to current imaging modalities. Several mixed applications performed with the FDXD system are presented including those where subtraction techniques were used. The dynamic aspects of the FDXD system are discussed in detail. In the fluoroscopic mode, images have been acquired with a dose-rate as low as 5 nGy per image using a frame rate of approx. 25 fps. Low dose fluoroscopic images will be presented and it will be confirmed that low readout noise of the detection system facilitates the clinical acceptability of the images, even without applying any noise reduction algorithms. Post-processing algorithms for exposures will also be discussed. It can be concluded that the results of the technical measurements, together with the clinical examinations, prove that in areas regarding dose efficiency and image quality, this new detector technology is superior to the current X-ray modalities in many aspects.

A timing sequence for operation of an amorphous silicon detector within a digital chest radiography system is evaluated. Measurements of large-area spatial non-uniformities (i.e., artifacts) resulting from various implementations of the sequence are presented. Based on these measurements, a method for operation is determined which successfully eliminates large-area artifacts and which satisfies several important system latency requirements. In addition, a method for quantifying artifacts of the type observed in these experiments is presented.

Conventional radiography is based on absorption contrast and geometrical (ray) optics. After an outline of the relevant theory, this article reports results displaying both phase- and absorption-contrast, collected with a technique which utilizes a micro-focus x-ray source to achieve a high degree of spatial coherence, and relatively large object-to-image distances to enable (wave) interference effects (Fresnel diffraction) to occur and manifest themselves as phase contrast in the image plane. Both soft tissue (chicken knee) and hard tissue (finger bone) samples are investigated for a range of source sizes and object-to-image distances, encompassing conditions somewhat analogous to conventional radiography. Variation in image contrast and resolution as a function of these variables is observed and discussed.

We report the results of quantitative hard X-ray phase- contrast microscopy and tomography using synchrotron radiation, in-line imaging geometry and a non-interferometric phase retrieval technique based on the Transport of Intensity equation. This quantitative imaging method is fast, simple, robust, does not require sophisticated X-ray optical elements and can potentially provide submicron spatial resolution over a field of view of the order of centimeters. In the present experiment a spatial resolution of approximately 0.8 micron has been achieved in images of a polystyrene sphere using 19.6 keV X-rays. We demonstrate that appropriate processing of phase-contrast images obtained in the in-line geometry can reveal important new information about the internal structure of weakly absorbing organic samples. We present some preliminary results of a phase-contrast tomographic reconstruction with and without phase retrieval in each X-ray projection. We believe that this method of quantitative X-ray phase-contrast imaging will find applications in biology and medicine, particularly for high-contrast imaging of soft tissues.

Human tissues obtained from cancerous kidneys fixed in formalin were observed with phase-contrast X-ray computed tomography (CT) using 17.7-keV synchrotron X-rays. By measuring the distributions of the X-ray phase shift caused by samples using an X-ray interferometer, sectional images that map the distribution of the refractive index were reconstructed. Because of the high sensitivity of phase- contrast X-ray CT, a cancerous lesion was differentiated from normal tissue and a variety of other structures were revealed without the need for staining.

Optical tomography has recently demonstrated the potential for deep imaging of tissue oxygenation and blood volume, non- invasively, using relatively simple and inexpensive instrumentation. Prior demonstrations of this form of tomography have relied on scanning and data collecting methods limited to imaging bandwidths of Hz or slower. Here we report on an approach that significantly accelerates the imaging rate of optical tomographs based on time-domain methods to well beyond the kHz range. Such high bandwidths are critical for extending the capabilities of optical tomographs to include deep imaging of blood flow and neural activity.

Fluorescence lifetime imaging is a useful tool for quantifying site-dependent environmental conditions in tissue. Fluorophores exist with known lifetime dependencies on factors such as concentrations of O₂ and other specific molecules, as well as on temperature and pH. Extracting fluorophore lifetime for deeply embedded sites in turbid media such as tissue is made difficult by the multiple scattering of photons traveling through tissue. This scattering introduces photon arrival delays that have similar characteristics to the delays resulting from the excitation and subsequent emission of photons by fluorophores. Random walk theory (RWT) provides a framework in which the two sources of diffusion-like delays can be separated so that the part due to fluorescent lifetime can be quantified. We derive a closed-form solution that predicts time-resolved photon arrivals from a deeply embedded fluorophore site. The solution requires that an average absorption coefficient be used. However, it is shown that this assumption introduces only a small error. This RWT-derived solution is also shown to be valid for a range of geometries in which the fluorophore site is embedded at least 10 mean scattering lengths and in which the fluorophore lifetime is less than 1 ns.

A receiver operating characteristic (ROC) experiment was conducted to evaluate the effects of pixel size on the characterization of mammographic microcalcifications. Digital mammograms were obtained by digitizing screen-film mammograms with a laser film scanner. One hundred twelve two-view mammograms with biopsy-proven microcalcifications were digitized at a pixel size of 35 micrometer X 35 micrometer. A region of interest (ROI) containing the microcalcifications was extracted from each image. ROI images with pixel sizes of 70 micrometers, 105 micrometers, and 140 micrometers were derived from the ROI of 35 micrometer pixel size by averaging 2 X 2, 3 X 3, and 4 X 4 neighboring pixels, respectively. The ROI images were printed on film with a laser imager. Seven MQSA-approved radiologists participated as observers. The likelihood of malignancy of the microcalcifications was rated on a 10-point confidence rating scale and analyzed with ROC methodology. The classification accuracy was quantified by the area, A_z, under the ROC curve. The statistical significance of the differences in the A_z values for different pixel sizes was estimated with the Dorfman-Berbaum-Metz (DBM) method for multi-reader, multi-case ROC data. It was found that five of the seven radiologists demonstrated a higher classification accuracy with the 70 micrometer or 105 micrometer images. The average A_z also showed a higher classification accuracy in the range of 70 to 105 micrometer pixel size. However, the differences in A(subscript z/ between different pixel sizes did not achieve statistical significance. The low specificity of image features of microcalcifications an the large interobserver and intraobserver variabilities may have contributed to the relatively weak dependence of classification accuracy on pixel size.

A digital mammography system in which the x-ray sensitive device is a solid-state direct conversion detector is under development. This detector is a 1 mm thick silicon photodiode array hybridized to a CCD read-out, with a 50 micrometer pixel pitch. The detector is designed to be used in a slot-scanned system using time-delay integration (TDI) for signal acquisition. To handle the large signal generated in the photodiode, a novel read-out technique was used, in which charge was integrated 'on-chip' over a small number of rows, and the output of each of these sections was digitally summed 'off-chip' to produce the total integrated signal for each pixel in the image. This two-stage integration process not only allows easy acquisition of large signals, it effectively increases bit depth from 12 bits (for a single section) to approximately 16 (for the total integrated signal). The image quality of the device has been measured and compared to predictions based on cascaded linear systems theory. The resolution of the new detector was determined from the modulation transfer function (MTF) which was obtained from over-sampled edge spread functions (ESF). The ESF was measured in both the scan and slot directions from four repeated images of a tantalum edge. Noise power spectra (NPS) were determined from 40 repeated flat-field images at each of several x-ray exposures. By combining the MTF and NPS measurements, the detective quantum efficiency (DQE) was also determined. The MTF in the non-scanned direction was found to greater than 20% at 10 mm^-1 and slightly lower in the scanned direction (approximately equals 10% at 10 mm^-1). In all cases, the DQE was at least comparable to film-screen mammography receptors. The DQE at 120 mR detector exposure at zero spatial frequency ranged from 0.4 to 0.6 depending on the sample tested. Electronic noise was fairly low, contributing to less than plus or minus 7 ADU (out of a possible 98304 ADU). Future work will involve re-designing the prototype to use a photoconductor with higher density and atomic number to improve quantum interaction efficiency and reduce geometric constraints on image quality.

We present a refractive x-ray lens to be used in a scanned- slit detector system for mammography. The objective is to increase the x-ray flux on the detector and thus reduce scan- times. The lens consists of 600 diamond-cut parabolic cylinders with a radius of curvature of 100 micrometer cut from a 60 mm long slab of PMMA. The refractive effect from each void is very small, but the cummulative effect gives a focal length of approximately 200 mm. A 50 micrometer source was projected onto a 30 micrometer slit 970 mm away with a gain in flux of 4.3 for 27 keV. Since the index of refraction depends on the energy, the gain reaches a peak value for a certain energy, and by changing the geometry and the parameters of the lens, this peak can be tuned to coincide with the optimal energy for the imaging task. In mammography, the favorable energy region is quite narrow, and the spectrum shaping will improve the signal-to-noise ratio. Calculations show about 20% dose reduction with preserved image quality. Furthermore, less or even no filtering of the beam is needed, which corresponds to an additional gain of flux of about a factor of 2.

In this paper, we report measurements from a prototype 1024 X 1024 selenium-based flat panel detector suited for interventional digital mammography applications. This detector is based on an amorphous silicon TFT array, with a pixel pitch of 85 micrometer and a fill factor of 70%. A 200 micrometer layer of amorphous selenium is used to directly convert the incident x-rays into electrical charges. The detector electronics, TFT array, and selenium converter structure are designed to operate at a frame rate of 10 images per second. Experimentally, this detector yields an x-ray sensitivity of nearly 290 electrons/absorbed x-ray nearly 100% absorption of x-rays at a beam energy of 18 keV, a high spatial resolution (limited only by the pixel pitch up to the Nyquist limit), and quantum-noise limited operation down to the lowest exposures currently investigated. Images from the ACR phantom and contrast detail phantom reveal all embedded targets in the phantoms, which indicates the potential of this technology for digital mammography.

Acoustic pressure waves are induced in soft tissue whenever time-varying radiation is absorbed. By recording these time- dependent pressure waves over a sufficient number of angles surrounding the tissue being imaged, it is possible to reconstruct the pattern of radiation absorption within the tissue in three dimensions with spatial resolution that is independent of the carrier frequency of the irradiating energy. We recently constructed the world's first thermoacoustic computed tomography (TACT) scanner, which exploits this physical interaction. Initial in vivo imaging of a human breast was performed using safe levels of 434 MHz radiation. Good soft tissue differentiation with 2 - 5 mm spatial resolution to a depth of 40 mm was achieved. The absorption properties of the breast and the irradiation pattern within the breast determined the TACT image contrast. The length of the RF pulse, the size of the transducers and their frequency response, the geometry of the detector array, and the reconstruction algorithm that was used determined the spatial resolution. We conclude that TACT imaging may have application to breast cancer detection.

The X-ray intensity pattern in a radiograph of the thorax was measured. The measurements were carried out in the detector plane with high spatial resolution. A special phantom prepared from the lung of a dead body in which all fine details were existing was used for these measurements. This phantom was exposed to X-rays applying the standard examination conditions for thoracic imaging according to the 'European Guidelines on Quality Criteria for Diagnostic Radiographic Images,' and the image was stored on industrial X-ray film normally used for non-destructive testing. These images were digitized with a high-resolution drum film scanner. It is shown that the image degradation by the industrial X-ray film and the scanner was very small in the range of the estimated spectrum of the intensity pattern and that no problems with aliasing due to the scanning process would arise. The characteristic curves of the film and the scanner were evaluated. The X-ray intensity pattern of the thorax was calculated on the basis of these curves. Different methods to evaluate the information content of the image in the frequency domain are presented. The results obtained by these methods have been used to propose requirements for digital detector systems.

A novel approach to patient dose and image quality optimization was developed and implemented for chest and lumbar spine radiography. A Monte Carlo model of the imaging chain, including an anthropomorphic voxel-phantom to simulate the patient, was utilized. Detector noise and system unsharpness were modeled and their influence on image quality considered. Image quality was quantified by the contrast ((Delta) OD) and the ideal observer signal-to-noise (SNR) for a number of relevant image details at various positions in the anatomy and measures of dynamic range (DR). Among systems evaluated in a clinical trial, a reference system, acknowledged to yield acceptable image quality, was selected. A large variety of other imaging conditions were simulated and compared to the reference system. Some of the simulated systems were found to give as good imaging performance but at substantially reduced patient doses: 35% and 50% reduction in the lumbar spine AP and the chest PA view, respectively. The model was also used to define a single-valued 'figure-of- merit,' the physical image quality score, PIQS, with the aim to make possible ranking of the imaging systems. By comparing the ranking according to PIQS with radiologists' ranking it was possible to analyze the features in the images which are clinically important.

We have developed a new detector for dual energy computed radiography that uses fast switching of the x-ray tube voltage. Voltage switching gives x-ray spectra with wider separation and lower average energy than conventional 'sandwich' detectors. When properly optimized, these better- conditioned spectra produce dual energy images with lower noise for the same dose. We constructed a prototype of the detector, optimized it, and used it to produce human subject images of ten human volunteers. We also compared the performance of the active detector with a commercial dual- energy, computed radiography system that uses a sandwich detector. The results are as follows: We were able to produce high quality images for all ten volunteers. There were slight artifacts at the edge of the heart in some of the images but these did not affect their diagnostic utility. The active detector required approximately 1/4 the entrance exposure of the commercial, dual-energy system to produce diagnostic quality images of an anthropomorphic chest phantom. A substantial decrease in radiation exposure can be obtained with the dual-energy active detector approach with a modest increase in the complexity of the acquisition equipment.

Twenty-five paired chest images (twenty Posterior-Anterior views and five Lateral views) were obtained under identical x- ray exposure conditions with both an amorphous silicon digital detector system and a thoracic film-screen system (InSight HC screen and InSight IT film; Kodak, Rochester, NY). The digital image data was transformed using non-linear algorithms to obtain a gray-scale appearance comparable to conventional film images. The digital images were printed at full resolution onto laser film (3M, 969). Each hard copy clinical image pair was evaluated by six radiologist experts in a blinded study. Two thirds of the radiologists' ratings showed the digital detector images 'Better' or 'Much Better' than screen-film images (p less than 0.05); Ninety-eight percent of the radiologists' ratings indicated the digital 'Equivalent,' 'Better' or 'Much Better' (p equals 0.0004) than analog film. The digital image system improvement can be explained by significantly higher DQE and an extended dynamic range. The study clearly demonstrates that clinically the new digital x- ray detector radiographic system can produce diagnostic images equivalent or superior to conventional screen film system.

A beam equalization system has been developed for a recently introduced digital slot scanner. Its equalization range is tailored to the dynamic range of digital camera's. Applied beam profiles are made available to be included in the image file for associated image processing. A scanning wheel in the primary beam plays a key role in the generation of beam profile signals and simplifies system alignment. Because of its simplicity, it can be added to already installed camera's Special measures have been taken to minimize X-ray tube loading.

An x-ray imaging detector designed for both radiographic and fluoroscopic medical applications has been developed. The requirement that the detector provide superior imaging in both fluoroscopic and radiographic operation put severe constraints on its design. User requirements and a translation of those requirements to detector performance parameters guided detector design. This paper reports on the performance of a 20.5 X 20.5 cm prototype detector which was a product of this design effort. The detector was tested using both physical measurements and clinical imaging trials. The frequency dependent DQE is used as a measure of contrast to noise performance. Measurements of DQE were made at both fluoroscopic and radiographic signal levels. In fluoroscopic operation, measurements of lag were also made. The detector performance is compared to that of existing and emerging technologies. Results of clinical studies in both radiographic mode (chest imaging) and fluoroscopic mode (cardiac imaging) are reported.

This paper describes a third-generation multi-mode x-ray imager whose applications include low-dose fluoroscopy, cine, spot films, and radiography. In addition, volumetric CT and applications whose environment includes a 2 tesla magnetic field are also in development. The VIP-9 is based on an amorphous silicon TFT/Photodiode array and x-ray conversion screen, which is optionally a deposited CsI(Tl) film or a removable Gd₂O₂S screen. There are three primary modes of operation: RAD for high resolution radiographs and spot films; Fluoro for video rate, low dose fluoroscopy as well as cine; Zoom for high resolution, limited field of view (FOV) fluoroscopy. Through improved electronics, the imager has greater sensitivity at low doses and far better rejection of correlated line noise than its predecessors. In addition, the VIP-9 incorporates many ease-of-use features absent from earlier prototype imagers. While previous reports have primarily focused on the imager construction and noise issues in large area sensing technology, in this paper the emphasis is on features which facilitate integration into a complete imaging system and measures of image quality.

Because of traps in the photodiodes, amorphous silicon detectors retain charge and release it slowly after exposure. One effect of this property is that after an exposure, a ghost image may appear in subsequent images. This is a particular problem when fluoroscopic imaging follows radiographic exposure. In this paper we propose a method for predicting the magnitude of the ghost image so that it can be eliminated. The method uses linear systems theory to model the time behavior of the signal. Then the model is used to predict the offset as a function of time so that offset correction can be performed.

We describe new amorphous silicon (a-Si:H) image sensor arrays which are the highest resolution imagers so far reported. The pixel sizes of 64 micrometer (resolution 8 lp/mm) and 75 micrometer (6.7 lp/mm) are made possible using a photodiode technology that enables high sensor fill factor even in very small pixels. This approach allows the a-Si:H imagers to satisfy high resolution requirements of digital mammography. Each array contains 512 X 512 pixels with matrix addressing provided by a-Si:H thin film transistors (TFT). The high fill factor structure contains a continuous a-Si:H photodiode layer grown on top of the TFT array, with contacts to each pixel through a patterned metal/n⁺ layer. X-ray detection is accomplished by use of a phosphor layer superimposed on the array. The continuous photodiode layer maximizes light absorption from the phosphor and provides high x-ray conversion efficiency. Since the photodiode forms a continuous layer, crosstalk between adjacent pixels due to the lack of isolation is a particular concern, and has been extensively studied. We find that the high fill factor structure can be made such that the lateral charge leakage is minimal in the dark or under moderate illumination, although small amount of charge spreading is observed under conditions of sensor saturation. The measured MTF for optical illumination exceeds 60% at the Nyquist frequency, even for long integration times.

A theoretical cascaded systems analysis of the performance limits of x-ray imagers based on thin-film, active matrix flat-panel technology is presented. This analysis specifically focuses upon an examination of the functional dependence of the detective quantum efficiency on exposure. While the DQE of AMFPI systems is relatively high at the large exposure levels associated with radiographic x-ray imaging, there is a significant decline in DQE with decreasing exposure over the medium and lower end of the exposure range associated with fluoroscopic imaging. This fall-off in DQE originates from the relatively large size of the additive noise of AMFPI systems compared to their overall system gain. Therefore, strategies to diminish additive noise and increase system gain should significantly improve performance. Potential strategies for noise reduction include the use of charge compensation lines while strategies for gain enhancement include continuous photodiodes, pixel amplification structures, or higher gain converters. The effect of the implementation of such strategies is examined for a variety for hypothetical imager configurations. Through the modeling of these configurations, such enhancements are shown to hold the potential of making low frequency DQE response large and essentially independent of exposure while greatly reducing the fall-off in DQE at higher spatial frequencies.

In MRI, gradient magnetic fields are used to obtain the spatial information by frequency modulation of the received signal. The gradient fields are generated by switching currents on the gradient coils, which generates acoustic noise due to Lorentzian force. In particular, fast imaging methods, which are usually used for fMRI, require fast switching of the gradient pulse, thereby generating large acoustic noise. The intensity of the acoustic noise depends on the imaging method and the pulse sequences. The acoustic noise induced by gradient pulsing may interfere for signal enhancement of brain areas with the presentation of auditory stimuli during fMRI. In this paper, the gradient pulsing effects on fMRI are analyzed for different combinations of gradients. The experimental results show that total activations by visual stimulation are slightly larger for a combination of Z readout and Y phase-encoding gradients than those for a combination of Y readout and Z phase-encoding gradients when sagittal-view fMRI is performed.