J. Boudry

Empirical and theoretical investigation of the noise performance of indirect detection, active matrix flat‐panel imagers (AMFPIs) for diagnostic radiology

Noise properties of active matrix, flat-panel imagers under conditions relevant to diagnostic radiology are investigated. These studies focus on imagers based upon arrays with pixels incorporating a discrete photodiode coupled to a thin-film transistor, both fabricated from hydrogenated amorphous silicon. These optically sensitive arrays are operated with an overlying x-ray converter to allow indirect detection of incident x rays. External electronics, including gate driver circuits and preamplification circuits, are also required to operate the arrays. A theoretical model describing the signal and noise transfer properties of the imagers under conditions relevant to diagnostic radiography, fluoroscopy, and mammography is developed. This frequency-dependent model is based upon a cascaded systems analysis wherein the imager is conceptually divided into a series of stages having intrinsic gain and spreading properties. Predictions from the model are compared with x-ray sensitivity and noise measurements obtained from individual pixels from an imager with a pixel format of 1536 x 1920 pixels at a pixel pitch of 127 microns. The model is shown to be in excellent agreement with measurements obtained with diagnostic x rays using various phosphor screens. The model is used to explore the potential performance of existing and hypothetical imagers for application in radiography, fluoroscopy, and mammography as a function of exposure, additive noise, and fill factor. These theoretical predictions suggest that imagers of this general design incorporating a CsI: Tl intensifying screen can be optimized to provide detective quantum efficiency (DQE) superior to existing screen-film and storage phosphor systems for general radiography and mammography. For fluoroscopy, the model predicts that with further optimization of a-Si:H imagers, DQE performance approaching that of the best x-ray image intensifier systems may be possible. The results of this analysis suggest strategies for future improvements of this imaging technology.

Demonstration of megavoltage and diagnostic x-ray imaging with hydrogenated amorphous silicon arrays.

Flat-panel imagers consisting of the first large area, self-scanning, pixelated, solid-state arrays made with hydrogenated amorphous silicon (a-Si:H) are under development by the authors for applications in diagnostic x-ray and megavoltage radiotherapy imaging. The arrays, designated by the acronym MASDA for multi-element amorphous silicon detector array, consist of a two-dimensional array of a-Si:H photodiodes and thin-film transistors and are used in conjunction with scintillating materials. Imagers utilizing MASDA arrays offer a variety of advantages over existing technologies. This article presents initial megavoltage and diagnostic-quality x-ray images taken with several such arrays including the first examples of anatomical-phantom images. The external readout electronics and imaging techniques required to obtain such images are outlined, the construction, operation, and advantages of the arrays briefly reviewed, and the future potential of this new technology discussed.

Radiation damage of amorphous silicon, thin-film, field-effect transistors.

The effect of 60Co radiation on the noise and drain-source current characteristics of hydrogenated amorphous silicon (alpha-Si:H) field-effect transistors (FETs) was examined as a function of dose to cumulative doses as high as approximately 2 x 10(4) Gy. Following these measurements, room-temperature and elevated-temperature annealing of induced radiation damage was examined. The FETs examined are representative of those incorporated in alpha-Si:H arrays under development for various x-ray medical imaging applications. No significant effect upon the noise characteristics of the FETs was observed as a result of the radiation. The predominant drain-source current effect with increasing dose was a shift of the transfer characteristic toward negative gate voltage and/or a decrease of the transfer characteristic subthreshold slope. This resulted in large increases in leakage current for gate voltages where the FETs were initially highly nonconducting. This leakage current increase was less pronounced for more negative gate voltages and was further diminished by maintaining the FETs at a more negative gate voltage during the irradiation. Following the radiation measurements, room-temperature annealing resulted in a 10% to 50% reduction in the leakage current in the first day followed by a logarithmic decrease thereafter. Elevated-temperature annealing for 2 h at 200 degrees C restored FET leakage current and threshold voltage properties to their preirradiation values. The irradiation effects are small for cumulative doses less than approximately 100 Gy, which is larger than the clinical lifetime dose for an imaging detector for chest radiography or for fluoroscopy (with infrequent exposure to the direct beam). For significantly higher dose applications such as mammography, fluoroscopy (with frequent direct beam exposure), and radiotherapy imaging, the results suggest that periodic elevated-temperature annealing or operation of the arrays at more negative gate voltages may be necessary.

Development of hydrogenated amorphous silicon sensors for high energy photon radiotherapy imaging

Measurements with a-Si:H n-i-p photodiodes with sensitive areas of approximately 0.6 mm/sup 2/ exposed to a 6 MV radiation therapy treatment beam have been performed. Such photodiodes can be configured into large two-dimensional arrays of addressable sensors suitable for real-time imaging of megavoltage treatment beams as well as other applications such as diagnostic X-ray imaging. Signal sizes per radiation pulse up to 3.9*10/sup 6/ and 69*10/sup 6/ electrons are observed when the sensors are used with cronex (CaWO/sub 4/) and lanex (gadolinium-oxysulfide) intensifying screens, respectively. For the cronex and lanex screens the size of the measured signals can be enhanced by reducing the thickness of the top p-layer, thereby reducing the attenuation of the incident light. However, this is a smaller effect for the lanex screen, which provides substantially more light signal. The variation in cronex signal size with p-layer thickness is consistent with calculations based on known attenuation coefficients. >

High-resolution, high-frame-rate, flat-panel TFT array for digital x-ray imaging

Progress toward the development of a large area, flat-panel imager for diagnostic x-ray imaging is described. The initial fabrication of a prototype array with a format of 1536 X 1920 pixels and a pixel pitch of 127 micrometers giving an active area of 19.5 X 24.4 cm2 is reported. With a total of approximately 2.9 million transistors, this ambitious array is on a par with modern microprocessors in terms of transistor count. This work builds upon our concurrent research into the development of a very large area, lower spatial resolution, flat-panel imager for radiotherapy. An overview of the anticipated imaging properties of these devices is presented and future prospects discussed.

Large-area flat-panel amorphous silicon imagers

Flat-panel x-ray imaging arrays based upon thin-film electronics are increasingly under development and investigation for a variety of applications. Our research has progressed to the point where three large area designs have now been fabricated, including a new 26 X 26 cm2 array. These arrays are the largest self-scanning, solid-state imaging arrays thus far reported. In all probability, they represent only the first examples of an entirely new class of real-time imaging devices whose properties offer significant advantages over current radiographic and fluoroscopic x-ray technologies. A general overview of the current state of this emerging imaging technology is presented. Our large area array designs are described and x-ray images from a high resolution array are presented. Future challenges as well as anticipated trends and developments are discussed.

Thin-film, flat-panel, composite imagers for projection and tomographic imaging

The recent development of large-area, flat-panel a-Si:H imaging arrays is generally expected to lead to real-time diagnostic and megavoltage X-ray projection imagers with film-cassette-like profiles. While such flat-panel imagers offer numerous advantages over existing fluoroscopic and radiographic imaging devices, the unique properties of the arrays also offer the prospect of detector configurations not previously possible with other real-time technologies. The thin, highly uniform profile of the arrays allows the creation of composite imaging devices in which a flat-panel detector overlies a second imaging detector. A dual-energy (diagnostic and megavoltage) composite imager consisting of a pair of stacked, flat-panel imagers would provide unique information helping to resolve the patient localization and verification problem in megavoltage radiotherapy. In PET or SPECT, attenuation corrections could be obtained by placing a flat-panel array for transmission measurements directly in front of the main emission detector. In this article, the concept of such real-time flat-panel composite imagers is proposed. Specific embodiments of this concept applied toward the resolution of outstanding problems in radiotherapy, PET and SPECT are outlined and calculations and data supporting the feasibility of the concept are presented.

Large area amorphous silicon photodiode arrays for radiotherapy and diagnostic imaging

Abstract Amorphous silicon imaging devices consisting of two-dimensional pixel arrays of photodiodes and field effect transistors can now be fabricated over areas as large as 30 cm by 30 cm. Such imagers can offer considerable advantages for real-time radiotherapy megavoltage and diagnostic X-ray imaging applications. The design, operation, and advantages of such imagers are discussed, and sensor signal data are presented.

Development of thin-film flat-panel arrays for diagnostic and radiotherapy imaging

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

Current‐noise‐power spectra of amorphous silicon thin‐film transistors

Current‐noise‐power spectra of thin‐film transistors (TFTs) fabricated from hydrogenated amorphous silicon were measured. TFTs with aspect ratios ranging from 1:1 to 16:1 were examined in both the conducting and nonconducting modes. In the conducting mode, the noise levels could be predicted to within an order of magnitude by theories developed for crystalline metal‐oxide field‐effect transistors. In the nonconducting mode, the noise was found to scale with the TFT leakage current in a power‐law fashion.