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Purpose In the spirit of overcoming the signal-to-noise limitations of active matrix, flat-panel imagers (AMFPIs) which employ array circuits based on a-Si:H thin-film transistors (TFTs), an empirical investigation of the noise properties of prototype active pixel arrays based on polycrystalline silicon (poly-Si) TFTs is reported. Like a-Si:H, poly-Si supports fabrication of large area, monolithic x-ray imaging arrays and offers good radiation damage resistance, while providing electron and hole mobility orders of magnitude higher. Compared to pixel circuits typically consisting of a single addressing switch in an AMFPI array, the pixel circuit of an active pixel array includes an amplifier that magnifies the imaging signal prior to readout by external acquisition electronics. Also, while readout erases signal stored in the pixels for AMFPI arrays, active pixel arrays allow multiple nondestructive readout, which can be used to reduce noise. The prototype arrays investigated in this paper were developed to explore the effect of variation in amplifier design on noise. Methods A pair of prototype arrays incorporating single-stage and two-stage poly-Si pixel amplifiers were examined. The arrays incorporate various amplifier designs in which dimensions of some of the three (or four) poly-Si TFTs per pixel circuit for the single-stage array, and some of the five poly-Si TFTs for the two-stage array, were varied. The arrays were operated using a recently developed electronic data acquisition system that allows variation of operational conditions such as voltages and timing of control signals. The arrays were operated in the absence of radiation in various correlated multiple sampling modes, with and without the injection of charge directly into the pixel circuits for measurements of in-pixel gain and pixel noise. Pixel noise, referred back to the input of the pixel amplifier, was compared to predictions generated by a sophisticated circuit simulation model. Results Across the various pixel circuit designs, the median in-pixel gain for the single-stage and two-stage arrays was measured to be ×9.3 and ×25, respectively. These gain levels were sufficient to reduce the contribution of external noise, defined as the electronic additive noise in the absence of noise contributions from circuitry in the pixel and referred back to the input of the pixel amplifier, to less than 340 e. As a result, median pixel noise results as low as ~695 e and 866 e, acquired using eight samples, were observed from the best-performing single-stage and two-stage designs, respectively. While the magnitude of pixel noise predicted by simulation was lower than the measured results, there was generally good agreement between simulation and measurement for the functional dependence of noise on operating voltages, timing, and sampling mode. Conclusions The single-stage and two-stage arrays examined in this study demonstrated pixel noise well below that typically demonstrated by AMFPIs. Through proper design, it should be possible to maintain the noise levels observed in this study irrespective of the size and pitch of an active pixel array. Further reduction in pixel noise may be possible through more optimized pixel circuit design, faster readout, or improvements in fabrication.

PURPOSE Photon counting arrays (PCAs), capable of measuring the spectral information of individual x-ray photons and recording that information digitally, provide a number of advantages compared to conventional, energy-integrating active matrix flat-panel imagers - such as reducing the undesirable effects of electronic readout noise and Swank noise. While contemporary PCAs are based on crystalline silicon, our group has been examining the use of polycrystalline silicon (poly-Si, a semiconductor material better-suited for the manufacture of large-area devices) for such arrays. In this study, a theoretical investigation of the front-end amplifiers of array pixels incorporating photon counting circuits is described - building upon circuit simulation techniques developed in a previous study. Results for amplifier circuit designs corresponding to prototype PCAs currently under development, as well as for hypothetical circuit designs identified in the study, are reported. In the simulations, performance metrics (such as signal gain, linearity of signal response, and energy resolution) as well as various measures of count rate are determined. METHODS The simulations employed various input energy distributions (i.e., a 72 kVp spectrum as well as monoenergetic x rays) in order to determine circuit performance. To make the results representative of the properties of poly-Si, the simulations incorporated transistor characteristics that were empirically obtained from test devices. Optimal operating conditions for the circuits were determined by applying criteria to the performance metrics and identifying which conditions minimized settling time. Once the optimal operating conditions were identified, trains of input pulses simulating x-ray flux were used to determine two measures of count rate corresponding to dead time losses of 10% and 30% (referred to as CR10 and CR30 , respectively). RESULTS The best-performing prototype amplifier design (implemented at a pixel pitch of 1 mm) exhibited CR10 and CR30 values (expressed in counts per second per pixel) of 8.4 and 21.6 kcps/pixel, respectively. A hypothetical amplifier design was derived by modifying transistor, resistor, and capacitor elements of the prototype amplifier designs. This hypothetical design (implemented at a pitch of 1 mm) exhibited CR10 and CR30 values of 154 and 381 kcps/pixel, respectively. When implemented at a pitch of 0.25 mm, the performance of that design increased to 210 and 491 kcps/pixel, respectively (corresponding to counts per second per unit area of 3.4 and 7.9 Mcps/mm2 ). CONCLUSIONS The simulation methodology described in this paper represents a useful tool for identifying promising designs for the amplifier component of photon counting arrays, as well as evaluating the analog signal and noise performance of those designs. The results obtained from the current study support the hypothesis that large-area, photon counting arrays based on poly-Si transistors can provide clinically useful count rates. Encouraged by these early results, further development of the methodology to assist in the identification and evaluation of even more promising designs, along with development and empirical characterization of prototype designs, is planned.

Purpose Active matrix flat-panel imagers, which typically incorporate a pixelated array with one a-Si:H thin-film transistor (TFT) per pixel, have become ubiquitous by virtue of many advantages, including large monolithic construction, radiation tolerance and high DQE. However, at low exposures such as those encountered in fluoroscopy, digital breast tomosynthesis and breast computed tomography, DQE is degraded due to the modest average signal generated per interacting x-ray relative to electronic additive noise levels of ~1000 e, or greater. A promising strategy for overcoming this limitation is to introduce an amplifier into each pixel, referred to as the active pixel (AP) concept. Such circuits provide in-pixel amplification prior to readout as well as facilitate correlated multiple sampling, enhancing signal-to-noise and restoring DQE at low exposures. In this study, a methodology for theoretically investigating the signal and noise performance of imaging array designs is introduced and applied to the case of AP circuits based on low-temperature polycrystalline silicon (poly-Si), a semiconductor suited to manufacture of large area, radiation tolerant arrays. Methods Computer simulations employing an analog circuit simulator and performed in the temporal domain were used to investigate signal characteristics and major sources of electronic additive noise for various pixel amplifier designs. The noise sources include photodiode shot noise and resistor thermal noise, as well as TFT thermal and flicker noise. TFT signal behavior and flicker noise were parameterized from fits to measurements performed on individual poly-Si test TFTs. The performance of three single-stage and three two-stage pixel amplifier designs were investigated under conditions relevant to fluoroscopy. The study assumes a 20×20 cm2, 150 μm pitch array operated at 30 fps and coupled to a CsI:Tl x-ray converter. Noise simulations were performed as a function of operating conditions, including sampling mode, of the designs. The total electronic additive noise included noise contributions from each circuit component. Results The total noise results were found to exhibit a strong dependence on circuit design and operating conditions, with TFT flicker noise generally found to be the dominant noise contributor. For the single-stage designs, significantly increasing the size of the source-follower TFT substantially reduced flicker noise -NDASH- with the lowest total noise found to be ~574 e [rms]. For the two-stage designs, in addition to tuning TFT sizes and introducing a low-pass filter, replacing a p-type TFT with a resistor (under the assumption in the study that resistors make no flicker noise contribution) results in significant noise reduction -NDASH- with the lowest total noise found to be ~336 e [rms]. Conclusions A methodology based on circuit simulations which facilitates comprehensive explorations of signal and noise characteristics has been developed and applied to the case of poly-Si AP arrays. The encouraging results suggest that the electronic additive noise of such devices can be substantially reduced through judicious circuit design, signal amplification and multiple sampling. This methodology could be extended to explore the noise performance of arrays employing other pixel circuitry such as that for photon counting as well as other semiconductor materials such as a-Si:H and a-IGZO. This article is protected by copyright. All rights reserved.

Photon counting detectors are of increasing interest for clinical imaging. By measuring and digitally recording the deposited energy of each detected x-ray photon, these detectors can decrease the effect of electronic readout noise and Swank noise, potentially leading to improved image quality at decreased dose. The energy information of each photon can also be used to perform advanced imaging techniques such as material separation and k-edge imaging. Photon counting detectors employing crystalline silicon for the detector backplane are currently used for mammographic imaging and are under active investigation for fan-beam breast computed tomography (BCT). Our group has been exploring the possibility of creating monolithic, large-area photon counting detectors in order to perform cone-beam CT (CBCT) for BCT and radiation therapy (kV CBCT) applications. The detectors employ polycrystalline silicon (poly-Si) – a thin-film material that can be used to economically manufacture monolithic, large-area backplanes. In addition, poly-Si transistors have demonstrated good radiation damage resistance. The introduction of poly-Si-based photon counting detectors to BCT and kV CBCT would combine the benefits of photon counting with those of CBCT. However, one major challenge is designing circuits that are capable of handling the x-ray fluxes associated with these applications. A previously developed simulation methodology has been employed to model various levels of input flux in order to evaluate the count rate capabilities of poly-Si circuit designs. In this paper, the count rate capabilities of a promising poly-Si amplifier design under BCT and kV CBCT conditions are reported.

The signal-to-noise properties of active matrix, flat-panel imagers (AMFPIs) limit the imaging performance of this x-ray imaging technology under conditions of low dose per image frame. This limitation can affect cone-beam computed tomography (CBCT) procedures where an AMFPI is used to acquire hundreds of image frames to form a single volumetric data set. An approach for overcoming this limitation is to replace the energy-integrating pixel circuits of AMFPI arrays with photon counting pixel circuits which examine the energy of each x-ray interaction and count those events whose signals exceed user-defined energy thresholds. A promising material for fabricating the circuits of such photon-counting detectors (PCDs) is polycrystalline silicon (poly-Si)-a semiconductor that facilitates economic manufacture of large area, monolithic arrays of the size presently provided by AMFPIs as well as provides good radiation damage resistance. In this paper, results are reported from a theoretical investigation of the potential for poly-Si PCDs to satisfy the count rate needs, while maintaining good energy resolution, of two CBCT applications-CBCT used for breast imaging and kilo-voltage CBCT used for providing localization information in image guided radiotherapy (referred to as BCT and kV-CBCT, respectively). The study focused on the performance of the critical first component of a PCD pixel circuit, the amplifier, under conditions relevant to the two applications. The study determined that, compared to the average input fluxes associated with BCT and kV-CBCT, a promising amplifier design employing poly-Si thin-film transistors can provide count rates two and four times in excess of those levels, respectively, assuming a dead time loss of 10%. In addition, calculational estimates based on foreseeable poly-Si circuit densities suggest that it should be possible to include sufficient circuitry to support 2 and 3 energy thresholds per pixel, respectively. Finally, prospects for further improvements are discussed.

Pixelated photon counting detectors with energy discrimination capabilities are of increasing clinical interest for x-ray imaging. Such detectors, presently in clinical use for mammography and under development for breast tomosynthesis and spectral CT, usually employ in-pixel circuits based on crystalline silicon – a semiconductor material that is generally not well-suited for economic manufacture of large-area devices. One interesting alternative semiconductor is polycrystalline silicon (poly-Si), a thin-film technology capable of creating very large-area, monolithic devices. Similar to crystalline silicon, poly-Si allows implementation of the type of fast, complex, in-pixel circuitry required for photon counting – operating at processing speeds that are not possible with amorphous silicon (the material currently used for large-area, active matrix, flat-panel imagers). The pixel circuits of two-dimensional photon counting arrays are generally comprised of four stages: amplifier, comparator, clock generator and counter. The analog front-end (in particular, the amplifier) strongly influences performance and is therefore of interest to study. In this paper, the relationship between incident and output count rate of the analog front-end is explored under diagnostic imaging conditions for a promising poly-Si based design. The input to the amplifier is modeled in the time domain assuming a realistic input x-ray spectrum. Simulations of circuits based on poly-Si thin-film transistors are used to determine the resulting output count rate as a function of input count rate, energy discrimination threshold and operating conditions.

Photon counting arrays (PCAs), defined as pixelated imagers which measure the absorbed energy of x-ray photons individually and record this information digitally, are of increasing clinical interest. A number of PCA prototypes with a 1 mm pixel-to-pixel pitch have recently been fabricated with polycrystalline silicon (poly-Si)-a thin-film technology capable of creating monolithic imagers of a size commensurate with human anatomy. In this study, analog and digital simulation frameworks were developed to provide insight into the influence of individual poly-Si transistors on pixel circuit performance-information that is not readily available through empirical means. The simulation frameworks were used to characterize the circuit designs employed in the prototypes. The analog framework, which determines the noise produced by individual transistors, was used to estimate energy resolution, as well as to identify which transistors contribute the most noise. The digital framework, which analyzes how well circuits function in the presence of significant variations in transistor properties, was used to estimate how fast a circuit can produce an output (referred to as output count rate). In addition, an algorithm was developed and used to estimate the minimum pixel pitch that could be achieved for the pixel circuits of the current prototypes. The simulation frameworks predict that the analog component of the PCA prototypes could have energy resolution as low as 8.9% full width at half maximum (FWHM) at 70 keV; and the digital components should work well even in the presence of significant thin-film transistor (TFT) variations, with the fastest component having output count rates as high as 3 MHz. Finally, based on conceivable improvements in the underlying fabrication process, the algorithm predicts that the 1 mm pitch of the current PCA prototypes could be reduced significantly, potentially to between ~240 and 290 μm.

Obstacles in elucidating the role of oxidative stress in aging include difficulties in (1) tracking in vivo oxidants, in (2) identifying affected proteins, and in (3) correlating changes in oxidant levels with life span. Here, we used quantitative redox proteomics to determine the onset and the cellular targets of oxidative stress during Caenorhabditis elegans' life span. In parallel, we used genetically encoded sensor proteins to determine peroxide levels in live animals in real time. We discovered that C. elegans encounters significant levels of oxidants as early as during larval development. Oxidant levels drop rapidly as animals mature, and reducing conditions prevail throughout the reproductive age, after which age-accompanied protein oxidation sets in. Long-lived daf-2 mutants transition faster to reducing conditions, whereas short-lived daf-16 mutants retain higher oxidant levels throughout their mature life. These results suggest that animals with improved capacity to recover from early oxidative stress have significant advantages later in life.

Active matrix, flat-panel imagers (AMFPIs) employing a 2D matrix of a-Si addressing TFTs have become ubiquitous in many x-ray imaging applications due to their numerous advantages. However, under conditions of low exposures and/or high spatial resolution, their signal-to-noise performance is constrained by the modest system gain relative to the electronic additive noise. In this article, a strategy for overcoming this limitation through the incorporation of in-pixel amplification circuits, referred to as active pixel (AP) architectures, using polycrystalline-silicon (poly-Si) TFTs is reported. Compared to a-Si, poly-Si offers substantially higher mobilities, enabling higher TFT currents and the possibility of sophisticated AP designs based on both n- and p-channel TFTs. Three prototype indirect detection arrays employing poly-Si TFTs and a continuous a-Si photodiode structure were characterized. The prototypes consist of an array (PSI-1) that employs a pixel architecture with a single TFT, as well as two arrays (PSI-2 and PSI-3) that employ AP architectures based on three and five TFTs, respectively. While PSI-1 serves as a reference with a design similar to that of conventional AMFPI arrays, PSI-2 and PSI-3 incorporate additional in-pixel amplification circuitry. Compared to PSI-1, results of x-ray sensitivity demonstrate signal gains of approximately 10.7 and 20.9 for PSI-2 and PSI-3, respectively. These values are in reasonable agreement with design expectations, demonstrating that poly-Si AP circuits can be tailored to provide a desired level of signal gain. PSI-2 exhibits the same high levels of charge trapping as those observed for PSI-1 and other conventional arrays employing a continuous photodiode structure. For PSI-3, charge trapping was found to be significantly lower and largely independent of the bias voltage applied across the photodiode. MTF results indicate that the use of a continuous photodiode structure in PSI-1, PSI-2, and PSI-3 results in optical fill factors that are close to unity. In addition, the greater complexity of PSI-2 and PSI-3 pixel circuits, compared to that of PSI-1, has no observable effect on spatial resolution. Both PSI-2 and PSI-3 exhibit high levels of additive noise, resulting in no net improvement in the signal-to-noise performance of these early prototypes compared to conventional AMFPIs. However, faster readout rates, coupled with implementation of multiple sampling protocols allowed by the nondestructive nature of pixel readout, resulted in a significantly lower noise level of approximately 560 e (rms) for PSI-3.

We present a 16-channel self triggered readout chip for simultaneous time and energy measurement, suitable for a wide range of applications. All circuit elements required for detector readout are integrated on the chip. A fast, low-noise discriminator detects hits at the differential inputs. The events are time stamped with an electronic resolution of 15 ps (single channel rms). The timing range is virtually infinite by means of a coarse counter and an overflow indicator. In parallel to the time stamping of the discriminated input signal, the analog input is integrated during a programmable time window and digitized with 8-bit integral resolution. Readout is achieved through a simple serial protocol. Several chips can be synchronized by means of a built in PLL circuit locking to a common reference clock. The 3 times 3 mm2 chip has been fabricated in 0.18 mum technology, employing a differential logic for all crucial parts. Possible applications of this ASIC are in the fields of high energy physics experiments, medical imaging, range finding, georadar and others.

Over the last ~15 years, the central goal in external beam radiotherapy of maximizing dose to the tumor while minimizing dose to surrounding normal tissues has been greatly facilitated by the development and clinical implementation of many innovations. These include megavoltage active matrix flat-panel imagers (MV AMFPIs) designed to image the treatment beam, and separate kilovoltage (kV) AMFPIs and x-ray sources designed to provide high-contrast projection and cone-beam CT images in the treatment room. While these systems provide clinically valuable information, a variety of advantages would accrue through introduction of the capability to produce clinically useful, high quality imaging information at multiple energies (e.g., kV and MV) from a single detector along the treatment beam direction. One possible approach for achieving this goal involves substitution of the x-ray converters used in conventional MV AMFPIs with thick, segmented crystalline scintillators designed for dual-energy operation, coupled with the addition of x-ray imaging beams that contain a significant diagnostic component. A second approach involves introduction of a large area, monolithic array of photon counting pixels with multiple energy thresholds and event counters, which could provide multi-spectral views of the treatment beam with improved contrast. In this paper, the motivations behind, and the merits of each approach are described. In addition, prospects for such dual-energy imagers and photon counting array designs are discussed in the context of the radiotherapy environment.

The thin-film semiconductor processing methods that enabled creation of inexpensive liquid crystal displays based on amorphous silicon transistors for cell phones and televisions, as well as desktop, laptop and mobile computers, also facilitated the development of devices that have become ubiquitous in medical x-ray imaging environments. These devices, called active matrix flat-panel imagers (AMFPIs), measure the integrated signal generated by incident X rays and offer detection areas as large as ~43×43 cm 2 . In recent years, there has been growing interest in medical x-ray imagers that record information from X ray photons on an individual basis. However, such photon counting devices have generally been based on crystalline silicon, a material not inherently suited to the cost-effective manufacture of monolithic devices of a size comparable to that of AMFPIs. Motivated by these considerations, we have developed an initial set of small area prototype arrays using thin-film processing methods and polycrystalline silicon transistors. These prototypes were developed in the spirit of exploring the possibility of creating large area arrays offering single photon counting capabilities and, to our knowledge, are the first photon counting arrays fabricated using thin film techniques. In this paper, the architecture of the prototype pixels is presented and considerations that influenced the design of the pixel circuits, including amplifier noise, TFT performance variations, and minimum feature size, are discussed.

Purpose: The contrast provided by conventional portal imagers is constrained by inefficient conversion of the incident radiation, and by the small diagnostic x-ray component of the treatment beam. Toward improved visualization of the tumor region prior to treatment, as well as improved tumor tracking during treatment, progress is reported on the design of a pair of novel detectors that would address these constraints. Methods: A dual-energy imager based on a segmented scintillator is being designed using a hybrid model to simulate radiation and optical transport. The simulations, which model operation with a megavoltage treatment beam and with a BEV imaging beam with a significant diagnostic component, employ parameters extracted from a previously developed prototype. Separately, the conceptual design of a second imager incorporating a 2D array of photon counting pixels is being explored through extrapolation of the design of the first prototype photon counting arrays fabricated using thin-film processing, a process scalable to very large area detectors. Results: The simulations identified a segmented scintillator imager design that, when operated with a BEV imaging beam, provides spatial resolution and contrast comparable to that of gantry-mounted kV imagers. When operated with a treatment beam, the design provides significantly improved DQE and contrast compared to conventional portal imagers. Extrapolation of initial photon counting array designs indicates that the circuitry necessary to provide energy-windowing of the diagnostic portion of a therapy beam can be achieved at the pixel pitch associated with current treatment room imagers. Conclusion: These early studies demonstrate potential for improved contrast in projection and volumetric images obtained with the megavoltage beam prior to treatment, while also providing good performance when used with BEV imaging beams. In addition, these studies also support the possibility of improved capability for tumor tracking with such dual-energy imagers as well as by photon counting array imagers. NIH grant R01-EB000558

Active matrix flat-panel imagers (AMFPIs) have become ubiquitous in medical imaging environments. AMFPIs are based on two-dimensional pixelated arrays coupled to various x-ray converter materials that provide either indirect or direct detection of the incident x-ray radiation. However, the capabilities of this technology are severely constrained by the underlying solid-state properties of the amorphous silicon semiconductor material employed in the thin-film transistors present in each array pixel. The considerably higher electron and hole mobilities of polycrystalline silicon, a semiconductor material that (like amorphous silicon) is well suited to fabrication of transistors for large area electronics, provide the potential to overcome these constraints by increasing the overall gain of the system relative to the electronic additive noise. To explore this potential, a series of prototype arrays based on increasingly complex pixel designs employing polycrystalline silicon transistors is under development by our collaboration. The designs include several generations of active pixel arrays that incorporate sophisticated pixel-level amplifier circuits with the goal of improving imaging performance. In this paper, an initial analysis of the noise and DQE performance of selected prototype pixel circuit designs will be presented. The results are based on a combination of Monte Carlo -based circuit simulations and cascaded systems analysis, supplemented with information obtained from measurements performed on poly-Si transistors. The paper concludes with a brief discussion of the potential for, and challenges associated with, the creation of single photon counting arrays based on poly-Si TFTs.

Purpose: Photon counting arrays (PCAs) offer several advantages over conventional, fluence-integrating x-ray imagers, such as improved contrast by means of energy windowing. For that reason, we are exploring the feasibility and performance of PCA pixel circuitry based on polycrystalline silicon. This material, unlike the crystalline silicon commonly used in photon counting detectors, lends itself toward the economic manufacture of radiation tolerant, monolithic large area (e.g., ∼43×43 cm2) devices. In this presentation, exploration of maximum count rate, a critical performance parameter for such devices, is reported. Methods: Count rate performance for a variety of pixel circuit designs was explored through detailed circuit simulations over a wide range of parameters (including pixel pitch and operating conditions) with the additional goal of preserving good energy resolution. The count rate simulations assume input events corresponding to a 72 kVp x-ray spectrum with 20 mm Al filtration interacting with a CZT detector at various input flux rates. Output count rates are determined at various photon energy threshold levels, and the percentage of counts lost (e.g., due to deadtime or pile-up) is calculated from the ratio of output to input counts. The energy resolution simulations involve thermal and flicker noise originating from each circuit element in a design. Results: Circuit designs compatible with pixel pitches ranging from 250 to 1000 µm that allow count rates over a megacount per second per pixel appear feasible. Such rates are expected to be suitable for radiographic and fluoroscopic imaging. Results for the analog front-end circuitry of the pixels show that acceptable energy resolution can also be achieved. Conclusion: PCAs created using polycrystalline silicon have the potential to offer monolithic large-area detectors with count rate performance comparable to those of crystalline silicon detectors. Further improvement through detailed circuit simulations and prototyping is expected. Partially supported by NIH grant R01-EB000558. This work was partially supported by NIH grant no. R01-EB000558.

A decade after the clinical introduction of active matrix, flat-panel imagers (AMFPIs), the performance of this technology continues to be limited by the relatively large additive electronic noise of these systems - resulting in significant loss of detective quantum efficiency (DQE) under conditions of low exposure or high spatial frequencies. An increasingly promising approach for overcoming such limitations involves the incorporation of in-pixel amplification circuits, referred to as active pixel architectures (AP) - based on low-temperature polycrystalline silicon (poly-Si) thin-film transistors (TFTs). In this study, a methodology for theoretically examining the limiting noise and DQE performance of circuits employing 1-stage in-pixel amplification is presented. This methodology involves sophisticated SPICE circuit simulations along with cascaded systems modeling. In these simulations, a device model based on the RPI poly-Si TFT model is used with additional controlled current sources corresponding to thermal and flicker (1/f) noise. From measurements of transfer and output characteristics (as well as current noise densities) performed upon individual, representative, poly-Si TFTs test devices, model parameters suitable for these simulations are extracted. The input stimuli and operating-point-dependent scaling of the current sources are derived from the measured current noise densities (for flicker noise), or from fundamental equations (for thermal noise). Noise parameters obtained from the simulations, along with other parametric information, is input to a cascaded systems model of an AP imager design to provide estimates of DQE performance. In this paper, this method of combining circuit simulations and cascaded systems analysis to predict the lower limits on additive noise (and upper limits on DQE) for large area AP imagers with signal levels representative of those generated at fluoroscopic exposures is described, and initial results are reported.

Active matrix, flat-panel imagers (AMFPIs) employing a 2D matrix of a-Si addressing TFTs have become ubiquitous in many x-ray imaging applications due to their numerous advantages. However, under conditions of low exposures and/or high spatial resolution, their signal-to-noise performance is constrained by the modest system gain relative to the electronic additive noise. In this article, a strategy for overcoming this limitation through the incorporation of in-pixel amplification circuits, referred to as active pixel (AP) architectures, using polycrystalline-silicon (poly-Si) TFTs is reported. Compared to a-Si, poly-Si offers substantially higher mobilities, enabling higher TFT currents and the possibility of sophisticated AP designs based on both n- and p-channel TFTs. Three prototype indirect detection arrays employing poly-Si TFTs and a continuous a-Si photodiode structure were characterized. The prototypes consist of an array (PSI-1) that employs a pixel architecture with a single TFT, as well as two arrays (PSI-2 and PSI-3) that employ AP architectures based on three and five TFTs, respectively. While PSI-1 serves as a reference with a design similar to that of conventional AMFPI arrays, PSI-2 and PSI-3 incorporate additional in-pixel amplification circuitry. Compared to PSI-1, results of x-ray sensitivity demonstrate signal gains of approximately 10.7 and 20.9 for PSI-2 and PSI-3, respectively. These values are in reasonable agreement with design expectations, demonstrating that poly-Si AP circuits can be tailored to provide a desired level of signal gain. PSI-2 exhibits the same high levels of charge trapping as those observed for PSI-1 and other conventional arrays employing a continuous photodiode structure. For PSI-3, charge trapping was found to be significantly lower and largely independent of the bias voltage applied across the photodiode. MTF results indicate that the use of a continuous photodiode structure in PSI-1, PSI-2, and PSI-3 results in optical fill factors that are close to unity. In addition, the greater complexity of PSI-2 and PSI-3 pixel circuits, compared to that of PSI-1, has no observable effect on spatial resolution. Both PSI-2 and PSI-3 exhibit high levels of additive noise, resulting in no net improvement in the signal-to-noise performance of these early prototypes compared to conventional AMFPIs. However, faster readout rates, coupled with implementation of multiple sampling protocols allowed by the nondestructive nature of pixel readout, resulted in a significantly lower noise level of approximately 560 e (rms) for PSI-3.

The near-ubiquity of large area, active matrix, flat-panel imagers (AMFPIs) in medical x-ray imaging applications is a testament to the usefulness and adaptability of the relatively simple concept of array pixels based on a single amorphous silicon (a-Si:H) TFT coupled to a pixel storage capacitor. Interestingly, the fundamental advantages of a-Si:H thin film electronics (including compatibility with very large area processing, high radiation damage resistance, and continued development driven by interest in mainstream consumer products) are shared by the rapidly advancing technology of polycrystalline silicon (poly-Si) TFTs. Moreover, the far higher mobilities of poly-Si TFTs, compared to those of a- Si:H, facilitate the creation of faster and more complex circuits than are possible with a-Si:H TFTs, leading to the possibility of new classes of large area, flat panel imagers. Given recent progress in the development of initial poly-Si imager prototypes, the creation of increasingly sophisticated active pixel arrays offering pixel-level amplification, variable gain, very high frame rates, and excellent signal-to-noise performance under all fluoroscopic and radiographic conditions (including very low exposures and high spatial frequencies), appears within reach. In addition, it is conceivable that the properties of poly-Si TFTs could allow the development of large area imagers providing single xray photon counting capabilities. In this article, the factors driving the possible realization of clinically practical active pixel and photon counting imagers based on poly-Si TFTs are described and simple calculational estimates related to photon counting imagers are presented. Finally, the prospect for future development of such imagers is discussed.

Conventional active matrix flat-panel imagers (AMFPIs), employing amorphous silicon (a-Si:H) semiconductors, are based on a relatively simple pixel architecture, commonly taking the form of a single, thin-film transistor (TFT) coupled to a pixel storage capacitor. Although this semiconductor-architecture combination has led to the successful creation of x-ray imagers for many applications, a variety of significant performance limitations related to DQE, frame rate and charge trapping have also become apparent. While prospects for designing solutions to these restrictions based on a-Si:H TFTs are uncertain, progress in the development of high-quality polycrystalline silicon (poly-Si) TFTs is opening up new possibilities for large area x-ray imager design. Recently, initial prototype imagers have been developed using poly-Si TFTs in the form of 1-stage and 2-stage pixel amplifiers-ircuit architectures that can generally be referred to as active pixel sensors (APS). The insight gained from empirical evaluations of such prototypes, coupled with theoretical studies, can inspire increasingly sophisticated APS architectures that overcome the limitations, while preserving the advantages, of conventional AMFPIs. In this paper, cascaded systems analysis and circuit simulation are used to explore potential performance improvements enabled by APS architectures based on poly-Si TFTs. These studies suggest that it is possible to achieve significant improvements in DQE at low exposures or very small pixel sizes, higher maximum frame rates, and reduced charge trapping effects through implementation of such architectures.

An examination of the noise of polycrystalline silicon thin film transistors, in the context of flat panel x-ray imager development, is reported. The study was conducted in the spirit of exploring how the 1/f, shot and thermal noise components of poly-Si TFTs, determined from current noise power spectral density measurements, as well as through calculation, can be used to assist in the development of imagers incorporating pixel amplification circuits based on such transistors.

Purpose: Megavoltage active matrix, flat‐panel imagers (MV AMFPIs), in which a phosphor screen/copper plate serves as the x‐ray detector, are the de facto gold standard in portal imaging. Nevertheless, the poor efficiency of conventional MV AMFPIs, which detect only ∼2% of the incident radiation at 6 MV, has stimulated efforts to develop more efficient detectors, potentially facilitating high‐contrast tomographic imaging at low, clinically practical doses. In one approach, multiple thick layers of polycrystalline mercuric iodide (HgI2) photoconductor replace the phosphor screen to increase x‐ray detection efficiency. In this presentation, theoretical limits to MTF and DQE for this approach are examined through variation of photoconductor thickness and the composition and thickness of the metal plate. Method and Materials: Monte Carlo simulations of the interaction of a 6 MV beam with various photoconductor/metal plate combinations were performed. From the resulting MTF and NPS for HgI2 (in the form of particle‐in‐binder, PIB) up to 6 mm thick, in combination with metal plates up to 3 mm thick, DQE was determined. Results: Various trends are apparent. Increasing the thickness of the PIB layer results in lower MTF and higher DQE, as expected. However, for a given thickness of PIB, MTF is observed to decrease with increasing plate thicknesses — with this effect becoming less pronounced as PIB thickness increases. Moreover, increasing plate thickness enhances DQE at low spatial frequencies while diminishing it at higher spatial frequencies — with the absolute changes in DQE becoming greater as PIB thickness increases. Conclusion: As thicker PIB detectors exhibiting good mechanical and signal properties are developed, the composition and thickness of the overlying metal plate (which also serves to absorb scattered radiation) will require optimization, based on considerations such as the trade‐off of DQE at low spatial frequencies against DQE at high spatial frequencies.

Purpose: Despite the many advantages of Active Matrix Flat‐Panel Imagers (AMFPIs), these devices suffer from modest system gain relative to additive noise, as well as restrictions on maximum frame rates and charge trapping. A quantitative comparison of the performance of two of the strategies being developed to overcome these limitations is presented. One strategy employs significant enhancement of direct detection converter properties while the other employs complex, polycrystallinesilicon (poly‐Si) pixel circuits to form active pixel sensor (APS) architectures. Method and Materials:Theoretical upper limits for DQE and for other metrics that determine maximum frame rate and charge trapping effects were computed under fluoroscopic and radiographic conditions using a combination of cascaded systems analysis and circuit simulation. The cascaded systems calculations employed empirical measurements, published data and analytical calculations, while the simulations used representative a‐Si:H and poly‐Si transistor models. Results: The potential of high sensitivity photoconductors, such as HgI 2 , to offer up to a factor of 10 increase in system gain results in significant improvement to DQE performance under conditions of low exposures and/or for very small pixel sizes. Poly‐Si APS designs are capable of equivalent, or even greater DQE improvement through a combination of gain provided by in‐pixel amplifiers, along with correlated double sampling of the pixel signal. Furthermore, APS circuit designs allow substantially higher frame rates as well as reduction in charge trapping effects such as ghosting. Conclusion: While both approaches offer substantial improvements in DQE, and thus imaging performance, under conditions of low exposure and/or for small pixel pitches, converter enhancement offers the potential advantage of compatibility with existing AMFPI array designs. Conversely, poly‐Si APS architectures offer flexibility of design, function and operation, providing for the possibility of variable gain as well as addressing frame rate limitations and charge trapping issues.

Purpose: Active matrix flat‐panel imagers (AMFPIs) — based on amorphous silicon thin‐film transistors(TFTs) — have been developed for a wide variety of x‐ray imaging applications at diagnostic energies. However, under conditions of low exposure or small pixel sizes, the additive electronic noise associated with readout of the pixel signal of these imagers leads to relatively low levels of signal‐to‐noise ratio — resulting in significant loss of detective quantum efficiency (DQE) and image quality. An increasingly promising approach for overcoming such limitations involves the incorporation of in‐pixel amplification circuits, referred to as active pixel (AP) architectures, based on low‐temperature polycrystallinesilicon (poly‐ Si) TFTs. Methods: Theoretical performance limits for additive noise and DQE for large area imagers employing various AP designs have been explored. The methodology involves SPICE simulations of pixel circuits in an array environment employing realistic inputs for signal, as well as noise derived from individual poly‐Si TFT test devices. Pixel circuit noise performance derived from these simulations, along with other imaging parameters, are input to cascaded systems models for calculations of the corresponding DQE performance. Results: For large area imagers employing architectures comprising one‐ and two‐stage in‐pixel amplification, the calculations suggest that the effect of additive noise can be significantly diminished, leading to substantial improvement in DQE compared to that of conventional AMFPIs, especially at the low exposures associated with fluoroscopy. Such improved performance is largely maintained even for more sophisticated pixel designs incorporating additional features such as higher frame rate and selectable amplification gain. Conclusions: Active pixel imagers based on poly‐Si TFTs offer a potential alternative to conventional AMFPIs for overcoming the decline of DQE at low exposures in fluoroscopy. The combination of detailed circuit simulations and cascaded systems modeling provides a powerful tool for guiding the development of future generations of ever more sophisticated imagers. Work Supported by NIH grant R01‐EB000558

Purpose: Megavoltage (MV) active matrix, flat‐panel imagers (AMFPIs) have become the gold standard in radiotherapy portal imaging by virtue of their many advantages. Nevertheless, conventional MV AMFPIs are very inefficient, detecting only ∼2% of the incident radiation at 6 MV. However, recent theoretical and empirical studies have demonstrated that the incorporation of thick, segmented scintillators can significantly improve the DQE of MV AMFPIs, leading to improved image quality at very low dose. This creates the possibility of using megavoltage cone‐beam computed tomography (MV CBCT) to provide soft‐tissue visualization at clinically practical doses.Method and Materials: Prototype AMFPIs incorporating segmented scintillators based on CsI:Tl or BGO crystals with thicknesses ranging from ∼11 to 40 mm have been constructed and are under evaluation for MV CBCT. Each prototype incorporates a detector consisting of a matrix of 120x60 scintillator elements separated by 50 μm‐thick, reflective septal walls, with an element‐to‐element pitch of 1.016 mm. The prototypes are being configured to allow acquisition of tomographic images of low‐contrast, soft‐tissue‐equivalent objects, embedded in a water‐equivalent phantom, at the lowest available dose (one beam pulse per image).Results: For projection imaging, all prototypes offer substantial improvement in contrast sensitivity at low dose compared to conventional MV AMFPIs. In particular, the BGO prototype with a zero‐frequency DQE of ∼20% provides comparable contrast resolution at ∼20 times less dose. In view of such good performance, it is anticipated that reconstructed tomographic images of the contrast phantom, obtained at practical doses, will demonstrate soft‐tissue delineation. Conclusion: Prototype AMFPIs incorporating thick segmented BGO and CsI:Tl scintillators provide significant improvement in image quality at extremely low dose, facilitating the visualization of soft‐tissue at clinically practical doses. It is expected that future, optimized scintillators will lead to highly useful MV AMFPIs for projection and CBCTimaging.

Purpose: Megavoltage active matrix flat‐panel imagers (AMFPIs) are routinely used for portal imaging in external beam radiotherapy. However, conventional MV AMFPIs are inefficient, utilizing only ∼2% of the incident radiation, which leads to a detective quantum efficiency (DQE) of only ∼1%. Recent theoretical studies have shown that incorporation of thick, segmented scintillating detectors in MV AMFPIs can significantly increase DQE performance, leading to improved image quality at low dose and the possibility for dose‐efficient, cone‐beam computed tomography (CBCT).Method and Materials: Building upon the findings of our earlier studies, four prototype AMFPIs have recently been constructed, each consisting of a segmented BGO (11.3 mm thick) or CsI:Tl (11.4, 25.6 and 40.0 mm thick) detector coupled to an indirect detection flat‐panel array. Each detector consists of a matrix of 120 × 60 scintillator elements at 1.016 mm pitch, covered by a black or mirror top reflector. X‐ray sensitivity, MTF, NPS, DQE and phantom images were obtained for each prototype using a 6 MV photon beam at extremely low doses (e.g., 1 beam pulse, equivalent to 0.028 MU). Results: The BGO prototype shows better MTF (∼20% at the Nyquist frequency) than the CsI:Tl prototype at similar thickness. The measured DQE(0) at 1 beam pulse for the BGO and the three CsI:Tl prototypes were ∼19%, 12%, 19% and 23%, respectively. Moreover, the BGO prototype offers higher DQE values compared to the CsI:Tl prototypes over most of the spatial frequency range. Images of a contrast detail phantom using the BGO prototype at 2 beam pulses are comparable to that obtained from a conventional AMFPI with 18 times more dose.Conclusion: Prototype AMFPIs employing thick, segmented BGO and CsI:Tl detectors offer significant improvement in image quality at extremely low doses, leading to the possibility of soft tissue visualization using MV CBCT at clinically practical doses.