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Purpose: Electronic portal imaging devices based on megavoltage (MV), active matrix, flat-panel imagers (AMFPIs) are presently regarded as the gold standard in portal imaging for external beam radiation therapy. These devices, employing indirect detection of incident radiation by means of a metal plate plus phosphor screen combination, offer a quantum efficiency of only ∼ 2 % at 6 MV, leading to a detective quantum efficiency (DQE) of only ∼ 1 % . In order to significantly improve the DQE performance of MV AMFPIs, a strategy based on the development of direct detectionimagers incorporating thick films of polycrystalline mercuric iodide ( HgI 2 ) photoconductor was undertaken and is reported. Methods: Two MV AMFPI prototypes, one incorporating an ∼ 300 μ m thick HgI 2 layer created through physical vapor deposition(PVD) and a second incorporating an ∼ 460 μ m thick HgI 2 layer created through screen-printing of particle-in-binder (PIB) material, were quantitatively evaluated using a 6 MV photon beam. The reported measurements include empirical determination of x-ray sensitivity, lag, modulation transfer function(MTF), noise power spectrum, and DQE. Results: For both prototypes, MTF and DQE results were found to be consistent with theoretical expectations and the MTFs were also found to be higher than that measured from a conventional MV AMFPI. In addition, the DQE results exhibit input-quantum-limited behavior, even at extremely low doses. Compared to PVD, the PIB prototype exhibits much lower dark current, slightly higher lag, and similar DQE. Finally, the challenges associated with this approach, as well as strategies for achieving considerably higher DQE through thicker HgI 2 layers, are discussed. Conclusions: The DQE of each of the prototypes is found to be comparable to that of conventional MV AMFPIs, commensurate with the modest photoconductor thicknesses of these early samples. It is anticipated that thicker layers of HgI 2 based on PIB deposition can provide higher DQE while maintaining good material properties.

Active matrix flat-panel imager (AMFPI) technology is being employed for an increasing variety of imaging applications. An important element in the adoption of this technology has been significant ongoing improvements in optical signal collection achieved through innovations in indirect detection array pixel design. Such improvements have a particularly beneficial effect on performance in applications involving low exposures and∕or high spatial frequencies, where detective quantum efficiency is strongly reduced due to the relatively high level of additive electronic noise compared to signal levels of AMFPI devices. In this article, an examination of various signal properties, as determined through measurements and calculations related to novel array designs, is reported in the context of the evolution of AMFPI pixel design. For these studies, dark, optical, and radiation signal measurements were performed on prototype imagers incorporating a variety of increasingly sophisticated array designs, with pixel pitches ranging from 75 to 127 μm. For each design, detailed measurements of fundamental pixel-level properties conducted under radiographic and fluoroscopic operating conditions are reported and the results are compared. A series of 127 μm pitch arrays employing discrete photodiodes culminated in a novel design providing an optical fill factor of ∼80% (thereby assuring improved x-ray sensitivity), and demonstrating low dark current, very low charge trapping and charge release, and a large range of linear signal response. In two of the designs having 75 and 90 μm pitches, a novel continuous photodiode structure was found to provide fill factors that approach the theoretical maximum of 100%. Both sets of novel designs achieved large fill factors by employing architectures in which some, or all of the photodiode structure was elevated above the plane of the pixel addressing transistor. Generally, enhancement of the fill factor in either discrete or continuous photodiode arrays was observed to result in no degradation in MTF due to charge sharing between pixels. While the continuous designs exhibited relatively high levels of charge trapping and release, as well as shorter ranges of linearity, it is possible that these behaviors can be addressed through further refinements to pixel design. Both the continuous and the most recent discrete photodiode designs accommodate more sophisticated pixel circuitry than is present on conventional AMFPIs – such as a pixel clamp circuit, which is demonstrated to limit signal saturation under conditions corresponding to high exposures. It is anticipated that photodiode structures such as the ones reported in this study will enable the development of even more complex pixel circuitry, such as pixel-level amplifiers, that will lead to further significant improvements in imager performance.

Megavoltage cone-beam computed tomography (MV CBCT) is a highly promising technique for providing volumetric patient position information in the radiation treatment room. Such information has the potential to greatly assist in registering the patient to the planned treatment position, helping to ensure accurate delivery of the high energy therapy beam to the tumor volume while sparing the surrounding normal tissues. Presently, CBCT systems using conventional MV active matrix flat-panel imagers (AMFPIs), which are commonly used in portal imaging, require a relatively large amount of dose to create images that are clinically useful. This is due to the fact that the phosphor screen detector employed in conventional MV AMFPIs utilizes only {approx}2% of the incident radiation (for a 6 MV x-ray spectrum). Fortunately, thick segmented scintillating detectors can overcome this limitation, and the first prototype imager has demonstrated highly promising performance for projection imaging at low doses. It is therefore of definite interest to examine the potential performance of such thick, segmented scintillating detectors for MV CBCT. In this study, Monte Carlo simulations of radiation energy deposition were used to examine reconstructed images of cylindrical CT contrast phantoms, embedded with tissue-equivalent objects. The phantoms were scanned at 6 MV using segmentedmore » detectors having various design parameters (i.e., detector thickness as well as scintillator and septal wall materials). Due to constraints imposed by the nature of this study, the size of the phantoms was limited to {approx}6 cm. For such phantoms, the simulation results suggest that a 40 mm thick, segmented CsI detector with low density septal walls can delineate electron density differences of {approx}2.3% and 1.3% at doses of 1.54 and 3.08 cGy, respectively. In addition, it was found that segmented detectors with greater thickness, higher density scintillator material, or lower density septal walls exhibit higher contrast-to-noise performance. Finally, the performance of various segmented detectors obtained at a relatively low dose (1.54 cGy) was compared with that of a phosphor screen similar to that employed in conventional MV AMFPIs. This comparison indicates that for a phosphor screen to achieve the same contrast-to-noise performance as the segmented detectors {approx}18 to 59 times more dose is required, depending on the configuration of the segmented detectors.« less

Empirical and theoretical investigations of the performance of a small-area, high-spatial-resolution, active matrix flat-panel imager, operated under mammographic conditions, is reported. The imager is based on an indirect detection array incorporating a continuous photodiode design, as opposed to the discrete photodiode design employed in conventional flat-panel imagers. Continuous photodiodes offer the prospect of higher fill factors, particularly for arrays with pixel pitches below ∼ 100 μ m . The array has a pixel-to-pixel pitch of 75 μ m and a pixel format of 512 × 512 , resulting in an active area of ∼ 3.8 × 3.8 cm 2 . The array was coupled to two commercially available, structured CsI : Tl scintillators of ∼ 150 μ m thickness: one optimized for high light output (FOS-HL) and the other for high spatial resolution (FOS-HR), resulting in a pair of imager configurations. Measurements of sensitivity, modulation transfer function(MTF),noise power spectra (NPS), and detective quantum efficiency (DQE) were performed with a 26 kVp mammography beam at exposures ranging from ∼ 0.5 to ∼ 19 mR . MTF results from both CsI : Tl scintillators show that the array demonstrates good spatial resolution, indicating effective isolation between adjacent pixels. The effect of additive noise of the system on DQE was observed to be significantly higher for the FOS-HR scintillator compared to the FOS-HL scintillator due to lower sensitivity of the former. For the FOS-HL scintillator, DQE performance was generally high at high exposures, limited by the x-ray quantum efficiency, Swank factor and the MTF of the scintillators. For both scintillators, the DQE performance degrades at lower exposures due to the relatively large contribution of additive noise. Theoretical calculations based on a cascaded systems model were found to be in general agreement with the empirically determined NPS and DQE values. Finally, such calculations were used to predict potential DQE performance for hypothetical 50 μ m pixel pitch imagers, employing similar continuous photodiode design and realistic inputs derived from the empirical measurements.

Modern-day radiotherapy relies on highly sophisticated forms of image guidance in order to implement increasingly conformal treatment plans and achieve precise dose delivery. One of the most important goals of such image guidance is to delineate the clinical target volume from surrounding normal tissue during patient setup and dose delivery, thereby avoiding dependence on surrogates such as bony landmarks. In order to achieve this goal, it is necessary to integrate highly efficient imaging technology, capable of resolving soft-tissue contrast at very low doses, within the treatment setup. In this paper we report on the development of one such modality, which comprises a nonoptimized, prototype electronic portal imaging device (EPID) based on a 40 mm thick, segmented crystalline CsI(Tl) detector incorporated into an indirect-detection active matrix flat panel imager (AMFPI). The segmented detector consists of a matrix of 160 × 160 optically isolated, crystalline CsI(Tl) elements spaced at 1016 μ m pitch. The detector was coupled to an indirect detection-based active matrix array having a pixel pitch of 508 μ m , with each detector element registered to 2 × 2 array pixels. The performance of the prototype imager was evaluated under very low-dose radiotherapy conditions and compared to that of a conventional megavoltage AMFPI based on a Lanex Fast-B phosphor screen. Detailed quantitative measurements were performed in order to determine the x-ray sensitivity, modulation transfer function, noise power spectrum, and detective quantum efficiency (DQE). In addition, images of a contrast-detail phantom and an anthropomorphic head phantom were also acquired. The prototype imager exhibited approximately 22 times higher zero-frequency DQE ( ∼ 22 % ) compared to that of the conventional AMFPI ( ∼ 1 % ) . The measured zero-frequency DQE was found to be lower than theoretical upper limits ( ∼ 27 % ) calculated from Monte Carlo simulations, which were based solely on the x-ray energy absorbed in the detector—indicating the presence of optical Swank noise. Moreover, due to the nonoptimized nature of this prototype, the spatial resolution was observed to be significantly lower than theoretical expectations. Nevertheless, due to its high quantum efficiency ( ∼ 55 % ) , the prototype imager exhibited significantly higher DQE than that of the conventional AMFPI across all spatial frequencies. In addition, the frequency-dependent DQE was observed to be relatively invariant with respect to the amount of incident radiation, indicating x-ray quantum limited behavior. Images of the contrast-detail phantom and the head phantom obtained using the prototype system exhibit good visualization of relatively large, low-contrast features, and appear significantly less noisy compared to similar images from a conventional AMFPI. Finally, Monte Carlo-based theoretical calculations indicate that, with proper optimization, further, significant improvements in the DQE performance of such imagers could be achieved. It is strongly anticipated that the realization of optimized versions of such very high-DQE EPIDs would enable megavoltage projection imaging at very low doses, and tomographic imaging from a “beam’s eye view” at clinically acceptable doses.

Electronic portal imaging devices (EPIDs) based on indirect detection, active matrix flat panel imagers (AMFPIs) have become the technology of choice for geometric verification of patient localization and dose delivery in external beam radiotherapy. However, current AMFPI EPIDs, which are based on powdered-phosphor screens, make use of only approximately 2% of the incident radiation, thus severely limiting their imaging performance as quantified by the detective quantum efficiency (DQE) (approximately 1%, compared to approximately 75% for kilovoltage AMFPIs). With the rapidly increasing adoption of image-guided techniques in virtually every aspect of radiotherapy, there exist strong incentives to develop high-DQE megavoltage x-ray imagers, capable of providing soft-tissue contrast at very low doses in megavoltage tomographic and, potentially, projection imaging. In this work we present a systematic theoretical and preliminary empirical evaluation of a promising, high-quantum-efficiency, megavoltage x-ray detector design based on a two-dimensional matrix of thick, optically isolated, crystalline scintillator elements. The detector is coupled with an indirect detection-based active matrix array, with the center-to-center spacing of the crystalline elements chosen to match the pitch of the underlying array pixels. Such a design enables the utilization of a significantly larger fraction of the incident radiation (up to 80% for a 6 MV beam), through increases in the thickness of the crystalline elements, without loss of spatial resolution due to the spread of optical photons. Radiation damage studies were performed on test samples of two candidate scintillator materials, CsI(Tl) and BGO, under conditions relevant to radiotherapy imaging. A detailed Monte Carlo-based study was performed in order to examine the signal, spatial spreading, and noise properties of the absorbed energy for several segmented detector configurations. Parameters studied included scintillator material, septal wall material, detector thickness, and the thickness of the septal walls. The results of the Monte Carlo simulations were used to estimate the upper limits of the modulation transfer function, noise power spectrum and the DQE for a select number of configurations. An exploratory, small-area prototype segmented detector was fabricated by infusing crystalline CsI(Tl) in a 2 mm thick tungsten matrix, and the signal response was measured under radiotherapy imaging conditions. Results from the radiation damage studies showed that both CsI(Tl) and BGO exhibited less than approximately 15% reduction in light output after 2500 cGy equivalent dose. The prototype CsI(Tl) segmented detector exhibited high uniformity, but a lower-than-expected magnitude of signal response. Finally, results from Monte Carlo studies strongly indicate that high scintillator-fill-factor configurations, incorporating high-density scintillator and septal wall materials, could achieve up to 50 times higher DQE compared to current AMFPI EPIDs.

Current electronic portal imaging devices (EPIDs) based on active matrix flat panel imager (AMFPI) technology use a metal plate + phosphor screen combination for x-ray conversion. As a result, these devices face a severe trade-off between x-ray quantum efficiency (QE) and spatial resolution, thus, significantly limiting their imaging performance. In this work, we present a novel detector design for indirect detection-based AMFPI EPIDs that aims to circumvent this trade-off. The detectors were developed using micro-electro-mechanical system (MEMS)-based fabrication techniques and consist of a grid of up to ∼ 2 mm tall, optically isolated cells of a photoresist material, SU-8. The cells are dimensionally matched to the pixels of the AMFPI array, and packed with a scintillating phosphor. In this paper, various design considerations for such detectors are examined. An empirical evaluation of three small-area ( ∼ 7 × 7 cm 2 ) prototype detectors is performed in order to study the effects of two design parameters—cell height and phosphor packing density, both of which are important determinants of the imaging performance. Measurements of the x-ray sensitivity, modulation transfer function(MTF) and noise power spectrum (NPS) were performed under radiotherapy conditions ( 6 MV ) , and the detective quantum efficiency (DQE) was determined for each prototype SU-8 detector. In addition, theoretical calculations using Monte Carlo simulations were performed to determine the QE of each detector, as well as the inherent spatial resolution due to the spread of absorbed energy. The results of the present studies were compared with corresponding measurements published in an earlier study using a Lanex Fast-B phosphor screen coupled to an indirect detection array of the same design. The SU-8 detectors exhibit up to 3 times higher QE, while achieving spatial resolution comparable or superior to Lanex Fast-B. However, the DQE performance of these early prototypes is significantly lower than expected due to high levels of optical Swank noise. Consequently, the SU-8 detectors presently exhibit DQE values comparable to Lanex Fast-B at zero spatial frequency and significantly lower than Fast-B at higher frequencies. Finally, strategies for reducing Swank noise are discussed and theoretical calculations, based on the cascaded systems model, are presented in order to estimate the performance improvement that can be achieved through such noise reduction.

Electronic portal imaging devices based on megavoltage (MV), active matrix, flat-panel imagers (AMFPIs) are presently regarded as the gold standard in portal imaging for external beam radiation therapy. These devices, employing indirect detection of incident radiation by means of a metal plate plus phosphor screen combination, offer a quantum efficiency of only approximately 2% at 6 MV, leading to a detective quantum efficiency (DQE) of only approximately 1%. In order to significantly improve the DQE performance of MV AMFPIs, a strategy based on the development of direct detection imagers incorporating thick films of polycrystalline mercuric iodide (HgI2) photoconductor was undertaken and is reported.Two MV AMFPI prototypes, one incorporating an approximately 300 microm thick HgI2 layer created through physical vapor deposition (PVD) and a second incorporating an approximately 460 microm thick HgI2 layer created through screen-printing of particle-in-binder (PIB) material, were quantitatively evaluated using a 6 MV photon beam. The reported measurements include empirical determination of x-ray sensitivity, lag, modulation transfer function (MTF), noise power spectrum, and DQE.For both prototypes, MTF and DQE results were found to be consistent with theoretical expectations and the MTFs were also found to be higher than that measured from a conventional MV AMFPI. In addition, the DQE results exhibit input-quantum-limited behavior, even at extremely low doses. Compared to PVD, the PIB prototype exhibits much lower dark current, slightly higher lag, and similar DQE. Finally, the challenges associated with this approach, as well as strategies for achieving considerably higher DQE through thicker HgI2 layers, are discussed.The DQE of each of the prototypes is found to be comparable to that of conventional MV AMFPIs, commensurate with the modest photoconductor thicknesses of these early samples. It is anticipated that thicker layers of HgI2 based on PIB deposition can provide higher DQE while maintaining good material properties.

Active matrix flat-panel imager (AMFPI) technology is being employed for an increasing variety of imaging applications. An important element in the adoption of this technology has been significant ongoing improvements in optical signal collection achieved through innovations in indirect detection array pixel design. Such improvements have a particularly beneficial effect on performance in applications involving low exposures and/or high spatial frequencies, where detective quantum efficiency is strongly reduced due to the relatively high level of additive electronic noise compared to signal levels of AMFPI devices. In this article, an examination of various signal properties, as determined through measurements and calculations related to novel array designs, is reported in the context of the evolution of AMFPI pixel design. For these studies, dark, optical, and radiation signal measurements were performed on prototype imagers incorporating a variety of increasingly sophisticated array designs, with pixel pitches ranging from 75 to 127 microm. For each design, detailed measurements of fundamental pixel-level properties conducted under radiographic and fluoroscopic operating conditions are reported and the results are compared. A series of 127 microm pitch arrays employing discrete photodiodes culminated in a novel design providing an optical fill factor of approximately 80% (thereby assuring improved x-ray sensitivity), and demonstrating low dark current, very low charge trapping and charge release, and a large range of linear signal response. In two of the designs having 75 and 90 microm pitches, a novel continuous photodiode structure was found to provide fill factors that approach the theoretical maximum of 100%. Both sets of novel designs achieved large fill factors by employing architectures in which some, or all of the photodiode structure was elevated above the plane of the pixel addressing transistor. Generally, enhancement of the fill factor in either discrete or continuous photodiode arrays was observed to result in no degradation in MTF due to charge sharing between pixels. While the continuous designs exhibited relatively high levels of charge trapping and release, as well as shorter ranges of linearity, it is possible that these behaviors can be addressed through further refinements to pixel design. Both the continuous and the most recent discrete photodiode designs accommodate more sophisticated pixel circuitry than is present on conventional AMFPIs--such as a pixel clamp circuit, which is demonstrated to limit signal saturation under conditions corresponding to high exposures. It is anticipated that photodiode structures such as the ones reported in this study will enable the development of even more complex pixel circuitry, such as pixel-level amplifiers, that will lead to further significant improvements in imager performance.

Megavoltage cone-beam computed tomography (MV CBCT) is a highly promising technique for providing volumetric patient position information in the radiation treatment room. Such information has the potential to greatly assist in registering the patient to the planned treatment position, helping to ensure accurate delivery of the high energy therapy beam to the tumor volume while sparing the surrounding normal tissues. Presently, CBCT systems using conventional MV active matrix flat-panel imagers (AMFPIs), which are commonly used in portal imaging, require a relatively large amount of dose to create images that are clinically useful. This is due to the fact that the phosphor screen detector employed in conventional MV AMFPIs utilizes only approximately 2% of the incident radiation (for a 6 MV x-ray spectrum). Fortunately, thick segmented scintillating detectors can overcome this limitation, and the first prototype imager has demonstrated highly promising performance for projection imaging at low doses. It is therefore of definite interest to examine the potential performance of such thick, segmented scintillating detectors for MV CBCT. In this study, Monte Carlo simulations of radiation energy deposition were used to examine reconstructed images of cylindrical CT contrast phantoms, embedded with tissue-equivalent objects. The phantoms were scanned at 6 MV using segmented detectors having various design parameters (i.e., detector thickness as well as scintillator and septal wall materials). Due to constraints imposed by the nature of this study, the size of the phantoms was limited to approximately 6 cm. For such phantoms, the simulation results suggest that a 40 mm thick, segmented CsI detector with low density septal walls can delineate electron density differences of approximately 2.3% and 1.3% at doses of 1.54 and 3.08 cGy, respectively. In addition, it was found that segmented detectors with greater thickness, higher density scintillator material, or lower density septal walls exhibit higher contrast-to-noise performance. Finally, the performance of various segmented detectors obtained at a relatively low dose (1.54 cGy) was compared with that of a phosphor screen similar to that employed in conventional MV AMFPIs. This comparison indicates that for a phosphor screen to achieve the same contrast-to-noise performance as the segmented detectors approximately 18 to 59 times more dose is required, depending on the configuration of the segmented detectors.

Empirical and theoretical investigations of the performance of a small-area, high-spatial-resolution, active matrix flat-panel imager, operated under mammographic conditions, is reported. The imager is based on an indirect detection array incorporating a continuous photodiode design, as opposed to the discrete photodiode design employed in conventional flat-panel imagers. Continuous photodiodes offer the prospect of higher fill factors, particularly for arrays with pixel pitches below approximately 100 microm. The array has a pixel-to-pixel pitch of 75 microm and a pixel format of 512 x 512, resulting in an active area of approximately 3.8 x 3.8 cm2. The array was coupled to two commercially available, structured CsI: Tl scintillators of approximately 150 microm thickness: one optimized for high light output (FOS-HL) and the other for high spatial resolution (FOS-HR), resulting in a pair of imager configurations. Measurements of sensitivity, modulation transfer function (MTF), noise power spectra (NPS), and detective quantum efficiency (DQE) were performed with a 26 kVp mammography beam at exposures ranging from approximately 0.5 to approximately 19 mR. MTF results from both CsI:Tl scintillators show that the array demonstrates good spatial resolution, indicating effective isolation between adjacent pixels. The effect of additive noise of the system on DQE was observed to be significantly higher for the FOS-HR scintillator compared to the FOS-HL scintillator due to lower sensitivity of the former. For the FOS-HL scintillator, DQE performance was generally high at high exposures, limited by the x-ray quantum efficiency, Swank factor and the MTF of the scintillators. For both scintillators, the DQE performance degrades at lower exposures due to the relatively large contribution of additive noise. Theoretical calculations based on a cascaded systems model were found to be in general agreement with the empirically determined NPS and DQE values. Finally, such calculations were used to predict potential DQE performance for hypothetical 50 microm pixel pitch imagers, employing similar continuous photodiode design and realistic inputs derived from the empirical measurements.

Electronic portal imaging devices (EPIDs) based on indirect detection, active matrix flat panel imagers (AMFPIs) have become the technology of choice for geometric verification of patient localization and dose delivery in external beam radiotherapy. However, current AMFPI EPIDs, which are based on powdered-phosphor screens, make use of only approximately 2% of the incident radiation, thus severely limiting their imaging performance as quantified by the detective quantum efficiency (DQE) (approximately 1%, compared to approximately 75% for kilovoltage AMFPIs). With the rapidly increasing adoption of image-guided techniques in virtually every aspect of radiotherapy, there exist strong incentives to develop high-DQE megavoltage x-ray imagers, capable of providing soft-tissue contrast at very low doses in megavoltage tomographic and, potentially, projection imaging. In this work we present a systematic theoretical and preliminary empirical evaluation of a promising, high-quantum-efficiency, megavoltage x-ray detector design based on a two-dimensional matrix of thick, optically isolated, crystalline scintillator elements. The detector is coupled with an indirect detection-based active matrix array, with the center-to-center spacing of the crystalline elements chosen to match the pitch of the underlying array pixels. Such a design enables the utilization of a significantly larger fraction of the incident radiation (up to 80% for a 6 MV beam), through increases in the thickness of the crystalline elements, without loss of spatial resolution due to the spread of optical photons. Radiation damage studies were performed on test samples of two candidate scintillator materials, CsI(Tl) and BGO, under conditions relevant to radiotherapy imaging. A detailed Monte Carlo-based study was performed in order to examine the signal, spatial spreading, and noise properties of the absorbed energy for several segmented detector configurations. Parameters studied included scintillator material, septal wall material, detector thickness, and the thickness of the septal walls. The results of the Monte Carlo simulations were used to estimate the upper limits of the modulation transfer function, noise power spectrum and the DQE for a select number of configurations. An exploratory, small-area prototype segmented detector was fabricated by infusing crystalline CsI(Tl) in a 2 mm thick tungsten matrix, and the signal response was measured under radiotherapy imaging conditions. Results from the radiation damage studies showed that both CsI(Tl) and BGO exhibited less than approximately 15% reduction in light output after 2500 cGy equivalent dose. The prototype CsI(Tl) segmented detector exhibited high uniformity, but a lower-than-expected magnitude of signal response. Finally, results from Monte Carlo studies strongly indicate that high scintillator-fill-factor configurations, incorporating high-density scintillator and septal wall materials, could achieve up to 50 times higher DQE compared to current AMFPI EPIDs.

Current electronic portal imaging devices (EPIDs) based on active matrix flat panel imager (AMFPI) technology use a metal plate+phosphor screen combination for x-ray conversion. As a result, these devices face a severe trade-off between x-ray quantum efficiency (QE) and spatial resolution, thus, significantly limiting their imaging performance. In this work, we present a novel detector design for indirect detection-based AMFPI EPIDs that aims to circumvent this trade-off. The detectors were developed using micro-electro-mechanical system (MEMS)-based fabrication techniques and consist of a grid of up to approximately 2 mm tall, optically isolated cells of a photoresist material, SU-8. The cells are dimensionally matched to the pixels of the AMFPI array, and packed with a scintillating phosphor. In this paper, various design considerations for such detectors are examined. An empirical evaluation of three small-area (approximately 7 x 7 cm2) prototype detectors is performed in order to study the effects of two design parameters--cell height and phosphor packing density, both of which are important determinants of the imaging performance. Measurements of the x-ray sensitivity, modulation transfer function (MTF) and noise power spectrum (NPS) were performed under radiotherapy conditions (6 MV), and the detective quantum efficiency (DQE) was determined for each prototype SU-8 detector. In addition, theoretical calculations using Monte Carlo simulations were performed to determine the QE of each detector, as well as the inherent spatial resolution due to the spread of absorbed energy. The results of the present studies were compared with corresponding measurements published in an earlier study using a Lanex Fast-B phosphor screen coupled to an indirect detection array of the same design. The SU-8 detectors exhibit up to 3 times higher QE, while achieving spatial resolution comparable or superior to Lanex Fast-B. However, the DQE performance of these early prototypes is significantly lower than expected due to high levels of optical Swank noise. Consequently, the SU-8 detectors presently exhibit DQE values comparable to Lanex Fast-B at zero spatial frequency and significantly lower than Fast-B at higher frequencies. Finally, strategies for reducing Swank noise are discussed and theoretical calculations, based on the cascaded systems model, are presented in order to estimate the performance improvement that can be achieved through such noise reduction.

Purpose: Active matrix flat‐panel imager technology is being introduced to an ever‐increasing variety of imaging applications. This has been greatly facilitated by improvements in optical signal collection achieved through innovations in array pixel design. Such improvements are particularly important for applications involving low exposures and high spatial frequencies, where DQE can be strongly attenuated due to relatively high additive electronic noise. In this presentation, recent innovations to pixel design are described and the effect on various signal properties, as determined through measurements and calculations related to novel prototypes, is reported. Method and Materials: Optical and radiation signal measurements were performed on prototype imagers incorporating a series of six increasingly sophisticated array designs, with pixel pitches ranging from 75 to 127 μm. The most recent discrete photodiode array employs aggressive design rules that significantly increase optical fill factor while also incorporating a clamp circuit. Two other array designs incorporate a continuous photodiode structure with the goal of providing an optical fill factor of 100%. Results: The new 127 μm pitch discrete photodiode design achieves a sensitivity consistent with its design goal of an ∼85% optical fill factor while the 75 and 90 μm pitch continuous photodiode designs demonstrate sensitivity corresponding to an ∼95% fill factor. These designs demonstrated no degradation in MTF due to charge sharing and the pixel clamp reduces memory effects at high signal levels. The effects of these enhanced sensitivities on DQE are illustrated and compared to the performance of arrays based on earlier generations of design. Conclusion: Aggressive application of design rules can achieve very high levels of optical fill factor and sensitivity resulting in improvements in DQE. Continuous photodiode designs extend optical fill factors almost to the theoretical limit, even for very small pixels, and are well suited to designs containing more complex pixel circuits.

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: A systematic theoretical investigation of Swank factor for thick, segmented crystalline scintillating detectors is reported. The Swank noise, originating from variation in the x‐ray‐to‐photon conversion gain, degrades the detective quantum efficiency (DQE) performance of radiotherapyimagers. Therefore, it is of great interest to examine the Swank factor, in particular the optical Swank factor, as a function of various detector design parameters. Method and Materials: Swank factor was investigated using a recently implemented Monte Carlo package (MANTIS) that simulates both radiation and optical transport. The radiation and optical Swank factors were examined using a 6 MV photon beam as a function of various parameters, such as the material and thickness of the scintillator, the element‐to‐element pitch of the detector, as well as the material and thickness of the septal wall separating detector elements. Moreover, the optical Swank factor was examined as a function of the optical properties of the scintillator, top reflector and septal wall. Results:Radiation Swank factor improved when thicker scintillators or higher density septal walls were employed. The optical Swank factor dropped dramatically when the scintillator surfaces facing the walls were not polished. Moreover, the results indicate that septal walls must be highly reflective, as the optical Swank factor was found to drop sharply with decreasing septal wall reflectivity, even for a relatively clear scintillator. Furthermore, the optical Swank factor was also strongly affected by the thickness, absorption and scattering of the scintillator, and modestly affected by the reflectivity of the top reflector. Conclusion: Simulation of radiation and optical transport in segmented scintillating detectors provides the means to examine properties that are critically important to performance, such as optical Swank factor. It is strongly anticipated that such studies will greatly assist in the design of detectors exhibiting very high DQE.

Purpose: The relatively high level of additive noise in active matrix flat‐panel imagers (AMFPIs) leads to significant loss of DQE under conditions of low exposures per frame and/or high spatial frequencies. One promising method for dealing with such performance loss involves in‐pixel signal amplification through incorporation of additional circuitry to each pixel — a concept generally referred to as an active pixel sensor. In this presentation, an examination of performance levels that can be achieved with active pixel sensors based on thin‐film electronics will be presented. Method and Materials: Calculations of the DQE of various active pixel sensor designs were performed using the cascaded systems formalism. Inputs to the calculations come from empirical measurements, published data, analytic calculations, and results derived from detailed circuit simulations. The calculations were performed for a series of designs, under fluoroscopic irradiation conditions, and using various readout protocols. Results: Our model calculations indicate that, under conditions of low exposure and/or high spatial frequency, substantial DQE restoration can be achieved via a combination of in‐pixel amplification circuitry and correlated double sampling protocols — each specifically selected to eliminate specific noise contributions. In particular, it appears feasible to reduce thermal noise associated with the pixel reset TFT and preamplifier noise to negligible levels and thereby increase DQE. However, the degree of performance improvement that can be practically realized will also depend upon the quality and properties of the thin film transistors used in the pixel circuit as well as selection of circuits that will yield well in manufacture. Conclusion: The degree of performance improvement made possible through array architectures comprising active pixel sensors is critically dependent on many factors including the properties of the transistors, the engineering rules governing pixel and array design, the choice of pixel circuit, and the method of array readout.

Purpose: The recent development of techniques to create high‐quality amorphous silicon (a‐Si:H) at relatively low deposition temperatures enables the creation of active matrix arrays of thin‐film transistors(TFTs) on very thin, flexible plastic sheets, rather than on thick, rigid glass substrates. In this presentation, an examination of the potential advantages and theoretical performance of indirect detectionmammographicimagers based upon such arrays will be reported. Methods and Materials: Prototype active matrix arrays of thin‐film transistors based on low‐temperature a‐Si:H and deposited on plastic substrates have demonstrated TFT performance essentially equivalent to that of devices produced with conventional a‐Si:H. In addition, other prototype imagers have demonstrated that the incorporation of continuous photodiode structures can provide improved signal gain compared to arrays with discrete photodiodes at small pixel pitches. Techniques based on Monte Carlo simulations and cascaded systems analysis, parameterized by empirical information obtained from these early prototypes, have been employed to explore the performance of optimized imaging array designs operated under mammographicimaging conditions. Results: The excellent performance of transistors and photodiodes fabricated from low temperature a‐Si:H, coupled with the incorporation of continuous photodiode structures, provides good signal and noise characteristics, even for sub‐100 μm pitch designs. In addition, the flexibility and x‐ray transparency of a thin plastic substrate allows for the possibility of conforming the shape of the detector to an arc and to position the scinitillator (and opposing active matrix array circuits) on the opposite side of the substrate relative to the x‐ray source — leading to improvements in spatial resolution and DQE. Conclusion: Early investigations of the potential performance of indirect detection active matrix mammographicimagers based on low‐temperature a‐Si:H and fabricated on plastic substrates suggest that significant advantages would accrue from the development and implementation of such devices.

Purpose: To investigate the potential of achieving significant improvements in the DQE performance of active‐matrix flat‐panel imagers at low fluoroscopic exposures and high spatial frequencies through incorporation of novel pixel architectures based on polycrystallinesilicon thin‐film transistors(TFTs). Methods and Materials: Detailed empirical studies have recently been performed on the signal and noise characteristics of a series of arrays incorporating poly‐Si TFTs. These indirect detection designs involved three pixel architectures employing either a single TFT switch, a single‐stage amplifier, or a dual‐stage amplifier — along with a continuous photodiode structure. Determinations of MTF, NPS, and DQE, as well as of individual pixel properties (sensitivity, linearity, trapping, noise) were performed under fluoroscopic and radiographic conditions. Circuit simulations were also performed to explore the potential performance of these and other hypothetical array designs. Results: The studies indicate that the high mobilities of poly‐Si lead to potential frame rates of at least an order of magnitude greater than those of conventional arrays with a‐Si:H TFTs. In addition, the single‐ and dual‐stage pixel‐amplifier arrays demonstrate signal gain (∼×10 and ∼×25, respectively) very close to design expectations. Furthermore, empirical data taken from these early prototypes demonstrate a small, but non‐negligible enhancement in signal‐to‐noise performance compared to that of similar arrays using conventional designs, as a result of pixel amplification and the use of repeated, non‐destructive readout. Analysis based on these empirical results and circuit simulations indicates that, with circuit design optimization and improved TFT quality, further significant enhancement of performance should be possible. Conclusion: These results indicate that substantial improvements in DQE performance are possible through incorporation of poly‐Si circuits in flat panel pixel designs. Factors limiting the performance of present designs will be described and future steps in the development of this technology will be discussed. This work is supported by NIH grant R01 EB000558.

Purpose: To investigate the potential for achieving significant improvements in DQE at low fluoroscopic exposures and high spatial frequencies, as well as at substantially higher frame rates, through the incorporation of novel pixel architectures based on polycrystallinesilicon thin‐film transistors (poly‐Si TFTs).Method and Materials: Detailed studies were performed on the signal and noise characteristics of a series of arrays incorporating poly‐Si TFTs. These indirect detection designs involved three pixel architectures with a standard circuit, with a single‐stage amplifier, and with a dual‐stage amplifier — along with a continuous photodiode structure. Determinations of MTF, NPS, and DQE, as well as of individual pixel properties (sensitivity, linearity, trapping, noise) were performed under fluoroscopic and radiographic conditions. Detailed studies of these and other hypothetical array designs were also performed using circuit simulation tools. Results: These studies indicate that the high mobilities of poly‐Si lead to potential frame rates at least an order of magnitude greater than those of conventional arrays with a‐Si:H TFTs. In addition, the single‐stage and dual‐stage pixel‐amplifier arrays exhibit signal gain (∼×11 and ∼×25, respectively) very close to that expected for these designs. Furthermore, while a net increase in the signal‐to‐noise performance was beyond the objective of these initial designs, analysis of the empirical data along with theoretical modeling strongly suggests that there are no intrinsic reasons precluding such performance enhancements. Finally, the non‐destructive nature of the readout for pixel‐amplifier designs enables repeated‐signal‐sampling, thereby creating new possibilities for noise reduction. Conclusion: These results encourage the hypothesis that substantial improvements in DQE performance and readout speed are possible through the incorporation of poly‐Si circuits into flat‐panel pixel designs. Factors limiting the performance of present designs will be described and future steps in the development of this technology will be discussed. This work is supported by NIH grant R01 EB000558.

Purpose: In order to explore the potential for significantly improving the DQE of megavoltage active matrix flat panel imagers (AMFPIs), two forms of thick, polycrystalline mercuric iodide ( HgI 2 ) photoconductors have been investigated. Method and Materials: Prototype direct‐detection AMFPIs with an effective pitch of 508 μm and a pixel format of 192×192 were coated with two types of HgI 2 : a “PVD” form created through physical vapor deposition and a “PIB” (particle‐in‐binder) form involving screen‐printing. Results were compared to those from a conventional indirect‐detection megavoltage AMFPI employing a phosphor screen. Data were obtained fluoroscopically, using a 6 MV beam at very low doses, equivalent to ∼0.004 to 0.04 cGy. Results: Compared to PVD, the PIB array exhibits much lower dark current, lower dark drift, slightly higher lag and similar non‐uniformity, linearity and DQE. For both PVD and PIB, MTF and DQE results are in a good agreement with theoretical expectations, and the MTF is higher than that from the conventional megavoltage AMFPI. Moreover, the DQE results show input‐quantum‐limited behavior, even at very low doses. Finally, zero‐frequency DQE values are ∼1.4% and 1.2% for PVD and PIB, respectively, matching the theoretical upper limits for the thickness of the converters used. Given the modest photoconductor thickness of these early prototypes, the DQE values compare favorably to values of ∼1% obtained from conventional megavoltage AMFPIs. Conclusion: These initial studies indicate that AMFPIs incorporating polycrystalline HgI 2 have the potential for significant dose reduction in megavoltage imaging. The use of HgI 2 offers the promise of increased quantum efficiencies through the development of substantially thicker converters than have thus far been achieved, without significant degradation of the spatial resolution. In addition, compared to PVD, PIB appears to provide better prospects for such thick coatings, leading to higher DQE. This work is supported by NIH grant R01‐CA51397.

Purpose: To characterize the performance of prototype direct‐detection active matrix flat‐panel imagers (AMFPIs) employing recently improved forms of polycrystalline mercuric iodide photoconductors as x‐ray converters for fluoroscopic imaging.Method and Materials: While direct and indirect detection flat‐panel imagers offer good performance for many applications, their modest signal per interacting x‐ray limits the DQE performance under imaging conditions of low exposure such as in fluoroscopy. One possible pathway to overcome this limitation involves the use of a high gain photoconductive x‐ray converter such as mercuric iodide (HgI2). Accordingly, performance results from a number of AMFPI prototypes employing two types of polycrystalline HgI2 (a form created through physical vapor deposition,PVD, and a particle‐in‐binder form involving screen printing, PIB) are reported. Each prototype incorporates an array with a pixel format of 768×768 and a pixel pitch of 127 μm, and was operated under fluoroscopic conditions. The measured performance was compared to theoretical calculations based on a cascaded systems model. Results: Performance of the prototypes is reported in terms of pixel properties as well as MTF, NPS and DQE. Recent significant improvements in the properties of HgI2photoconductors, such as chemical stability, low dark current (< 10 pA/mm2), low lag, and high x‐ray sensitivity (corresponding to an effective ionization energy approaching the theoretical limit of ∼5 eV) resulted in a high DQE performance at low exposures. Pixel‐to‐pixel non‐uniformities, which tend to reduce the dynamic range, remain high (∼10% to 30%) and thus require further optimization. Conclusions: While the development of polycrystalline HgI2 as an x‐ray converter for AMFPIs has achieved important milestones related to various material properties, various challenges remain. Nevertheless, the high x‐ray sensitivity and DQE observed at low exposures demonstrate the potential for input‐quantum‐limited fluoroscopic operation. This work was supported by NIH grant R01‐EB000558.