Minimization of image lag in polycrystalline mercuric iodide converters through incorporation of Frisch grid structures for digital breast tomosynthesis.

Objective. Polycrystalline mercuric iodide photoconductive converters fabricated using particle-in-binder techniques (PIB HgI2) provide significantly more detected charge per x-ray interaction than from a-Se and CsI:Tl converters commonly used with active matrix flat-panel imagers (AMFPIs). This enhanced sensitivity makes PIB HgI2an interesting candidate for applications involving low x-ray exposures-since the relatively high levels of additive electronic noise exhibited by AMFPIs incorporating a-Se and CsI:Tl reduce detective quantum efficiency (DQE) performance under such conditions. A theoretical study is reported on an approach for addressing a major challenge impeding practical use of PIB HgI2converters-the high lag exhibited by the material (over 10%) which would lead to undesirable image artifacts in applications involving acquisition of consecutive images such as digital breast tomosynthesis.Approach. Charge transport modeling accounting for the trapping and release of holes (thought to be the primary contributor to lag) was used to examine signal properties, including lag, of pillar-supported Frisch grids embedded in the photoconductor for 100µm pitch AMFPI pixels. Performance was examined as a function of electrode voltage, grid pitch (center-to-center distance between neighboring grid wires) and the ratio of grid wire width to grid pitch.Main results. Optimum grid designs maximizing suppression of signal generated by hole transport, without significantly affecting the total signal due to electron and hole transport, were identified and MTF was determined. For the most favorable designs, additional modeling was used to determine DQE. The results indicate that, through judicious choice of grid design and operational conditions, first frame lag can be significantly reduced to below 1%-less than the low levels exhibited by a-Se. DQE performance is shown to be largely maintained as exposure decreases-which should help to maintain good image quality.Significance. Substantial reduction of lag in PIB HgI2converters via incorporation of Frisch grids has been demonstrated through modeling.

Theoretical investigation of the signal performance of HgI2x-ray converters incorporating a Frisch grid structure at mammographic energies.

Active matrix, flat-panel imagers (AMFPIs) suffer from decreased detective quantum efficiency under conditions of low dose per image frame (such as for digital breast tomosynthesis, fluoroscopy and cone-beam CT) due to low signal compared to the additive electronic noise. One way to address this challenge is to introduce a high-gain x-ray converter called particle-in-binder mercuric iodide (PIB HgI2) which exhibits 3-10 times higher x-ray sensitivity compared to that of a-Se and CsI:Tl converters employed in commercial AMFPI systems. However, a remaining challenge for practical implementation of PIB HgI2is the high level of image lag, which is believed to largely originate from the trapping of holes. Towards addressing this challenge, this paper reports a theoretical investigation of the use of a Frisch grid structure embedded in the converter to suppress hole signal-which would be expected to reduce image lag. The grid acts as a third electrode sandwiched between a continuous top electrode and pixelated bottom electrodes having a 100µm pitch. Signal properties of such a detector are investigated as a function of VDR (the ratio of the voltage difference between the electrodes in the region below the grid to that above the grid), grid pitch (the center-to-center distance between two neighboring grid wires) andRGRID(the ratio of grid wire width to grid pitch) for mammographic x-ray energies. The results show that smaller grid pitch suppresses hole signal to a higher degree (up to ∼96%) while a larger gap between grid wires and higher VDR provide minimally impeded electron transport. Examination of the tradeoff between maximizing electron signal and minimizing hole signal indicates that a grid design having a grid pitch of 20µm withRGRIDof 50% and 65% provides hole signal suppression of ∼93% and ∼95% for VDR of 1 and 3, respectively.

Performance evaluation of polycrystalline HgI2 photoconductors for radiation therapy imaging.

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 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. METHODS: 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. 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 HgI2 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 HgI2 based on PIB deposition can provide higher DQE while maintaining good material properties.

Empirical noise performance of prototype active pixel arrays employing polycrystalline silicon thin-film transistors.

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.

Count rate capabilities of polycrystalline silicon photon counting detectors for CBCT applications-a theoretical study.

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.

A Monte Carlo investigation of Swank noise for thick, segmented, crystalline scintillators for radiotherapy imaging.

Thick, segmented scintillating detectors, consisting of 2D matrices of scintillator crystals separated by optically opaque septal walls, hold considerable potential for significantly improving the performance of megavoltage (MV) active matrix, flat-panel imagers (AMFPIs). Initial simulation studies of the radiation transport properties of segmented detectors have indicated the possibility of significant improvement in DQE compared to conventional MV AMFPIs based on phosphor screen detectors. It is therefore interesting to investigate how the generation and transport of secondary optical photons affect the DQE performance of such segmented detectors. One effect that can degrade DQE performance is optical Swank noise (quantified by the optical Swank factor I(opt)), which is induced by depth-dependent variations in optical gain. In this study, Monte Carlo simulations of radiation and optical transport have been used to examine I(opt) and zero-frequency DQE for segmented CsI:Tl and BGO detectors at different thicknesses and element-to-element pitches. For these detectors, I(opt) and DQE were studied as a function of various optical parameters, including absorption and scattering in the scintillator, absorption at the top reflector and septal walls, as well as scattering at the side surfaces of the scintillator crystals. The results indicate that I(opt) and DQE are only weakly affected by absorption and scattering in the scintillator, as well as by absorption at the top reflector. However, in some cases, these metrics were found to be significantly degraded by absorption at the septal walls and scattering at the scintillator side surfaces. Moreover, such degradations are more significant for detectors with greater thickness or smaller element pitch. At 1.016 mm pitch and with optimized optical properties, 40 mm thick segmented CsI:Tl and BGO detectors are predicted to provide DQE values of approximately 29% and 42%, corresponding to improvement by factors of approximately 29 and 42, respectively, compared to that of conventional MV AMFPIs.

High-DQE EPIDs based on thick, segmented BGO and CsI:Tl scintillators: performance evaluation at extremely low dose.

PURPOSE: Electronic portal imaging devices (EPIDs) based on active matrix, flat-panel imagers (AMFPIs) have become the gold standard for portal imaging and are currently being investigated for megavoltage cone-beam computed tomography (CBCT) and cone-beam digital tomosynthesis (CBDT). However, the practical realization of such volumetric imaging techniques is constrained by the relatively low detective quantum efficiency (DQE) of AMFPI-based EPIDs at radiotherapy energies, approximately 1% at 6 MV. In order to significantly improve DQE, the authors are investigating thick, segmented scintillators, consisting of 2D matrices of scintillating crystals separated by septal walls. METHODS: A newly constructed segmented BGO scintillator (11.3 mm thick) and three segmented CsI:Tl scintillators (11.4, 25.6, and 40.0 mm thick) were evaluated using a 6 MV photon beam. X-ray sensitivity, modulation transfer function, noise power spectrum, DQE, and phantom images were obtained using prototype EPIDs based on the four scintillators. RESULTS: The BGO and CsI:Tl prototypes were found to exhibit improvement in DQE ranging from approximately 12 to 25 times that of a conventional AMFPI-based EPID at zero spatial frequency. All four prototype EPIDs provide significantly improved contrast resolution at extremely low doses, extending down to a single beam pulse. In particular, the BGO prototype provides contrast resolution comparable to that of the conventional EPID, but at 20 times less dose, with spatial resolution sufficient for identifying the boundaries of low-contrast objects. For this prototype, however, the BGO scintillator exhibited an undesirable radiation-induced variation in x-ray sensitivity. CONCLUSIONS: Prototype EPIDs based on thick, segmented BGO and CsI:T1 scintillators provide significantly improved portal imaging performance at extremely low dose (i.e., down to 1 beam pulse corresponding to approximately 0.022 cGy), creating the possibility of soft-tissue visualization using MV CBCT and CBDT at clinically practical dose.

Active pixel imagers incorporating pixel-level amplifiers based on polycrystalline-silicon thin-film transistors.

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.

An investigation of signal performance enhancements achieved through innovative pixel design across several generations of indirect detection, active matrix, flat-panel arrays.

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.

Monte Carlo investigations of megavoltage cone-beam CT using thick, segmented scintillating detectors for soft tissue visualization.

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.

Investigation of the signal behavior at diagnostic energies of prototype, direct detection, active matrix, flat-panel imagers incorporating polycrystalline HgI2.

Active matrix, flat-panel x-ray imagers based on a-Si:H thin-film transistors offer many advantages and are widely utilized in medical imaging applications. Unfortunately, the detective quantum efficiency (DQE) of conventional flat-panel imagers incorporating scintillators or a-Se photoconductors is significantly limited by their relatively modest signal-to-noise ratio, particularly in applications involving low x-ray exposures or high spatial resolution. For this reason, polycrystalline HgI2 is of considerable interest by virtue of its low effective work function, high atomic number and the possibility of large-area deposition. In this study, a detailed investigation of the properties of prototype, flat-panel arrays coated with two forms of this high-gain photoconductor are reported. Encouragingly, high x-ray sensitivity, low dark current and spatial resolution close to the theoretical limits were observed from a number of prototypes. In addition, input-quantum-limited DQE performance was measured from one of the prototypes at relatively low exposures. However, high levels of charge trapping, lag and polarization, as well as pixel-to-pixel variations in x-ray sensitivity are of concern. While the results of the current study are promising, further development will be required to realize prototypes exhibiting the characteristics necessary to allow practical implementation of this approach.

Theoretical investigation of the count rate capabilities of in-pixel amplifiers for photon counting arrays based on polycrystalline silicon TFTs.

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.

Slit design for efficient and accurate MTF measurement at megavoltage x-ray energies.

Empirical determination of the modulation transfer function (MTF) for analog and digital mega-voltage x-ray imagers is a challenging task. The most common method used to determine MTF at megavoltage x-ray energies employs a long, narrow slit formed by two parallel, metal blocks in order to form a "slit beam." In this work, a detailed overview of some of the important considerations of slit design is presented. Based on these considerations, a novel, compact slit, using 19 cm thick tungsten blocks, was designed. The prototype slit was configured to attach to the accessory slot of the gantry of a linear accelerator, which greatly simplified the measurement process. Measurements were performed to determine the presampling MTF at 6 MV for an indirect detection active matrix flat panel imager prototype previously developed for megavoltage imaging applications. In addition, the effects of two important slit design parameters, material type and thickness, on the accuracy of MTF determination were investigated via a Monte Carlo-based theoretical study. Empirically determined MTFs obtained from the prototype slit closely match those from an earlier, less compact slit design based on 40 cm thick steel blocks. The results of the Monte Carlo-based theoretical studies indicate that the prototype slit achieves close-to-ideal performance in terms of accurately determining the MTF by virtue of practically 100% beam attenuation in regions other than the slit gap. Furthermore, the theoretical results suggest that it may be possible to achieve even further reductions in slit thickness without compromising measurement accuracy.

Performance of a high fill factor, indirect detection prototype flat-panel imager for mammography.

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.

Performance of a direct-detection active matrix flat panel dosimeter (AMFPD) for IMRT measurements.

The dosimetric performance of a direct-detection active matrix flat panel dosimeter (AMFPD) is reported for intensity modulated radiation therapy (IMRT) measurements. The AMFPD consists of a-Si : H photodiodes and thin-film transistors deposited on a glass substrate with no overlying scintillator screen or metal plate. The device is operated at 0.8 frames per second in a continuous acquisition or fluoroscopic mode. The effect of the applied bias voltage across the photodiodes on the response of the AMFPD was evaluated because this parameter affects dark signal, lag contributions, and pixel sensitivity. In addition, the AMPFD response was evaluated as a function of dose, dose rate, and energy, for static fields at 10 cm depth. In continuous acquisition mode, the AMFPD maintained a linear dose response (r2 > 0.99999) up to at least 1040 cGy. In order to obtain reliable integrated dose results for IMRT fields, the effects of lag on the radiation signal were minimized by operating the system at the highest frame rate and at an appropriate reverse bias voltage. Segmental MLC and dynamic MLC IMRT fields were measured with the AMFPD, and the results were compared to film, using standard methods for reliable film dosimetry. Both AMFPD and film measurements were independently converted to dose in cGy. Gamma and chi values were calculated as indices of agreement. The results from the AMFPD were in excellent agreement with those from film. When 2% of D(max) and 2 mm of distance to agreement were used as the criteria, 98% of the region of interest (defined as the region where dose is greater than 5% of D(max)) satisfied [chi] < or = 1 on average across the cases that were tested.

Theoretical investigation of the noise performance of active pixel imaging arrays based on polycrystalline silicon thin film transistors.

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 - 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) resulted in significant noise reduction - 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.

Segmented crystalline scintillators: empirical and theoretical investigation of a high quantum efficiency EPID based on an initial engineering prototype CsI(TI) detector.

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 x 160 optically isolated, crystalline CsI(Tl) elements spaced at 1016 microm pitch. The detector was coupled to an indirect detection-based active matrix array having a pixel pitch of 508 microm, with each detector element registered to 2 x 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 (approximately 22%) compared to that of the conventional AMFPI (approximately 1%). The measured zero-frequency DQE was found to be lower than theoretical upper limits (approximately 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 (approximately 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.

Performance of in-pixel circuits for photon counting arrays (PCAs) based on polycrystalline silicon TFTs.

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.

Segmented crystalline scintillators: an initial investigation of high quantum efficiency detectors for megavoltage x-ray imaging.

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.

An Active Matrix Flat Panel Dosimeter (AMFPD) for in-phantom dosimetric measurements.

An a-Si Active Matrix Flat Panel Imager (AMFPI) prototype developed in-house has been modified to function as an in-phantom dosimetry system providing high resolution two-dimensional (2-D) data. This Active Matrix Flat Panel Dosimeter (AMFPD) system can be used as a replacement device for standard in-phantom dosimeters, such as scanning ion chambers in water, or film in solid water. The initial characterization of the device demonstrates a wide dynamic range (up to 160 cGy), a stable calibration curve (less than 1.5% variation over 1 year), dose rate independence (less than 1%), and excellent agreement of output factors with ion chamber measurements for a range of field sizes (less than 2%). The device also compares well to film for 2-D planar dose distributions. It is expected that the AMFPD system will be useful for beam commissioning, algorithm verification test data, and routine IMRT quality assurance dosimetry.