Jin Yamamoto

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.

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.

Systematic investigation of the signal properties of polycrystalline HgI2 detectors under mammographic, radiographic, fluoroscopic and radiotherapy irradiation conditions.

The signal properties of polycrystalline mercuric iodide (HgI2) film detectors, under irradiation conditions relevant to mammographic, radiographic, fluoroscopic and radiotherapy x-ray imaging, are reported. Each film detector consists of an approximately 230 to approximately 460 microm thick layer of HgI2 (fabricated through physical vapour deposition or a screen-print process) and a thin barrier layer, sandwiched between a pair of opposing electrode plates. The high atomic number, high density and low effective ionization energy, W(EFF), of HgI2 make it an attractive candidate for significantly improving the performance of active matrix, flat-panel imagers (AMFPIs) for several x-ray imaging applications. The temporal behaviour of current from the film detectors in the presence and in the absence of radiation was used to examine dark current levels, the lag and reciprocity of the signal response, x-ray sensitivity and W(EFF). The results are discussed in the context of present AMFPI performance. This study provides performance data for a wide range of potential medical x-ray imaging applications from a single set of detectors and represents the first investigation of the signal properties of polycrystalline mercuric iodide for the radiotherapy application.

Segmented phosphors: MEMS-based high quantum efficiency detectors for megavoltage x-ray imaging.

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.

Examination of HgI/sub 2/ and PbI/sub 2/ photoconductive materials for direct detection, active matrix, flat-panel imagers for diagnostic X-ray imaging

In this paper, the results of initial empirical studies of the dark current and X-ray signal response of thick, polycrystalline films of lead iodide (PbI/sub 2/) and mercuric iodide (HgI/sub 2/) are reported. These studies are being carried out as part of an extensive, integrated program of research to develop and incorporate such photoconductive, X-ray detection materials into direct detection, active matrix flat-panel imagers (AMFPIs) for applications in diagnostic imaging, as well as for radiotherapy imaging. Simple detector configurations incorporating these materials were prepared by physical vapor deposition (PVD) with thicknesses ranging from approximately 90 to 500 urn. In the present study, dark current, X-ray sensitivity and temporal response of the detectors were quantitatively evaluated under irradiation conditions relevant to radiography, fluoroscopy and mammography. Measurements were performed for externally applied bias voltages ranging from 0.1 to 2.0V//spl mu/m for both negative and positive polarities.In this paper, the results of empirical studies of the dark current and X-ray signal response of thick, polycrystalline films of lead iodide (PbI/sub 2/) and mercuric iodide (HgI/sub 2/) are reported. These studies are being carried out as part of an extensive, integrated program of research to develop and incorporate such photoconductive, X-ray detection materials into direct detection, active matrix flat-panel imagers (AMFPIs) for applications in diagnostic imaging, as well as for radiotherapy imaging. Simple detector configurations incorporating these materials were prepared by physical vapor deposition (PVD) with thicknesses ranging from approximately 90 to 500 /spl mu/m. For these detectors, the temporal behavior of the dark current and of the X-ray-induced photocurrent, under irradiation conditions relevant to fluoroscopy, radiography and mammography, were quantitatively examined and are reported. In addition, X-ray sensitivity results are also presented for a variety of conditions. The measurements were performed for externally applied electric fields ranging from 0.2 to 2.0 V//spl mu/m for both negative and positive polarities.

Performance evaluation of polycrystalline HgI2 photoconductors for radiation therapy imaging

PURPOSEnElectronic 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 detection imagers incorporating thick films of polycrystalline mercuric iodide (HgI2) photoconductor was undertaken and is reported.nnnMETHODSnTwo MV AMFPI prototypes, one incorporating an ∼300μm thick HgI2 layer created through physical vapor deposition (PVD) and a second incorporating an ∼460μm 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.nnnRESULTSnFor 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.nnnCONCLUSIONSnThe 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.

Evaluation of novel direct- and indirect-detection active matrix flat-panel imagers (AMFPIs) for mammography

A performance evaluation of small-area, high-spatial-resolution, active matrix flat-panel imager (AMFPI) prototypes, operated under mammographic conditions, is reported. These prototypes are based on two 512 x 512 pixel imagers employing novel designs to enhance signal performance for direct and indirect detection. The indirect detection prototype is based on a 75 μm pixel pitch array incorporating a continuous photodiode design, as opposed to the discrete photodiode design used in conventional flat-panel imagers. This array was coupled to a pair of commercially-available x-ray converters: (1) a 34 mg/cm2 Gd2O2S:Tb phosphor screen (Min-R, Kodak) and (2) a 150 μm thick structured CsI:Tl scintillator on a fiber-optic plate (FOS-HL, Hamamatsu). The direct detection prototype is based on a 100 μm pixel pitch array and uses a 240 μm thick, high gain mercuric iodide (HgI2) photoconductor. Measurements of sensitivity, MTF, NPS and DQE were performed with a 26 kVp mammography beam attenuated by a 4 cm BR-12 breast phantom at various radiation exposures. Results from empirical studies of sensitivity indicate that these imagers offer a substantial enhancement in signal over conventional flat-panel imagers. Measurements of DQE for the imagers show values greater than those obtained from high performance mammographic film-screen systems, under some conditions. These studies also show that the FOS-HL imager configuration despite its lower MTF, exhibits DQE performance (up to approximately 0.77) superior or equivalent to that of the Min-R configuration due to better optical properties of the converter. In addition, despite a smaller pixel pitch, both of these indirect detection configurations exhibit improved DQE in comparison to similar configurations employing a 97 μm pitch discrete photodiode design, especially at low exposures. Results of DQE measurements from the HgI2 photoconductor prototype are promising (DQE values up to approximately 0.6). Finally, calculations of potential DQE performance for hypothetical 50 μm pitch imagers, employing similar novel designs, were performed. These calculations were based on the cascaded systems formalism and used realistic inputs derived from empirical measurements. The results predict that the HgI2 configuration would provide high DQE performance (up to approximately 0.9), which would be largely unaffected by the magnitude of exposure, due to the high gain of the photoconductor. These calculations also indicate that the continuous photodiode configuration would provide high DQE (up to approximately 0.8), degraded only at low exposure by the effect of additive noise.

Investigation of strategies to achieve optimal DQE performance from indirect-detection active-matrix flat-panel imagers (AMFPIs) through novel pixel amplification architectures

The numerous merits of x-ray imagers based on active matrix, flat-panel array technology have led to their introduction in a wide variety of x-ray imaging applications. However, under certain conditions, the performance of direct and indirect detection AMFPIs is significantly limited by the relatively modest ratio of singal to noise provided by conventional systems. While substantial reduction in the additive noise of such systems is difficult, significant enhancement of signal can be achieved through the incorporation of an amplification circuit in each pixel. In addition, innovative photodiode structures can be incorporated into indirect detection designs to enhance optical signal collection efficiency. In this paper, an investigation of these strategies, involving the design, fabrication and performance evaluation of a variety of novel, small area, indirect detection arrays, is described. Each prototype array incorporates innovative features, such as continuous photodiodes and single-stage and dual-stage in-pixel amplifiers, that are designed to provide insight into promising avenues for achieving significant singal-to-noise enhancement. This information will assist in the realization of a next generation of highly-optimized AMFPI pixel architectures whose DQE performance will be limited only by x-ray noise and x-ray converter properties under a very wide range of conditions. In this paper, the design and operation of the present prototype arrays are described and initial performance results are reported. In addition, the benefits of significant improvements to the signal-to-noise properties of AMFPIs are illustrated through cascaded systems calculations of the DQE performance of hypothetical systems.

Systematic development of input-quantum-limited fluoroscopic imagers based on active-matrix flat-panel technology

The development of fluoroscopic imagers exhibiting performance that is primarily limited by the noise of the incident x-ray quanta, even at very low exposures, remains a highly desirable objective for active matrix flat-panel technology. Previous theoretical and empirical studies have indicated that promising strategies to acheiving this goal include the development of array designs incorporating improved optical collection fill factors, pixel-level amplifiers, or very high-gain photoconductors. Our group is pursuing all three strategies and this paper describes progress toward the systematic development of array designs involving the last approach. The research involved the iterative fabrication and evaluation of a series of prototype imagers incorporating a promising high-gain photoconductive material, mercuric iodide (HgI2). Over many cycles of photoconductor deposition and array evaluation, improvements ina variety of properties have been observed and remaining fundamental challenges have become apparent. For example, process compatibility between the deposited HgI2 and the arrays have been greatly improved, while preserving efficient, prompt signal extraction. As a result, x-ray sensitivities within a factor of two of the nominal limit associated with the single-crystal form of HgI2 have been observed at relatively low electric fields (~0.1 to 0.6 V/μm), for some iterations. In addition, for a number of iterations, performance targets for dark current stability and range of linearity have been met or exceeded. However, spotting of the array, due to localized chemical reactions, is still a concern. Moreover, the dark current, uniformity of pixel response, and degree of charge trapping, though markedly improved for some iterations, require further optimization. Furthermore, achieving the desired performance for all properties simultaneously remains an important goal. In this paper, a broad overview of the progress of the research will be presented, remaining challenges in the development of this photoconductive material will be outlined, and prospects for further improvement will be discussed.

Exploring new frontiers in x-ray quantum limited portal imaging using active matrix flat-panel imagers (AMFPIs)

In recent years, indirect detection active matrix flat-panel imagers (AMFPIs) have become the gold standard in radiotherapy imaging. The excellent imaging performance of AMFPI-based electronic portal imaging devices (EPIDs) can be attributed to their ability to perform x-ray quantum limited imaging under radiotherapy conditions. However, all current commercial (AMFPI and non-AMFPI) EPIDs use only approximately 1% to 2% of the incident radiation. In this work, strategies to significantly improve the overall performance of indirect detection AMFPI-based EPIDs through novel designs, are presented. Specifically, the focus of this work is on thick, structured scintillators, which achieve high x-ray detection efficiency while simultaneously maintaining spatial resolution. Fundamental signal and noise measurements using prototype, small area, structured scintillators are reported. In addition, theoretical calculations were performed in order to estimate the signal and noise properties of various structured scintillator configurations. Results from theory and experiments were used to estimate the frequency-dependent detective quantum efficiency (DQE). Results suggest that these designs could yield DQE values that are significantly higher than those for current commercial EPID technologies.