Conventional Active Matrix Flat-panel Imagers Research Articles

Development of active matrix flat panel imagers incorporating thin layers of polycrystalline HgI2 for mammographic x-ray imaging

Active matrix flat-panel imagers (AMFPIs) offer many advantages and have become ubiquitous across a wide variety of medical x-ray imaging applications. However, for mammography, the imaging performance of conventional AMFPIs incorporating CsI:Tl scintillators or a-Se photoconductors is limited by their relatively modest signal-to-noise ratio (SNR), particularly at low x-ray exposures or high spatial resolution. One strategy for overcoming this limitation involves the use of a high gain photoconductor such as mercuric iodide (HgI2) which has the potential to improve the SNR by virtue of its low effective work function (WEFF). In this study, the performance of direct-detection AMFPI prototypes employing relatively thin layers of polycrystalline HgI2 operated under mammographic irradiation conditions over a range of 0.5 to 16.0 mR is presented. High x-ray sensitivity (corresponding to WEFF values of ∼19 eV), low dark current (<0.1 pA mm−2) and good spatial resolution, largely limited by the size of the pixel pitch, were observed. For one prototype, a detective quantum efficiency of ∼70% was observed at an x-ray exposure of ∼0.5 mR at 26 kVp.

SU‐D‐301‐01: Performance Limits of Large Area Poly‐Si X‐Ray Imagers Employing In‐Pixel Amplification

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

SU‐C‐220‐05: Investigation of Novel, Focused, Segmented Scintillator Geometries for High DQE Megavoltage Active Matrix Imagers

Purpose: Megavoltage active matrix, flat‐panel imagers (AMFPIs) have become ubiquitous in modern radiotherapy environments. However, their imaging performance is severely constrained by very low DQE (∼1%). While significant performance improvements have been demonstrated through introduction of thick, precision‐machined matrices of optically‐ isolated crystalline detector elements, the benefits are constrained by geometric beam divergence. It is therefore of interest to examine the potential performance of focused, segmented scintillator geometries specifically designed to circumvent DQE losses caused by divergence. Methods: The potential detection efficiency and DQE performance of hypothetical detector geometries was examined by means of Monte Carlo simulation of radiation transport. The choice of geometries examined was governed by considerations including: maximization of DQE through geometrically focused elements; manufacturability of the focused matrix; and feasibility of the underlying active matrix of indirect detection imaging pixels. Calculations were performed for a variety of candidate scintillator materials up to 6 cm thick. Results: The most promising detector geometries identified in the study consisted of matrices of scintillator elements located along the surface of a plane, as well as along the surface of a spherical cap. For cap detectors, elements arranged in a warped‐rectangular matrix, concentric rings, and a geodesic are of interest — and geometries employing a single element shape are particularly favored, although at the cost of fill factor. While focused planar detectors would be compatible with conventional AMFPI arrays, cap detectors would involve active matrix arrays in the shape of an approximately spherically curved surface created through cut and bend techniques. Overall, depending upon detector geometry, scintillator material and scintillator thickness, calculations indicate that DQE improvements of up to a factor of ∼50 are conceivable. Conclusions: This initial theoretical study suggests that the promise of very high DQE performance from thick, segmented scintillators could be realized through novel, focused detector geometries. Work supported by NIH grant R01‐CA051397

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.

SU‐HH‐AUD C‐01: Empirical Investigation of Flat‐Panel Imagers Incorporating High‐Efficiency, Segmented BGO and CsI:Tl Detectors for Radiotherapy Imaging

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

WE-C-342-02: The Future of Flat Panel Imagers: From Active Matrix to Active Pixel Architectures and Many Possibilities in Between

Since the turn of the century, the phrase “Flat Panel Imager” has been increasingly used in modern x-ray imaging venues ranging from large, institutional cardiac care, radiology and radiotherapy settings to community oral surgery offices. The phrase most commonly refers to those technologies employing a large area, monolithic array consisting of a two-dimensional grid of imaging pixels fabricated on a thin glass substrate. Individual pixels are made addressable by means of an “active matrix” of switches — usually, in each pixel, taking the form of a single amorphous silicon (a-Si:H) thin film transistor (TFT) coupled to some form of storage capacitor. Two variations of this relatively simple architecture (based on so-called indirect or direct detection of the incident radiation by means of a scintillator or a photoconductor, respectively) have become almost ubiquitous for a wide variety of projection (e.g. radiography, fluoroscopy, mammography) and volumetric (e.g., CBCT, tomosynthesis) imaging applications. While offering many advantages, such Active Matrix Flat Panel Imagers (AMFPIs) are restrictive in terms of signal-to-noise performance, maximum frame rate, image artifacts, configurability and cost. These limitations are inspiring considerable innovation and creativity in imager development. Some approaches involve: high-gain photoconductors such as HgI2 or avalanche-gain with a-Se (to improve system gain and DQE); active pixel circuits involving the inclusion of amplifiers in each pixel (to increase gain and DQE, frame rate, and to reduce artifacts); thick segmented scintillating converters (to increase x-ray quantum detection efficiency and DQE at megavoltage energies); flexible substrates (to provide lighter, flexible and more x-ray transparent substrates); and subtractive and additive printing of a-Si:H or organic TFTs (to reduce costs). In this talk, a broad overview of the state of conventional AMFPI technology, its limitations, the potential for improvement, and some of the avenues being pursued to achieve these improvements will be presented. In addition, challenges for some of these approaches, along with the long-term prospects for this general area of technology, will be reviewed, and the effect of large performance improvements on the practical implementation of advanced applications will be discussed. Educational Objectives: 1. Review the fundamental concepts behind the technology of flat panel imagers based on active matrix addressing. 2. Provide an understanding of the performance limitations on such active matrix, flat panel imagers (AMFPIs). 3. Outline the general approaches for achieving significant improvements over conventional AMFPI performance. 4. Detail specific improvement strategies involving front-end enhancement of converter signal and pixel level circuit modifications, which preserve the major advantages of conventional AMFPIs. 5. Discuss the long-term prospects for, and implications of flat panel imager performance improvement.