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

. Polycrystalline mercuric iodide photoconductive converters fabricated using particle-in-binder techniques (PIB HgI) 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 HgIan 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.

Theoretical investigation of the signal performance of HgIx-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 HgI) 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 HgIis the high level of image lag, which is believed to largely originate from the trapping of holes.

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.

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.

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.

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.

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.

Theoretical investigation of the design and performance of a dual energy (kV and MV) radiotherapy imager.

Purpose: In modern radiotherapy treatment rooms, megavoltage (MV) portal imaging and kilovoltage (kV) cone-beam CT (CBCT) imaging are performed using various active matrix flat-panel imager (AMFPI) designs. To expand the clinical utility of MV and kV imaging, MV AMFPIs incorporating thick, segmented scintillators and, separately, kV imaging using a beam's eye view geometry have been investigated by a number of groups. Motivated by these previous studies, it is of interest to explore to what extent it is possible to preserve the benefits of kV and MV imaging using a single AMFPI design, given the considerably different x ray energy spectra used for kV and MV imaging.

Optimization of the design of thick, segmented scintillators for megavoltage cone-beam CT using a novel, hybrid modeling technique.

Purpose: Active matrix flat-panel imagers (AMFPIs) incorporating thick, segmented scintillators have demonstrated order-of-magnitude improvements in detective quantum efficiency (DQE) at radiotherapy energies compared to systems based on conventional phosphor screens. Such improved DQE values facilitate megavoltage cone-beam CT (MV CBCT) imaging at clinically practical doses. However, the MV CBCT performance of such AMFPIs is highly dependent on the design parameters of the scintillators.

Optimization of the performance of segmented scintillators for radiotherapy imaging through novel binning techniques.

Thick, segmented crystalline scintillators have shown increasing promise as replacement x-ray converters for the phosphor screens currently used in active matrix flat-panel imagers (AMFPIs) in radiotherapy, by virtue of providing over an order of magnitude improvement in the detective quantum efficiency (DQE). However, element-to-element misalignment in current segmented scintillator prototypes creates a challenge for optimal registration with underlying AMFPI arrays, resulting in degradation of spatial resolution. To overcome this challenge, a methodology involving the use of a relatively high resolution AMFPI array in combination with novel binning techniques is presented.

Development of active matrix flat panel imagers incorporating thin layers of polycrystalline HgI(2) 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 (HgI(2)) which has the potential to improve the SNR by virtue of its low effective work function (W(EFF)).

Countering beam divergence effects with focused segmented scintillators for high DQE megavoltage active matrix imagers.

The imaging performance of active matrix flat-panel imagers designed for megavoltage imaging (MV AMFPIs) is severely constrained by relatively low x-ray detection efficiency, which leads to a detective quantum efficiency (DQE) of only ∼1%. Previous theoretical and empirical studies by our group have demonstrated the potential for addressing this constraint through the utilization of thick, two-dimensional, segmented scintillators with optically isolated crystals. However, this strategy is constrained by the degradation of high-frequency DQE resulting from spatial resolution loss at locations away from the central beam axis due to oblique incidence of radiation.

Low-dose megavoltage cone-beam CT imaging using thick, segmented scintillators.

Megavoltage, cone-beam computed tomography (MV CBCT) employing an electronic portal imaging device (EPID) is a highly promising technique for providing soft-tissue visualization in image-guided radiotherapy. However, current EPIDs based on active matrix flat-panel imagers (AMFPIs), which are regarded as the gold standard for portal imaging and referred to as conventional MV AMFPIs, require high radiation doses to achieve this goal due to poor x-ray detection efficiency (∼2% at 6 MV). To overcome this limitation, the incorporation of thick, segmented, crystalline scintillators, as a replacement for the phosphor screens used in these AMFPIs, has been shown to significantly improve the detective quantum efficiency (DQE) performance, leading to improved image quality for projection imaging at low dose.

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.

Monte Carlo investigations of the effect of beam divergence on thick, segmented crystalline scintillators for radiotherapy imaging.

The use of thick, segmented scintillators in electronic portal imagers offers the potential for significant improvement in x-ray detection efficiency compared to conventional phosphor screens. Such improvement substantially increases the detective quantum efficiency (DQE), leading to the possibility of achieving soft-tissue visualization at clinically practical (i.e.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.