a-Si:H TFT-Based Active Matrix Flat-Panel Imagers For Medical X-Ray Applications

Antonuk, Larry E.

doi:10.1007/978-1-4615-0397-2_10

Cited by 14 publications

(15 citation statements)

References 135 publications

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“…For each resistor and photodiode, the following expressions for spectral density were used to account for thermal noise associated with the current through the resistor, and shot noise associated with the leakage current of the photodiode, respectively:

S_{i r e s} (f) = \frac{4 k_{b} T}{R}

S_{i l e a k} (f) = 2 q I_{l e a k}

where R is the resistance of the resistor and I leak is the photodiode leakage current based on the assumption of 1 pA/mm 2 . Equations and correspond to Eqs.…”

Section: Methodsmentioning

confidence: 99%

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

et al. 2017

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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 elect...

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S_{i r e s} (f) = \frac{4 k_{b} T}{R}

S_{i l e a k} (f) = 2 q I_{l e a k}

where R is the resistance of the resistor and I leak is the photodiode leakage current based on the assumption of 1 pA/mm 2 . Equations and correspond to Eqs.…”

Section: Methodsmentioning

confidence: 99%

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

et al. 2017

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“…Each of the properties chosen for examination significantly influences the operation and performance of an AMFPI and is particularly sensitive to the details of a design. 3 The measured properties include dark current, the degree of linearity in pixel response, the amount of charge lost to trapping, the amount of trapped charge released per frame, and optical fill factor. In addition, an analysis of those components of pixel-level noise related to array design is presented.…”

Section: Iic Techniques For Empirical Performance Evaluationsmentioning

confidence: 99%

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

et al. 2009

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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.

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“…Both values are much higher than that exhibited by high quality conventional AMFPIs ͑typically less than ϳ5%͒. 57 For subsequent frames, the lag for the PIB prototype decreases faster than that of the PVD prototype. For example, at 0.031 MU, Lag 30 ͑obtained ϳ10.5 s after the last image frame͒ for the PIB prototype is ϳ0.6% whereas that for the PVD prototype is ϳ1.3%.…”

Section: Iiic Lagmentioning

confidence: 78%

Section: Iiic Lagmentioning

confidence: 78%

“…In the case of lag, both prototypes exhibited values of first frame lag ͑20% and 15% for the PIB and PVD prototypes, respectively͒ that are much higher than the values of less than ϳ5% typically exhibited by conventional AMFPIs. 57 The presence of such high levels of lag is likely to intensify undesirable image artifacts such as ghosting and memory and thus degrade fluoroscopic image quality. Therefore, a reduction in the amount of image lag ͑and in the corresponding trapping of charge͒ in the photoconductor is required to improve prospects for practical use of devices based on polycrystalline HgI 2 .…”

Section: Summary and Discussionmentioning

confidence: 99%

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Performance evaluation of polycrystalline photoconductors for radiation therapy imaging

et al. 2010

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Purpose: Electronic portal imaging devices based on megavoltage ͑MV͒, active matrix, flat-panel imagers ͑AMFPIs͒ 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 ϳ2% at 6 MV, leading to a detective quantum efficiency ͑DQE͒ of only ϳ1%. 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 ͑HgI 2 ͒ photoconductor was undertaken and is reported. Methods: Two MV AMFPI prototypes, one incorporating an ϳ300 m thick HgI 2 layer created through physical vapor deposition ͑PVD͒ and a second incorporating an ϳ460 m thick HgI 2 layer created through screen-printing of particle-in-binder ͑PIB͒ material, were quantitatively evaluated using a 6 MV photon beam. The reported measurements include empirical determination of x-ray sensitivity, lag, modulation transfer function ͑MTF͒, noise power spectrum, and DQE. Results: For both prototypes, MTF and DQE results were found to be consistent with theoretical expectations and the MTFs were also found to be higher than that measured from a conventional MV AMFPI. In addition, the DQE results exhibit input-quantum-limited behavior, even at extremely low doses. Compared to PVD, the PIB prototype exhibits much lower dark current, slightly higher lag, and similar DQE. Finally, the challenges associated with this approach, as well as strategies for achieving considerably higher DQE through thicker HgI 2 layers, are discussed. Conclusions:The DQE of each of the prototypes is found to be comparable to that of conventional MV AMFPIs, commensurate with the modest photoconductor thicknesses of these early samples. It is anticipated that thicker layers of HgI 2 based on PIB deposition can provide higher DQE while maintaining good material properties.

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a-Si:H TFT-Based Active Matrix Flat-Panel Imagers For Medical X-Ray Applications

Cited by 14 publications

References 135 publications

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

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

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

Performance evaluation of polycrystalline photoconductors for radiation therapy imaging

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