Empirical investigation of the signal performance of a high‐resolution, indirect detection, active matrix flat‐panel imager (AMFPI) for fluoroscopic and radiographic operation

Antonuk, Larry E.; El‐Mohri, Youcef; Siewerdsen, Jeffrey H.; Yorkston, J.; Huang, Wei; Scarpine, V.; Street, R. A.

doi:10.1118/1.597918

Cited by 115 publications

(135 citation statements)

References 41 publications

(63 reference statements)

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“…35 In order to control the amount of light received by array pixels, the LED was placed a short distance above each array and was flashed a specific number of times between frames, with each flash lasting a few microseconds. 34 For measurements with the incandescent lamp, there was no synchronization between light emission and array readout. In this case, the light source was continuously "on" and the desired amount of light signal per image frame was achieved through a combination of appropriate optical collimation and positioning of the light source above an array.…”

Section: Iid Experimental Methodologymentioning

confidence: 99%

“…Data frames were acquired either in the absence of radiation ͑resulting in dark frames͒ or in the presence of radiation using X rays or optical illumination ͑resulting in image frames͒. Image frames were acquired both radiographically and fluoroscopically 34 with the delivery of radiation synchronized with array readout.…”

Section: Iic Electronic Acquisition Systemmentioning

confidence: 99%

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

El‐Mohri¹,

Antonuk²,

Koniczek³

et al. 2009

Medical Physics

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Active matrix, flat-panel imagers ͑AMFPIs͒ 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 ͑AP͒ architectures, using polycrystalline-silicon ͑poly-Si͒ 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-1͒ that employs a pixel architecture with a single TFT, as well as two arrays ͑PSI-2 and PSI-3͒ 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 ϳ10.7 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 ϳ560 e ͑rms͒ for PSI-3.

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Section: Iid Experimental Methodologymentioning

confidence: 99%

Section: Iic Electronic Acquisition Systemmentioning

confidence: 99%

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

El‐Mohri¹,

Antonuk²,

Koniczek³

et al. 2009

Medical Physics

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“…A correction for lag ͑i.e., frame-to-frame signal carryover, estimated to be ϳ3%͒ was applied to each NPS, yielding a lag-corrected NPS, NPS L . 5,36,37,44 In this article, the normalized NPS ͑NNPS͒ was determined from…”

Section: Iib Measurement Methodsmentioning

confidence: 99%

High‐DQE EPIDs based on thick, segmented BGO and CsI:Tl scintillators: Performance evaluation at extremely low dose

et al. 2009

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Purpose: Electronic portal imaging devices ͑EPIDs͒ based on active matrix, flat-panel imagers ͑AMFPIs͒ have become the gold standard for portal imaging and are currently being investigated for megavoltage cone-beam computed tomography ͑CBCT͒ and cone-beam digital tomosynthesis ͑CBDT͒. However, the practical realization of such volumetric imaging techniques is constrained by the relatively low detective quantum efficiency ͑DQE͒ of AMFPI-based EPIDs at radiotherapy energies, ϳ1% 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 thick͒ and three segmented CsI:Tl scintillators ͑11.4, 25.6, and 40.0 mm thick͒ 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 ϳ12 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:Tl scintillators provide significantly improved portal imaging performance at extremely low dose ͑i.e., down to 1 beam pulse corresponding to ϳ0.022 cGy͒, creating the possibility of soft-tissue visualization using MV CBCT and CBDT at clinically practical dose.

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“…Signal and noise data, including sensitivity, lag, and NPS, were acquired in fluoroscopic mode, while MTF data were acquired in radiographic mode. 13,50,51 In fluoroscopic mode, a sequence of image frames was acquired in synchronization with beam pulses ͑using the "Target I" signal provided by the linac͒, so that every image frame was read out after delivery of a predetermined number of pulses. Dark frames were acquired in synchronization with pulses generated by a pulse generator ͑33250A, Agilent Technologies, Inc., USA͒ operated at the same frequency as the beam pulses.…”

Section: Iib Measurement Methodsmentioning

confidence: 99%

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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Empirical investigation of the signal performance of a high‐resolution, indirect detection, active matrix flat‐panel imager (AMFPI) for fluoroscopic and radiographic operation

Cited by 115 publications

References 41 publications

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

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

High‐DQE EPIDs based on thick, segmented BGO and CsI:Tl scintillators: Performance evaluation at extremely low dose

Performance evaluation of polycrystalline photoconductors for radiation therapy imaging

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