Performance of a high fill factor, indirect detection prototype flat‐panel imager for mammography

El‐Mohri, Youcef; Antonuk, Larry E.; Zhao, Qihua; Wang, Yi; Li, Yixin; Du, Hong; Sawant, Amit

doi:10.1118/1.2403967

Cited by 40 publications

(63 citation statements)

References 54 publications

(78 reference statements)

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“…This results in a significant reduction in detective quantum efficiency ͑DQE͒ under conditions of low exposure or high spatial frequencies 1 and is particularly pronounced for fluoroscopy 1,2 as well as for applications requiring small pixels, such as mammography and breast tomosynthesis. 3,4 In the case of fluoroscopy, it has been shown that the DQE of AMFPIs is inferior to that of image intensifiers ͑XRIIs͒ at the lower end of the exposure range ͑0.1-1 R͒ 5 -where the limited system gain of AMFPIs results in the additive noise 6 becoming significant relative to x-ray quantum noise.…”

Section: Introductionmentioning

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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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: Introductionmentioning

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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“…2 Given the low quantum efficiency ͑QE͒ of such detectors at radiotherapy energies ͑ϳ2% at 6 MV͒, the detective quantum efficiency ͑DQE͒ of conventional EPIDs is only ϳ1%, compared to ϳ40% -80% for kV AMFPIs. [3][4][5] The low DQE of conventional EPIDs constrains the practical execution of volumetric imaging techniques, such as MV cone-beam computed tomography ͑CBCT͒ 6-13 and cone-beam digital tomosynthesis ͑CBDT͒. 14,15 These techniques are under examination for providing 3D visualization of soft tissues -information that could help ensure accurate execution of advanced treatment plans for 3D conformal radiotherapy 16 and intensity modulated radiotherapy ͑IMRT͒, 17 in which dose delivery is precisely shaped to the tumor treatment volume.…”

Section: Introductionmentioning

confidence: 99%

“…Those detectors that directly detect MV X rays include a 1D arc array of tungsten cavities filled with high-pressure xenon gas, 21 2D gas chambers formed by microstructured tungsten spacer plates 22 and thick HgI 2 photoconductors. 23 For indirect detection, some of the detectors considered include thick optical fibers detecting Cerenkov radiation, 24 2D polymer matrices filled with Gd 2 O 2 S : Tb phosphor, 25 as well as 1D and 2D crystalline scintillators ͑e.g., CsI:Tl, Bi 4 Ge 3 O 12 , CdWO 4 , and ZnWO 4 ͒, employing the concept of segmentation. [26][27][28][29][30][31][32][33][34] A series of theoretical and empirical studies has been previously reported on 2D segmented crystalline scintillators, which consists of matrices of scintillating crystals separated by septal walls to limit the lateral spread of optical photons.…”

Section: Introductionmentioning

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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“…As a result, the detective quantum efficiency ͑DQE͒ for conventional MV AMFPIs is only ϳ1%, 3 which is much lower than that for kilovoltage AMFPIs ͑ϳ40% to 80%͒. [4][5][6] In order to significantly improve portal imaging performance, 7 as well as reduce the dose requirement for MV CBCT imaging, [7][8][9] it is necessary to substantially increase the DQE of MV AMFPIs.…”

Section: Introductionmentioning

confidence: 99%

A Monte Carlo investigation of Swank noise for thick, segmented, crystalline scintillators for radiotherapy imaging

et al. 2009

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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 ͑MV͒ active matrix, flat-panel imagers ͑AMFPIs͒. 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. One effect that can degrade DQE performance is optical Swank noise ͑quantified by the optical Swank factor I opt ͒, which is induced by depth-dependent variations in optical gain. In this study, Monte Carlo simulations of radiation and optical transport have been used to examine I opt and zero-frequency DQE for segmented CsI:Tl and BGO detectors at different thicknesses and element-to-element pitches. For these detectors, I opt and DQE were studied as a function of various optical parameters, including absorption and scattering in the scintillator, absorption at the top reflector and septal walls, as well as scattering at the side surfaces of the scintillator crystals. The results indicate that I opt and DQE are only weakly affected by absorption and scattering in the scintillator, as well as by absorption at the top reflector. However, in some cases, these metrics were found to be significantly degraded by absorption at the septal walls and scattering at the scintillator side surfaces. Moreover, such degradations are more significant for detectors with greater thickness or smaller element pitch. At 1.016 mm pitch and with optimized optical properties, 40 mm thick segmented CsI:Tl and BGO detectors are predicted to provide DQE values of ϳ29% and 42%, corresponding to improvement by factors of ϳ29 and 42, respectively, compared to that of conventional MV AMFPIs.

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Performance of a high fill factor, indirect detection prototype flat‐panel imager for mammography

Cited by 40 publications

References 54 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

A Monte Carlo investigation of Swank noise for thick, segmented, crystalline scintillators for radiotherapy imaging

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