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Performance evaluation of polycrystalline HgI 2 photoconductors for radiation therapy imaging
Author(s) -
Zhao Qihua,
Antonuk Larry E.,
ElMohri Youcef,
Wang Yi,
Du Hong,
Sawant Amit,
Su Zhong,
Yamamoto Jin
Publication year - 2010
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3416924
Subject(s) - detective quantum efficiency , optical transfer function , optics , materials science , optoelectronics , quantum efficiency , x ray detector , radiation , photon , image quality , physics , detector , computer science , artificial intelligence , image (mathematics)
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 ∼ 2 % at 6 MV, leading to a detective quantum efficiency (DQE) of only ∼ 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(HgI 2 )photoconductor was undertaken and is reported. Methods Two MV AMFPI prototypes, one incorporating an ∼ 300 μ m thickHgI 2layer created through physical vapor deposition (PVD) and a second incorporating an ∼ 460 μ m thickHgI 2layer created through screen‐printing of particle‐in‐binder (PIB) material, were quantitatively evaluated using a 6 MV photon beam. The reported measurements include empirical determination of x‐ray sensitivity, lag, modulation transfer function (MTF), 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 thickerHgI 2layers, 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 ofHgI 2based on PIB deposition can provide higher DQE while maintaining good material properties.