Performance evaluation of polycrystalline photoconductors for radiation therapy imaging

Abstract

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.

A novel multilayer MV imager computational model for component optimization

“…Therefore, when all of the MLI layers are combined, the total efficiency is five times that of the AS1200. A more complete presentation of the imaging performance characteristics of the MLI and comparison with the AS1200 can be found in Rottmann et al 8 The signal and noise performance of an indirect imaging system (such as the MLI) can be fully characterized in terms of MTF, NPS, and DQE. 37 The Swank factor was implied in the measurement and calculation of NPS and DQE.…”

“…Previous efforts to improve EPID imaging performance characteristics have mainly focused on replacing the phosphor material with segmented crystalline scintillators, [3][4][5][6] exploiting Cerenkov radiation 7 or employing direct detection methods. 8 Such studies often demonstrate very high DQE; for example, Star-Lack et al 5 reported a DQE approximately 25 times higher than that of a conventional EPID. The main drawbacks are the cost and complexity of manufacturing and implementation, which can be challenging obstacles to clinical translation.…”

Characterization of polycrystalline lead oxide for application in direct conversion X-ray detectors

“…The later can be improved by developing a PbO blocking structure, as it was successfully done for a-Se 16 . However, HgI 2 layers are toxic in the sense that they chemically react with imaging array electronics, thus creating additional technological challenges 39 , 40 . Currently, the major competitors of PbO seems to be perovskite photoconductors, for example methylammonium lead iodide (CH 3 NH 3 PbI 3 ).…”

Development of active matrix flat panel imagers incorporating thin layers of polycrystalline HgI2for mammographic x-ray imaging

“…In previous studies, AMFPI prototypes employing ~210 to 635 μm of polycrystalline HgI 2 material prepared using both the PVD and PIB methods have been examined under radiographic, fluoroscopic and radiotherapy imaging conditions. (Du et al , 2008; Zhao et al , 2010; Zentai et al , 2004) In addition, an early investigation of the performance of an AMFPI prototype coated with 240 μm of PVD HgI 2 under mammographic imaging conditions demonstrated DQE values up to ~60%. (El-Mohri et al , 2003) However, the performance of prototype imagers incorporating PIB HgI 2 has not been previously investigated for mammographic imaging.…”