title: Performance evaluation of polycrystalline HgI2 photoconductors for radiation therapy imaging.
abstract: OBJECTIVE
  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 approximately 2% at 6 MV, leading to a detective quantum efficiency (DQE) of only approximately 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 (HgI2) photoconductor was undertaken and is reported.
  
  
  METHODS
  Two MV AMFPI prototypes, one incorporating an approximately 300 microm thick HgI2 layer created through physical vapor deposition (PVD) and a second incorporating an approximately 460 microm thick HgI2 layer 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 thicker HgI2 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 HgI2 based on PIB deposition can provide higher DQE while maintaining good material properties.
authors.affiliation: Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan 48019, USA. qizhao@umich.edu
authors.forename: Qihua
authors.initials: Q
authors.lastname: Zhao
authors.forename: Larry E
authors.initials: LE
authors.lastname: Antonuk
authors.forename: Youcef
authors.initials: Y
authors.lastname: El-Mohri
authors.forename: Yi
authors.initials: Y
authors.lastname: Wang
authors.forename: Hong
authors.initials: H
authors.lastname: Du
authors.forename: Amit
authors.initials: A
authors.lastname: Sawant
authors.forename: Zhong
authors.initials: Z
authors.lastname: Su
authors.forename: Jin
authors.initials: J
authors.lastname: Yamamoto
mesh_terms: D017076:Computer-Aided Design; D003460:Crystallization / Q000379:methods; D004867:Equipment Design; D019544:Equipment Failure Analysis; D011874:Radiometry / Q000295:instrumentation*; D011879:Radiotherapy Dosage; D020266:Radiotherapy, Conformal / Q000295:instrumentation*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014963:X-Ray Intensifying Screens