TY - JOUR
T1 - Performance evaluation of polycrystalline HgI2 photoconductors for radiation therapy imaging
AU - Zhao, Qihua
AU - Antonuk, Larry E.
AU - El-Mohri, Youcef
AU - Wang, Yi
AU - Du, Hong
AU - Sawant, Amit
AU - Su, Zhong
AU - Yamamoto, Jin
N1 - Funding Information:
The authors would like to thank Yixin Li for his assistance with the measurements and data analysis software. This work was supported by NIH Grant No. R01-CA51397.
PY - 2010/6
Y1 - 2010/6
N2 - 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 (HgI2) photoconductor was undertaken and is reported. Methods: Two MV AMFPI prototypes, one incorporating an ∼300 μm thick HgI2 layer created through physical vapor deposition (PVD) and a second incorporating an ∼460 μm 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.
AB - 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 (HgI2) photoconductor was undertaken and is reported. Methods: Two MV AMFPI prototypes, one incorporating an ∼300 μm thick HgI2 layer created through physical vapor deposition (PVD) and a second incorporating an ∼460 μm 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.
KW - DQE
KW - EPID
KW - Flat-panel imager
KW - Megavoltage x-ray imaging
KW - PIB
KW - PVD
KW - Polycrystalline mercuric iodide
UR - http://www.scopus.com/inward/record.url?scp=77953513430&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=77953513430&partnerID=8YFLogxK
U2 - 10.1118/1.3416924
DO - 10.1118/1.3416924
M3 - Article
C2 - 20632584
AN - SCOPUS:77953513430
SN - 0094-2405
VL - 37
SP - 2738
EP - 2748
JO - Medical physics
JF - Medical physics
IS - 6
ER -