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Sci—Thur PM: YIS — 08: The effect of copper conversion plates on low‐Z target image quality
Author(s) -
Parsons D,
Robar JL
Publication year - 2012
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 - 10.1118/1.4740105
Subject(s) - optics , detector , copper , materials science , planar , contrast to noise ratio , image sensor , image quality , cone beam computed tomography , photon , nuclear medicine , physics , computed tomography , medicine , radiology , image (mathematics) , computer graphics (images) , artificial intelligence , computer science , metallurgy
Current generation electronic portal imaging devices (EPID) contain a 1.0 mm copper conversion plate to increase detection efficiency of a therapeutic megavoltage spectrum. When using these EPIDs for low‐Z target imaging, the conversion plate largely attenuates the large populations of diagnostic energy photons, thereby decreasing the benefits of low‐Z target imaging. In this work we measure directly the effect the variation in thickness of a copper conversion plate has on image quality in planar and cone beam computed tomography imaging. Monte Carlo modeling was used to quantify changes to the diagnostic spectrum and detector response for low‐Z target beams generated with 2.35 and 7.00 MeV electrons incident on a carbon target. Planar contrast‐to‐noise ratio (CNR) measurements were made as a function of copper thickness. Cone beam computed tomography (CBCT) image CNR measurements were made as a function of dose both with and without the copper plate present in the EPID. The presence of copper in the EPID decreased the diagnostic photon population by up to 20% and suppressed the peak detector response at 60 kV by a factor of 6.4. Planar CNR was increased by a factor ranging from 1.4 to 4.0 with no copper present compared to 1.0 mm thickness. Increases in CBCT image CNR ranged from a factor of 1.3 to 2.1 with the copper plate removed. As a result of this we suggest that the copper conversion plate be removed from the EPID when used for low‐Z target planar or CBCT imaging.