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Measured and calculated K‐fluorescence effects on the MTF of an amorphous‐selenium based CCD x‐ray detector
Author(s) -
M. Hunter David,
Belev George,
Kasap Safa,
J. Yaffe Martin
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 - 10.1118/1.3673957
Subject(s) - optics , x ray detector , detector , optical transfer function , monochromator , physics , monochromatic color , detective quantum efficiency , synchrotron , optoelectronics , image quality , wavelength , artificial intelligence , computer science , image (mathematics)
Purpose: Theoretical reasoning suggests that direct conversion digital x‐ray detectors based upon photoconductive amorphous‐selenium (a‐Se) could attain very high values of the MTF (modulation transfer function) at spatial frequencies well beyond 20 cycles mm −1 . One of the fundamental factors affecting resolution loss, particularly at x‐ray energies just above the K ‐edge of selenium (12.66 keV), is the K ‐fluorescence reabsorption mechanism, wherein energy can be deposited in the detector at locations laterally displaced from the initial x‐ray interaction site. This paper compares measured MTF changes above and below the Se K ‐edge of a CCD based a‐Se x‐ray detector with theoretical expectations. Methods: A prototype 25 μm sampling pitch (Nyquist frequency = 20 cycles mm −1 , 200 μm thick a‐Se layer based x‐ray detector, utilizing a specialized CCD readout device (200 × 400 area array), was used to make edge images with monochromatic x‐rays above and below the K ‐edge of Se. A vacuum double crystal monochromator, exposed to polychromatic x‐rays from a synchrotron, formed the monochromatic x‐ray source. The monochromaticity of the x‐rays was 99% or better. The presampling MTF was determined using the slanted edge method. The theory modeling the MTF performance of the detector includes the basic x‐ray interaction physics in the a‐Se layer as well as effects related to the operation of the CCD and charge trapping at a blocking layer present at the CCD/a‐Se interface. Results: The MTF performance of the prototype a‐Se CCD was reduced from the theoretical value prescribed by the basic Se x‐ray interaction physics, principally by the presence of a blocking layer. Nevertheless, the K‐fluorescence reduction in the MTF was observed, approximately as predicted by theory. For the CCD prototype detector, at five cycles mm −1 , there was a 14% reduction of the MTF, from a value of 0.7 below the K ‐edge of Se, to 0.6 just above the K ‐edge. Conclusions: The MTF of an a‐Se x‐ray detector has been measured using monochromatic x‐rays above and below the K ‐edge of selenium. The MTF is poorer above the K ‐edge by an amount consistent with theoretical expectations.