Measured and calculated K-fluorescence effects on the MTF of an amorphous-selenium based CCD x-ray detector

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 affect...

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Bibliographic Details
Published in:Medical physics (Lancaster) 2012-02, Vol.39 (2), p.608-622
Main Authors: M. Hunter, David, Belev, George, Kasap, Safa, J. Yaffe, Martin
Format: Article
Language:English
Subjects:
amorphous state
CCD
Charge coupled devices
Charge transfer
Compton scattering
Computer-Aided Design
Devices sensitive to very short wavelength, e.g. x‐rays, gamma‐rays or corpuscular radiation
Digital radiography
Equipment Design
Equipment Failure Analysis
Fluorescence
K-fluorescence
mammography
Medical imaging
Models, Theoretical
Modulation transfer functions
MTF
optical transfer function
photoconductor
resolution
selenium
Selenium - radiation effects
Signal Processing, Computer-Assisted - instrumentation
Space charge effects
Tomography, X-Ray Computed - instrumentation
Transforming x‐rays
x-ray
X‐ and γ‐ray sources, mirrors, gratings, and detectors
X‐ray detection
X‐ray detectors
X‐ray effects
X‐ray imaging
X‐ray monochromators
X‐ray scattering
X‐ray technique
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Summary: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 theK-edge of selenium. The MTF is poorer above the K-edge by an amount consistent with theoretical expectations.
ISSN:0094-2405
2473-4209
DOI:10.1118/1.3673957