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Medical Physics / Volume 39 / Issue 2 / RADIATION IMAGING PHYSICS

Med. Phys. 39, 608 (2012); http://dx.doi.org/10.1118/1.3673957 (15 pages)

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

David M. Hunter

Sunnybrook Health Sciences Centre, SG-17, 2075 Bayview Avenue, Toronto, Ontario, M4N 3M5, Canada

George Belev

Canadian Light Source, Inc., University of Saskatchewan, 101 Perimeter Road, Saskatoon, Saskatchewan K, S7N 0X4, Canada

Safa Kasap

Department of Electrical and Computer Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, Saskatchewan, S7N 5A9, Canada

Martin J. Yaffe

Sunnybrook Health Sciences Centre, S6-57, 2075 Bayview Avenue, Toronto, Ontario, M4N 3M5 and Department of Medical Biophysics, University of Toronto, Ontario, M4N 3M5, Canada

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(Received 17 September 2011; accepted 12 December 2011; revised 10 November 2011; published online 11 January 2012)

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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.

© 2012 American Association of Physicists in Medicine

ACKNOWLEDGMENTS

A special thanks to Vladimir Verpakhovski for electronic circuit design and testing. Thanks to Giovanni DeCrescenzo and James Mainprize for helpful comments. Part of the research described in this paper was performed at the Canadian Light Source, which is supported by the Natural Sciences and Engineering Research Council of Canada, The National Research Council of Canada, the Canadian Institutes of Health Research, the Province of Saskatchewan, Western Economic Diversification of Canada, and the University of Saskatchewan.

Article Outline

  1. INTRODUCTION
  2. THEORY
    1. Photo-electron/auger-electron range
    2. K-fluorescence reabsorption mechanism
    3. Coherent and incoherent scatter
    4. Other effects: Diffusion, blocking layer, CCD charge transfer efficiency
  3. MATERIALS AND METHODS
    1. a-Se base CCD detector
    2. Control electronics
    3. Selenium deposition
    4. Monochromatic x-ray source and imaging procedure
  4. RESULTS AND DISCUSSION
    1. a-Se CCD operating characteristics
      1. CCD: dark response, x-ray signal transfer, CTE
      2. CCD image results
    2. MTF results
  5. CONCLUSIONS
Digital Object Identifier
http://dx.doi.org/10.1118/1.3673957

KEYWORDS and PACS

Keywords

amorphous state, charge-coupled devices, mammography, optical transfer function, selenium, X-ray detection, X-ray monochromators, x-ray, MTF, resolution, K-fluorescence, selenium, photoconductor, CCD

PACS

  • 87.59.bf

    Digital radiography

  • 07.85.Fv

    X- and γ-ray sources, mirrors, gratings, and detectors

PUBLICATION DATA

ISSN

0094-2405 (print)  

Publisher

$abstract.society.value is a Member of CrossRef American Association of Physicists in Medicine

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Medical Physics is an official science journal of the AAPM and of the COMP/CCPM/IOMP
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Departments of Radiology, Radiation Oncology, Biophysics, Community and Public Health, Medical College of Wisconsin
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