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Title: The x-ray time of flight method for investigation of ghosting in amorphous selenium-based flat panel medical x-ray imagers

Abstract

Amorphous selenium (a-Se) based real-time flat-panel imagers (FPIs) are finding their way into the digital radiology department because they offer the practical advantages of digital x-ray imaging combined with an image quality that equals or outperforms that of conventional systems. The temporal imaging characteristics of FPIs can be affected by ghosting (i.e., radiation-induced changes of sensitivity) when the dose to the detector is high (e.g., portal imaging and mammography) or the images are acquired at a high frame rate (e.g., fluoroscopy). In this paper, the x-ray time-of-flight (TOF) method is introduced as a tool for the investigation of ghosting in a-Se photoconductor layers. The method consists of irradiating layers of a-Se with short x-ray pulses. From the current generated in the a-Se layer, ghosting is quantified and the ghosting parameters (charge carrier generation rate and carrier lifetimes and mobilities) are assessed. The x-ray TOF method is novel in that (1) x-ray sensitivity (S) and ghosting parameters can be measured simultaneously (2) the transport of both holes and electrons can be isolated, and (3) the method is applicable to the practical a-Se layer structure with blocking contacts used in FPIs. The x-ray TOF method was applied to an analysis of ghostingmore » in a-Se photoconductor layers under portal imaging conditions, i.e., 1 mm thick a-Se layers, biased at 5 V/{mu}m, were irradiated using a 6 MV LINAC x-ray beam to a total dose (ghosting dose) of 30 Gy. The initial sensitivity (S{sub 0}) of the a-Se layers was 63{+-}2 nC cm{sup -2} cGy{sup -1}. It was found that S decreases to 30% of S{sub 0} after a ghosting dose of 5 Gy and to 21% after 30 Gy at which point no further change in S occurs. At an x-ray intensity of 22 Gy/s (instantaneous dose rate during a LINAC x-ray pulse), the charge carrier generation rate was 1.25{+-}0.1x10{sup 22} ehp m{sup -3} s{sup -1} and, to a first approximation, independent of the ghosting dose. However, both hole and electron transport showed a strong dependence on the ghosting dose: hole transport decreased by 61%, electron transport by up to {approx}80%. Therefore, degradation of both hole and electron transport due to the recombination of mobile charge carriers with trapped carriers (of opposite polarity) were identified as the main cause of ghosting in this study.« less

Authors:
; ;  [1]
  1. Department of Medical Biophysics, University of Toronto, 2075 Bayview Avenue, Toronto, Ontario, M4N 3M5 (Canada)
Publication Date:
OSTI Identifier:
20726388
Resource Type:
Journal Article
Journal Name:
Medical Physics
Additional Journal Information:
Journal Volume: 32; Journal Issue: 10; Other Information: DOI: 10.1118/1.2042248; (c) 2005 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0094-2405
Country of Publication:
United States
Language:
English
Subject:
62 RADIOLOGY AND NUCLEAR MEDICINE; CARRIER LIFETIME; DOSE RATES; FLUOROSCOPY; IMAGE PROCESSING; IMAGES; LINEAR ACCELERATORS; MAMMARY GLANDS; RADIATION DOSES; SENSITIVITY; TIME-OF-FLIGHT METHOD; X RADIATION

Citation Formats

Rau, A W, Bakueva, L, Rowlands, J A, Imaging Research, Sunnybrook and Women's College Health Sciences Centre, 2075 Bayview Avenue, Toronto, Ontario, M4N 3M5, and Department of Medical Biophysics, University of Toronto, 2075 Bayview Avenue, Toronto, ON, M4N 3M5. The x-ray time of flight method for investigation of ghosting in amorphous selenium-based flat panel medical x-ray imagers. United States: N. p., 2005. Web. doi:10.1118/1.2042248.
Rau, A W, Bakueva, L, Rowlands, J A, Imaging Research, Sunnybrook and Women's College Health Sciences Centre, 2075 Bayview Avenue, Toronto, Ontario, M4N 3M5, & Department of Medical Biophysics, University of Toronto, 2075 Bayview Avenue, Toronto, ON, M4N 3M5. The x-ray time of flight method for investigation of ghosting in amorphous selenium-based flat panel medical x-ray imagers. United States. https://doi.org/10.1118/1.2042248
Rau, A W, Bakueva, L, Rowlands, J A, Imaging Research, Sunnybrook and Women's College Health Sciences Centre, 2075 Bayview Avenue, Toronto, Ontario, M4N 3M5, and Department of Medical Biophysics, University of Toronto, 2075 Bayview Avenue, Toronto, ON, M4N 3M5. Sat . "The x-ray time of flight method for investigation of ghosting in amorphous selenium-based flat panel medical x-ray imagers". United States. https://doi.org/10.1118/1.2042248.
@article{osti_20726388,
title = {The x-ray time of flight method for investigation of ghosting in amorphous selenium-based flat panel medical x-ray imagers},
author = {Rau, A W and Bakueva, L and Rowlands, J A and Imaging Research, Sunnybrook and Women's College Health Sciences Centre, 2075 Bayview Avenue, Toronto, Ontario, M4N 3M5 and Department of Medical Biophysics, University of Toronto, 2075 Bayview Avenue, Toronto, ON, M4N 3M5},
abstractNote = {Amorphous selenium (a-Se) based real-time flat-panel imagers (FPIs) are finding their way into the digital radiology department because they offer the practical advantages of digital x-ray imaging combined with an image quality that equals or outperforms that of conventional systems. The temporal imaging characteristics of FPIs can be affected by ghosting (i.e., radiation-induced changes of sensitivity) when the dose to the detector is high (e.g., portal imaging and mammography) or the images are acquired at a high frame rate (e.g., fluoroscopy). In this paper, the x-ray time-of-flight (TOF) method is introduced as a tool for the investigation of ghosting in a-Se photoconductor layers. The method consists of irradiating layers of a-Se with short x-ray pulses. From the current generated in the a-Se layer, ghosting is quantified and the ghosting parameters (charge carrier generation rate and carrier lifetimes and mobilities) are assessed. The x-ray TOF method is novel in that (1) x-ray sensitivity (S) and ghosting parameters can be measured simultaneously (2) the transport of both holes and electrons can be isolated, and (3) the method is applicable to the practical a-Se layer structure with blocking contacts used in FPIs. The x-ray TOF method was applied to an analysis of ghosting in a-Se photoconductor layers under portal imaging conditions, i.e., 1 mm thick a-Se layers, biased at 5 V/{mu}m, were irradiated using a 6 MV LINAC x-ray beam to a total dose (ghosting dose) of 30 Gy. The initial sensitivity (S{sub 0}) of the a-Se layers was 63{+-}2 nC cm{sup -2} cGy{sup -1}. It was found that S decreases to 30% of S{sub 0} after a ghosting dose of 5 Gy and to 21% after 30 Gy at which point no further change in S occurs. At an x-ray intensity of 22 Gy/s (instantaneous dose rate during a LINAC x-ray pulse), the charge carrier generation rate was 1.25{+-}0.1x10{sup 22} ehp m{sup -3} s{sup -1} and, to a first approximation, independent of the ghosting dose. However, both hole and electron transport showed a strong dependence on the ghosting dose: hole transport decreased by 61%, electron transport by up to {approx}80%. Therefore, degradation of both hole and electron transport due to the recombination of mobile charge carriers with trapped carriers (of opposite polarity) were identified as the main cause of ghosting in this study.},
doi = {10.1118/1.2042248},
url = {https://www.osti.gov/biblio/20726388}, journal = {Medical Physics},
issn = {0094-2405},
number = 10,
volume = 32,
place = {United States},
year = {2005},
month = {10}
}