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Proceedings Paper

Parameter investigation and first results from a digital flat panel detector with forward bias capability
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Paper Abstract

Digital flat panel a-Si x-ray detectors can exhibit image lag of several percent. The image lag can limit the temporal resolution of the detector, and introduce artifacts into CT reconstructions. It is believed that the majority of image lag is due to defect states, or traps, in the a-Si layer. Software methods to characterize and correct for the image lag exist, but they may make assumptions such as the system behaves in a linear time-invariant manner. The proposed method of reducing lag is a hardware solution that makes few additional hardware changes. For pulsed irradiation, the proposed method inserts a new stage in between the readout of the detector and the data collection stages. During this stage the photodiode is operated in a forward bias mode, which fills the defect states with charge. Parameters of importance are current per diode and current duration, which were investigated under light illumination by the following design parameters: 1.) forward bias voltage across the photodiode and TFT switch, 2.) number of rows simultaneously forward biased, and 3.) duration of the forward bias current. From measurements, it appears that good design criteria for the particular imager used are 8 or fewer active rows, 2.9V (or greater) forward bias voltage, and a row frequency of 100 kHz or less. Overall, the forward bias method has been found to reduce first frame lag by as much as 95%. The panel was also tested under x-ray irradiation. Image lag improved (94% reduction), but the temporal response of the scintillator became evident in the turn-on step response.

Paper Details

Date Published: 18 March 2008
PDF: 9 pages
Proc. SPIE 6913, Medical Imaging 2008: Physics of Medical Imaging, 69130L (18 March 2008); doi: 10.1117/12.772356
Show Author Affiliations
Jared Starman, Stanford Univ. (United States)
Carlo Tognina, Varian Medical Systems (United States)
Gary Virshup, Varian Medical Systems (United States)
Josh Star-lack, Varian Medical Systems (United States)
Ivan Mollov, Varian Medical Systems (United States)
Rebecca Fahrig, Stanford Univ. (United States)

Published in SPIE Proceedings Vol. 6913:
Medical Imaging 2008: Physics of Medical Imaging
Jiang Hsieh; Ehsan Samei, Editor(s)

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