title: Digital radiology using active matrix readout of amorphous selenium: detectors with high voltage protection.
abstract: A flat-panel x-ray imaging detector is being investigated for digital radiography and fluoroscopy. The detector uses a layer of amorphous selenium (a-Se) to convert x rays to a charge image, which is then electronically read out with a two-dimensional array of thin film transistors (TFTs). In order to sensitize the a-Se layer to x rays, a high voltage (of the order of several thousand volts) is applied to its top surface. The TFTs, which are at the bottom surface of the a-Se layer, are not subjected to any high voltage under normal radiological operational conditions since the pixel potential is < 10 V. However under a fault condition where these two events occur simultaneously: (1) suspended detector scan; and (2) an x-ray exposure more than ten times higher than normal, the voltage on the TFTs could rise to a damaging value. This paper describes a method for protecting the TFTs from high voltage damage under this fault condition. It employs a dual-gate TFT structure, one gate is for scanning control and the other is connected to the pixel electrode for high voltage protection. Before the pixel potential reaches a damaging value, the protection gate turns on the TFT automatically and drains excess charge away from the pixel thus providing a safe pixel saturation potential. In this paper, the characteristic curves of dual-gate TFTs are studied both theoretically and experimentally. The pixel x-ray response for imaging detectors with high voltage protection are predicted, and it is shown that with practical TFT designs the detector can provide a safe pixel saturation potential as well as satisfy the dynamic range required for diagnostic x-ray imaging applications.
authors.affiliation: Department of Medical Biophysics, University of Toronto, Sunnybrook Health Science Centre, Ontario, Canada.
authors.forename: W
authors.initials: W
authors.lastname: Zhao
authors.forename: J
authors.initials: J
authors.lastname: Law
authors.forename: D
authors.initials: D
authors.lastname: Waechter
authors.forename: Z
authors.initials: Z
authors.lastname: Huang
authors.forename: J A
authors.initials: JA
authors.lastname: Rowlands
mesh_terms: D004867:Equipment Design; D005260:Female; D005471:Fluoroscopy / Q000379:methods*; D006801:Humans; D008327:Mammography / Q000379:methods; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011835:Radiation Protection; D011857:Radiographic Image Interpretation, Computer-Assisted / Q000295:instrumentation / Q000379:methods; D011859:Radiography / Q000379:methods*; D013902:Radiography, Thoracic / Q000379:methods; D015203:Reproducibility of Results; D012643:Selenium; D014965:X-Rays
chemical_list: D012643:Selenium