Characterization of mercuric iodide photoconductor for radiographic and fluoroscopic medical imagers

Abstract

Photoconductive polycrystalline mercuric iodide deposited on flat panel thin film transistor (TFT) arrays is one of the best candidates for direct digital X-ray detectors for radiographic and fluoroscopic medical imaging. The mercuric iodide is vacuum deposited by Physical Vapor Deposition (PVD). This deposition technology has been scaled up to the 20cmX25cm size required in common medical imaging applications. A TFT array with a pixel pitch of 127 microns is used for these imagers. In addition to successful imager scale up, non-TFT based detectors were developed in order to improve analysis methods of the mercuric iodide photoconductor itself. These substrates consist of an array of palladium or ITO stripes on a glass substrate. Following deposit of the photoconductor, striped bias electrodes are deposited on top of the photoconductor at a 90 degree orientation to the bottom electrodes. These substrates provide more information than was previously available on the dark current and signal uniformity of the mercuric iodide photoconductor without the use of expensive TFT arrays. Mercuric iodide photoconductor thicknesses between 110 microns and 300 microns were tested with beam energy between 40 kVp and 120 kVp utilizing exposure ranges typical for both fluoroscopic and radiographic imaging. Diagnostic quality radiographic and fluoroscopic images at up to 15 pulses per second were demonstrated. Resolution tests on resolution target phantoms were performed and performance close to the theoretical sinc function up to the Nyquist frequency of ~3.9 lp/mm is shown (127 micron pixel pitch).