Abstract:
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Purpose: To achieve x-ray input-quantum-limited operation under fluoroscopic irradiation conditions, the performance of novel, direct detection, active matrix flat-panel imagers (AMFPIs), using a high gain photoconductive material, has been studied.
Methods and Materials: Current AMFPI technologies employing direct and indirect detection of the incident radiation exhibit limited system gain and comparatively large electronic noise. This results in a relatively low signal-to-noise ratio at the low exposures encountered in fluoroscopy, which reduces DQE performance. In order to overcome these limitations, a newly designed, prototype imager based upon a 127 μm pixel pitch direct detection array, incorporating a high gain HgI2 photoconductor, has been developed. This imager has a pixel format of 768x768 and was operated at 30 fps over the fluoroscopic exposure range. For comparison, the performance of a conventional indirect detection prototype AMFPI based on a 1536x1920 pixel array with a 127 μm pitch was measured under similar conditions. This imager employed two types of x-ray converters: a 70 mg/cm2 Gd2O2S:Tb phosphor screen and a 600 μm thick CsI:Tl scintillator. The performance of both imager designs were compared to each other and to theoretical calculations based on a cascaded systems model.
Results: Characterization of both direct and indirect detection imagers in terms of fundamental pixel signal properties as well as observer-independent performance variables (MTF, NPS, and DQE) was performed. The direct detection HgI2 imager exhibits very high sensitivity, providing over an order of magnitude larger signal than that of conventional direct and indirect detection AMFPIs. The DQE performance of the direct detection prototype is high across the fluoroscopic exposure range and is limited mainly by the quantum efficiency of the photoconductor. This level of performance is superior to that of the indirect detection imager with the CsI:Tl scintillator. For all measurements, the results are in reasonable agreement with theoretical predictions from the cascaded systems model.
Conclusion: Direct detection prototype imagers employing a HgI2 photoconductor provide high system gain and a high DQE over the fluoroscopic exposure range. Such performance matches or exceeds that of conventional direct and indirect detection flat-panel imagers and demonstrates the potential for input-quantum-limited fluoroscopic operation. This work is supported by NIH grant R01-EB000558.
Zhao PhD, Q,
Progress Toward Input-Quantum Limited Performance of Active Matrix Flat-Panel Fluoroscopic Imagers. Radiological Society of North America 2003 Scientific Assembly and Annual Meeting, November 30 - December 5, 2003 ,Chicago IL.
http://archive.rsna.org/2003/3107793.html
