Systematic development of input-quantum-limited fluoroscopic imagers based on active matrix, flat-panel technology

The development of fluoroscopic imagers exhibiting performance that is primarily limited by the noise of the incident x-ray quanta, even at very low exposures, remains a highly desirable objective for active matrix flat-panel technology. Previous theoretical and empirical studies have indicated that promising strategies to achieving this goal include the development of array designs incorporating improved optical collection fill factors, pixel-level amplifiers, or very high-gain photoconductors. Our group is pursuing all three strategies and this paper describes progress toward the systematic development of array designs involving the last approach. The research involved the iterative fabrication and evaluation of a series of prototype imagers incorporating a promising high-gain photoconductive material, mercuric iodide (HgI ₂). Over many cycles of photoconductor deposition and array evaluation, improvements in a variety of properties have been observed and remaining fundamental challenges have become apparent. For example, process compatibility between the deposited HgI ₂ and the arrays has been greatly improved, while preserving efficient, prompt signal extraction. As a result, x-ray sensitivities within a factor of two of the nominal limit associated with the single-crystal form of HgI ₂ have been observed at relatively low electric fields (-0.1 to 0.6 V/μm), for some iterations. In addition, for a number of iterations, performance targets for dark current stability and range of linearity have been met or exceeded. However, spotting of the array, due to localized chemical reactions, is still a concern. Moreover, the dark current, uniformity of pixel response, and degree of charge trapping, though markedly improved for some iterations, require further optimization. Furthermore, achieving the desired performance for all properties simultaneously remains an important goal. In this paper, a broad overview of the progress of the research will be presented, remaining challenges in the development of this photoconductive material will be outlined, and prospects for further improvement will be discussed.