TH-C-I-611-09: Development of Direct Detection Active Matrix Flat-Panel Imagers Employing Mercuric Iodide for Diagnostic Imaging

Purpose: To characterize the performance of prototype direct-detection active matrix flat-panel imagers (AMFPIs) employing recently improved forms of polycrystalline mercuric iodide photoconductors as x-ray converters for fluoroscopic imaging. Method and Materials: While direct and indirect detection flat-panel imagers offer good performance for many applications, their modest signal per interacting x-ray limits the DQE performance under imaging conditions of low exposure such as in fluoroscopy. One possible pathway to overcome this limitation involves the use of a high gain photoconductive x-ray converter such as mercuric iodide (HgI2). Accordingly, performance results from a number of AMFPI prototypes employing two types of polycrystalline HgI2 (a form created through physical vapor deposition, PVD, and a particle-in-binder form involving screen printing, PIB) are reported. Each prototype incorporates an array with a pixel format of 768×768 and a pixel pitch of 127 μm, and was operated under fluoroscopic conditions. The measured performance was compared to theoretical calculations based on a cascaded systems model. Results: Performance of the prototypes is reported in terms of pixel properties as well as MTF, NPS and DQE. Recent significant improvements in the properties of HgI2 photoconductors, such as chemical stability, low dark current (< 10 pA/mm2), low lag, and high x-ray sensitivity (corresponding to an effective ionization energy approaching the theoretical limit of ∼5 eV) resulted in a high DQE performance at low exposures. Pixel-to-pixel non-uniformities, which tend to reduce the dynamic range, remain high (∼10% to 30%) and thus require further optimization. Conclusions: While the development of polycrystalline HgI2 as an x-ray converter for AMFPIs has achieved important milestones related to various material properties, various challenges remain. Nevertheless, the high x-ray sensitivity and DQE observed at low exposures demonstrate the potential for input-quantum-limited fluoroscopic operation. This work was supported by NIH grant R01-EB000558.