Comparison of mercuric iodide and lead iodide X-ray detectors for X-ray imaging applications

Mercuric iodide (HgI2) and lead iodide (PbI2) have been under development for several years as direct converter layers for digital x-ray imaging. We deposited these materials on flat panel thin film transistor (TFT) arrays and found that they are good candidates for x-ray imaging applications in medical imaging, security applications, and Non Destructive Testing (NDT). This paper present basic imaging parameters and compares both lead iodide and mercuric iodide imagers. A difficult challenge of both lead iodide and mercuric iodide is the higher than desired leakage currents. These currents are influenced by factors such as applied electrical field, layer thickness, layer density, electrode structure and material purity. Minimizing the leakage current must also be achieved without adversely affecting charge transport, which plays a large role in gain and is influenced by these parameters. New deposition and annealing technologies [1] have been developed through which the leakage current has now decreased by more than an order of magnitude while showing no negative affects on gain. Other challenges relate to increasing film thickness without degrading electrical properties. Both lead iodide and mercuric iodide were vacuum deposited by Physical Vapor Deposition (PVD) on a-Si TFT arrays with 127 mu m pixel pitch. This coating technology is scalable to sizes required in common x-ray imaging applications, as proven by the present 4" x 4" and 8" x 10" imager results [4]. The imagers were evaluated for both radiographic and fluoroscopic imaging. Modulation Transfer Function (MTF) was measured as a function of the spatial frequency. The MTF data were compared to values published in the literature for indirect detectors (01). Resolution tests on resolution target phantoms showed that resolution is limited by the TFT array Nyquist frequency (similar to 3.9 lp/mm). The ability to operate at moderate voltages (similar to 0.5-1.0 V/mu m) provides acceptable dark current for most applications and permits low voltage electronics design. Image lag characteristics of mercuric iodide appear adequate for fluoroscopic rates. The structure and x-ray diffraction data of the two materials were compared to explain the difference in image lag between them.