Signal behavior of polycrystalline HgI₂at diagnostic energies of prototype, direct detection, active matrix, flat-panel imagers

Abstract

Active matrix, flat-panel x-ray imagers based on a-Si:H thin film transistors offer many advantages and are widely utilized in medical imaging applications. Unfortunately, the detective quantum efficiency (DQE) of conventional flat-panel imagers incorporating scintillators or a-Se photoconductors is significantly limited by their relatively modest signal to noise ratio, particularly in applications involving low x-ray exposures or high spatial resolution. For this reason, polycrystalline HgI 2 is of considerable interest by virtue of its low effective work function, high atomic number, and the possibility of large-area deposition. In this study, a detailed investigation of the properties of prototype, flat-panel arrays coated with two forms of this high-gain photoconductor are reported. Encouragingly, high x-ray sensitivity, low dark current, and spatial resolution close to the theoretical limits were observed from a number of prototypes. In addition, input-quantum-limited DQE performance was measured from one of the prototypes at relatively low exposures. However, high levels of charge trapping, lag, and polarization, as well as pixel-to-pixel variations in x-ray sensitivity are of concern. While the results of the current study are promising, further development will be required to realize prototypes exhibiting the characteristics necessary to allow practical implementation of this approach.

Development of active matrix flat panel imagers incorporating thin layers of polycrystalline HgI2for mammographic x-ray imaging

“…While prototype 9B exhibits a dark current of a few pA/mm 2 , the dark current of prototype 11B is much lower, less than 0.1 pA/mm 2 – which is slightly lower than the best results obtained from previous PIB prototypes examined by our group. (Du et al , 2008)…”

“…These calculations are based on cascaded systems models similar to those used in previous studies. (Du et al , 2008; El-Mohri et al , 2007) The calculations illustrate that, compared to zero-frequency DQE values at high exposures (e.g., above ~2 mR) which approach an upper limit given by the product of the x-ray detection efficiency and the Swank factor of a given converter, the zero-frequency DQE of mammographic AMFPI systems drops more than 44% at low exposures (e.g., less than ~0.2 mR). Such DQE loss is even more pronounced at higher (i.e., non-zero) spatial frequencies.…”

“…In response, a number of candidate photoconductive materials, including PbI 2 , PbO and HgI 2 have been examined. (Street et al , 2002; Zuck et al , 2003; Hartsough et al , 2004; Kang et al , 2005; Simon et al , 2005; Su et al , 2005; Zentai et al , 2007; Du et al , 2008)…”

“…In previous studies, AMFPI prototypes employing ~210 to 635 μm of polycrystalline HgI 2 material prepared using both the PVD and PIB methods have been examined under radiographic, fluoroscopic and radiotherapy imaging conditions. (Du et al , 2008; Zhao et al , 2010; Zentai et al , 2004) In addition, an early investigation of the performance of an AMFPI prototype coated with 240 μm of PVD HgI 2 under mammographic imaging conditions demonstrated DQE values up to ~60%. (El-Mohri et al , 2003) However, the performance of prototype imagers incorporating PIB HgI 2 has not been previously investigated for mammographic imaging.…”

Photoconductor Digital X‐Ray Imaging

“…Du and Antonuk et al [72] gave a summary of their HgI 2 imagers ' evaluations. All of their imagers were prepared by RTR either by a PVD or PIB process.…”