Effect of Charge Trapping and Release on the Performance of Polycrystalline Mercuric Iodide X-ray Converters Incorporating Frisch Grid Structures for Digital Breast Tomosynthesis

While active matrix flat-panel imagers (AMFPIs) are commonly used in low dose-per-frame applications such as digital breast tomosynthesis, DQE performance under such irradiation conditions is constrained by the large electronic additive noise relative to imaging signal. Replacement of the CsI:Tl and a-Se converters typically used in AMFPIs with polycrystalline mercuric iodide fabricated using particle-in-binder techniques (PIB-HgI2) would largely overcome this limitation due to the 3 to 10 times higher signal per interacting x-ray offered by this photoconductive material. However, practical implementation of PIB-HgI2 converters in clinical systems requires significant reduction of the high levels of image lag associated with the material. A promising strategy for significantly reducing lag is to diminish the contribution of signal induced by hole transport (which is believed to be a principal source of lag) through incorporation of a grid structure (referred to as a Frisch grid) into the converter material - with previous theoretical studies indicating hole signal reductions of up to similar to 95%. In this paper, an initial examination of the direct influence of the grid on image lag is reported. This involved modeling the trapping and release of holes in the converter for grids supported by insulating pillars. Signal properties, including line spread function, MTF, and image lag, were examined as a function of grid design parameters and operational conditions. The modeling shows that, when charge trapping and release are accounted for, favorably high suppression of image lag as well as good MTF is possible for some grid designs.