Theoretical Investigation Of Detector Mtf Of Polycrystalline Mercuric Iodide X-Ray Converters Incorporating Frisch Grid Structures For Digital Breast Tomosynthesis

The DQE performance of active matrix flat-panel imagers (AMFPIs) degrades under low dose conditions - such as those encountered in digital breast tomosynthesis where electronic additive noise becomes comparable to imaging signal. Compared to commercially available x-ray converter materials such as CsI:Tl and a-Se, particle-in-binder polycrystalline mercuric iodide (PIB HgI2) offers imaging signal 3 to 10 times larger. However, PIB HgI2 exhibits an unacceptably high degree of image lag, believed to originate from the trapping of holes. To suppress hole signal contribution (with the expectation of decreasing image lag), a Frisch grid structure embedded within a PIB HgI2 detector is under investigation. In this theoretical study involving finite element analysis modeling, the effects of grid design and charge carrier lifetimes on the line spread function and modulation transfer function (MTF) were investigated. Two design parameters were considered: grid pitch (defined as the center-to-center distance between adjacent grid elements) and R-GRID (defined as the ratio of grid element width to grid pitch). Results show that for a grid pitch comparable to the pixel pitch (i.e., 100 mu m) and high R-GRID, MTF is significantly degraded compared to a detector without a grid while, for a grid pitch of 20 mu m, MTF is largely maintained and is almost independent of R-GRID. The best identified design is a grid pitch of 20 mu m and R-GRID of 45% - providing an MTF similar to that of a detector without a grid while suppressing hole signal by 78%. This work is supported by NIH grant R01-EB022028.