Room: Room 202
Purpose: Practical application of the superior x-ray sensitivity of polycrystalline mercuric iodide to improve the signal-to-noise performance of active matrix, flat-panel imagers requires reduction of the significant charge trapping exhibited by this material. In this presentation, we report results from an examination of how the introduction of a third electrode, referred to as a Frisch Grid, suppresses signal arising from hole transport (hypothesized to reduce trapping) while promoting signal arising from electron transport. The complex dependence of signal performance on grid design and operational conditions is explored and insights on optimizing detector performance are discussed.
Methods: Detector signal generated by charge transport was examined through analysis of electric field distributions created using finite element analysis techniques for a wide variety of grid designs. The resulting field maps were used to track charge transport of both electrons and holes through the converter, assuming an energy deposition profile created under irradiation conditions relevant to digital breast tomosynthesis. Trajectory information obtained from this modeling was used to calculate various metrics that illuminate different aspects of a grid's effect on signal performance.
Results: Maximizing signal collected from electron transport (to enhance DQE), while minimizing signal collected from hole transport involves a trade-off. Larger gaps between grid elements promote more efficient collection of electron signal, while smaller gaps can more effectively suppress hole signal (depending on the grid pitch). These effects are enhanced as the ratio of the electric field across the bottom of the converter compared to that across the top is increased.
Conclusion: Through judicious selection of grid design and operational conditions, a reduction of more than two-thirds of the signal contribution from holes (producing a proportionate reduction in charge trapping) is predicted to be possible - without degradation of the DQE due to signal loss. Supported by NIH grant R01-EB022028.
Tomosynthesis, Flat-panel Imagers, Simulation