Purpose: The theoretical signal and spatial resolution performance of particle-in-binder mercuric iodide (PIB HgI2) x-ray converters incorporating pillar-supported Frisch grid structures are reported. The grid serves to reduce hole signal contribution, and thus image lag, under digital breast tomosynthesis (DBT) irradiation conditions.
Methods: Each converter employs a top, continuous electrode and 100 µm pitch, bottom pixelated electrodes. Finite element analysis was used to calculate electric field and weighting potential maps in the converter. A trajectory tracking algorithm used these maps to follow the transit of electrons and holes through the converter and determine induced signal. The signal performance of the converter along with the modulation transfer function (MTF) were investigated as a function of various grid parameters in order to identify grid designs that optimize performance.
Results: Examination of MTF and signal performance as a function of grid pitch (the center-to-center distance between two neighboring grid wires), RGRID (the ratio of grid wire width to grid pitch) and projection image number in a DBT scan, indicates the following. (1) Higher RGRID and smaller grid pitch provide better suppression of hole signal, but diminish total signal due to narrower gaps between grid wires. (2) Grid pitches considerably smaller than the pixel pitch are more effective for maintaining MTF close to the ideal of a sinc function corresponding to the pixel aperture – irrespective of RGRID. (3) For a total DBT scan exposure of 15 mR, charge accumulation on the insulator pillars has insignificant effect upon performance – other than a slight improvement of total signal across consecutive images for high RGRID.
Conclusion: PIB HgI2 converters incorporating a pillar-supported Frisch grid at a grid pitch of 20 µm and RGRID of 35% suppress up to ~76.5% of hole signal, while preserving total signal and MTF compared to a detector without a grid.
Funding Support, Disclosures, and Conflict of Interest: NIH grant 1R01-EB022028
MTF, Finite Element Analysis, Flat-panel Imagers
Not Applicable / None Entered.