Theoretical Examination of Lag Suppression in Detectors Employing Polycrystalline HgI2 Converters Through Incorporation of Frisch Grid Structures for Digital Breast Tomosynthesis
L Shen1*, Y El-Mohri1, Q Zhao1, L Antonuk1, (1) University of Michigan - Ann Arbor, Ann Arbor, MI
Presentations
PO-GePV-I-21 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: Reduction of the undesirably high level of lag exhibited by x-ray converters based on particle-in-binder mercuric iodide (PIB HgI₂) was investigated through modeling of Frisch grids embedded in the material. PIB HgI₂ is an attractive candidate for replacing a-Se and CsI:Tl converters in active matrix, flat-panel imagers (AMFPIs) due to the large signal generated per interacting x-ray which helps to preserve high DQE at low exposures.
Methods: The modeled detectors consist of a pillar-supported Frisch grid embedded in a 300 μm thick PIB HgI₂ layer which is in contact with pixelated electrodes that conceptually correspond to the top contacts of an underlying, 100 μm pitch, AMFPI array. The modeling employs finite element analysis to create maps of electric field and weighting potential as well as a trajectory tracking algorithm (which accounts for trapping and release of holes) to follow the transit of electrons and holes through the converter – allowing determination of induced signal. Assuming a distribution of electrons and holes created under irradiation conditions relevant to digital breast tomosynthesis, signal properties including lag, MTF and DQE were calculated.
Results: Results were obtained as a function of the pitch of the grid wires and the ratio of the width of grid wires to grid pitch (RGRID) – allowing identification of optimal designs that minimize hole signal (which is responsible for lag) and maximize total induced signal. For first frame lag, the modeling shows that (1) lag decreases with increasing RGRID and decreasing grid pitch, (2) lag can be reduced to less than 1% (compared to levels typically greater than 10% for PIB HgI₂) while largely preserving total induced signal, thereby helping to maintain DQE.
Conclusion: Introduction of an optimally designed, pillar-supported Frisch grid into a PIB HgI₂ converter can strongly suppress lag while preserving DQE at low exposures.
Keywords
Finite Element Analysis, Flat-panel Imagers, DQE
Taxonomy
IM- Breast X-Ray Imaging: Detector development & evaluation
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