Charge Sharing Corrections for a Photon-Counting Detector (PCD) and Their Impact on PCD-CT
Abstract
Purpose
To compare digital coincidence detection (DCD) with analog charge summation (ACS) for a cadmium zinc telluride (CZT) photon counting detector (PCD) and evaluate the impact on spatial resolution in PCD computed tomography (CT) as a function of imaging beam energy.
Methods
A CZT PCD with 330 µm pixels equipped with DCD and ACS and six energy bins was used with a 120kVp x-ray source. Charge-sharing effects were quantified by acquiring PCD-CT images of a spatial resolution piece featuring 0.25mm-1mm hole patterns on a table-top system. The thresholds were set at 25, 33, 41, 50, 59, and 68keV, balancing the number of counts in each bin first without additional filtration. To simulate the human body, 0-25cm of acrylic slabs at an air gap of 64cm were added, with the tube current increasing proportionally. Quantification of spatial resolution was performed through the modulation transfer function (MTF) for DCD and ACS.
Results
Bins 3 and 4 were omitted from the study because of a 50% and 100% discrepancy in MTF in comparison to the other bins for the same energy thresholds. Overall, both DCD and ACS experienced up to a 25% loss in MTF as attenuation increased to 25cm of acrylic. Without additional filtration, the MTF trends were consistent between DCD and ACS, with the MTF decreasing with energy. With thicker filtration, DCD showed higher MTF values at the higher spatial frequencies by 10% to 20% for energy bin 1 where charge sharing is more prominent. Additionally, the overall spatial resolution for all energies was maintained between DCD and ACS.
Conclusion
Both ACS and DCD are overall comparable in MTF in spatial resolution performance for most energy bins. However, DCD shows a distinct advantage in the lower energy bins, particularly in the higher spatial frequencies where charge sharing has a greater impact.