Saturday, May 17, 2025
5/17/2025
Improving the dose efficiency of photon counting CT - PROJECT SUMMARY/ABSTRACT Radiation dose is a major drawback of computed tomography (CT) imaging. While the long-term effects of radiation are difficult to quantify, there is a continued desire to reduce radiation dose through technical improvement. In theory, photon counting detectors (PCDs) provide several opportunities to reduce CT radiation dose. In practice, however, PCD CT scanners today have shown only modest gains in dose efficiency compared to scanners from prior generations for most scanning protocols. Several components of the CT scanner must be re-engineered to benefit from the dose saving potential of PCDs. The objective of this work is to perform this re-engineering by designing new components that can take advantage of radiation dose reduction opportunities with PCD CT. In Specific Aim 1, we will develop hardware that could improve quantum efficiency, or the fraction of incident photons that are successfully detected. Today, about 40% of the x-rays that arrive at the detector are never absorbed by the sensor. 30% of the x-rays are stopped by the anti-scatter grid, and 10% of the x-rays penetrate the sensor without interaction due to the relatively low stopping power of the sensor. First, we will experimentally demonstrate a lightweight, striped anti-scatter grid that will increase the number of primary photons detected while providing a means for estimating residual scatter. The geometry of this grid introduces striping in the detector signal, with stripe contrast proportional to the level of remaining scatter. Therefore, the images themselves will contain a means to subtract the remaining scatter. Second, we will develop a guardrail that can fluoresce a fraction of the punch-through photons and return them to the sensor. We will demonstrate the guardrail experimentally in a CdTe benchtop PCD. In Specific Aim 2, we turn to spectral nonidealities in PCD CT. PCDs suffer from pulse pileup and charge sharing. We will experimentally test a pileup detection circuit that reroutes suspected pileup events into separate counters. We show that this substantially reduces the impact of pileup and will simulate its use together with charge summing modes to reduce charge sharing effects. Historically, charge summing modes have not been used in CT applications because of their pileup penalty. We hypothesize that this mode could be enabled by stronger pileup corrections, such as the one we will demonstrate. Charge summing modes also improve dose efficiency for grayscale (non-spectral) tasks by eliminating double counting effects. Successful completion of these Aims could allow PCDs to unlock their dose reduction potential and could improve the dose efficiency of PCD CT by 40% for non-spectral imaging tasks and 200% for spectral imaging tasks.
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