Taly Gilat Schmidt PhD, Presenter: Nothing to Disclose

The purpose of this work is to quantify the impact of energy weighting on the scatter-to-primary ratio (SPR) of x-ray projections.

Conventional x-ray and CT systems utilize energy-integrating detectors that more heavily weight higher energy photons. Photon counting detectors can increase the SNR by weighting all photons equally. Energy-discriminating detectors enable task-dependant, optimal weighting by assigning more weight to low energy photons. Because scatter has a softer spectrum than the primary beam, optimal weighting schemes may amplify scatter. To quantify this effect, we generated primary and scatter projections of a 20-cm-diameter water cylinder using the Geant4 Monte Carlo toolkit. We investigated five weighting schemes: energy integrating, photon counting, optimal weighting for the task of differentiating adipose tissue and water, optimal weighting for the task of differentiating calcium and water, and a general optimal weighting (1/energy^3). The cone-angle was varied from 0.5 - 9 degrees to simulate a range of scatter conditions. SPR was calculated as the ratio of the weighted scatter and primary projections. The simulations were performed with 80 kVp and 120 kVp incident spectrums. We examined the contrast in the projection by simulating a 1-cm-diameter sphere of adipose tissue embedded in the cylinder center.

Compared to energy integrating, optimal energy weighting increased the SPR at 120 kVp by 60-117% depending on the task and by 50-70% at 80 kVp. Photon counting increased the SPR by 15% and 11% at 120 kVp and 80 kVp respectively. Results were independent of SPR magnitude. In the absence of scatter, the contrast of the adipose sphere was increased by 10% with photon counting and by 33% with optimal energy weighting. When scatter from the 9-degree cone-beam is included, the improvement in contrast was 4% for photon counting and 7% for optimal energy weighting.

Photon counting and optimal energy weighting increase the SPR compared to energy-integrating weighting. Scatter rejection methods must be implemented in order to fully realize the benefits of optimal energy weighting. Future work is planned to investigate the effects of energy weighting and scatter on the CNR in CT reconstructions.

Photon counting and energy-discriminating detectors, capable of improved photon weighting schemes, are being implemented for x-ray and CT applications.

Schmidt, T, The Effects of X-ray Energy Weighting on the Detected Scatter Signal: A Simulation Study.  Radiological Society of North America 2008 Scientific Assembly and Annual Meeting, February 18 - February 20, 2008 ,Chicago IL. http://archive.rsna.org/2008/6019407.html