Scatter Rejection In X-Ray CT Using Time-of-Flight Information
Abstract
Purpose
Time of flight (TOF) x-ray imaging is an emerging technology that combines high temporal resolution photon counting detector and very short x-ray pulses to provide additional information about photon interactions. For example, due to longer path lengths, scattered radiation arrives at the detector with a time delay relative to primary (unscattered) photons. This work investigates the potential for scatter rejection in simulated TOF computed tomography (CT) for varied temporal resolution and x-ray pulse width.
Methods
A Monte Carlo simulator was developed to simulate photon interactions and record detection events and the arrival time. Experiments using a 10 mm radius water cylinder, a 90 kVp source, and 180 projections with 2-degree sampling steps were conducted. Assuming that both the source pulse shape and the detector timing jitter have Gaussian distributions, we perform repeated simulations with a combined FWHM temporal resolution of 50, 100 and 200 ps. We explored multiple delay thresholds to reject late-arriving photons, such that the retained fraction of primary (signal efficiency, SE) is in the range 56-99%. FBP reconstructions were performed for each case and the RMSE compared to a scatter-free reconstruction was reported.
Results
Simulations show an overlap between the measured time delay of primary and scattered photons, which is increasing with the total FHWM. As a result, for increasing SE, the fraction of scattered radiation correctly discarded (rejection efficiency, RE) is decreasing. The RMSE metric indicates that there is an optimal operating region, around 70-80% SE for each FWHM considered, where a balance between primary signal retention and scatter suppression is achieved. Deviations towards either lower RE or SE leads to degraded image quality.
Conclusion
These results show that time-delay thresholding can improve image quality in TOF CT and some of the tradeoffs that need to be considered for TOF CT design.