Characterization and correction of pulse pile‐up in simultaneous emission–transmission computed tomography

Abstract

We have developed an emission-transmission CT (ETCT) system capable of both single-photon emission computed tomography (SPECT) imaging and x-ray transmission CT imaging using a common photon counting detector. In principle, SPECT and x-ray CT projection data can be acquired simultaneously with the ETCT system; however, doing so results in contamination of the SPECT projection data due to pulse pile-up caused by the relatively high x-ray fluence rate. In this study, we characterize the effects of pulse pile-up for simultaneous ETCT imaging through computer simulation and experimental studies. We demonstrate that pulse pile-up in the SPECT energy window can be well approximated by a simple quadratic relationship between the pile-up rate and the x-ray fluence rate for sufficiently small x-ray fluence rates. Using this quadratic relationship, we developed a simple pile-up correction scheme that subtracts the pile-up counts from the emission data and also truncates the exterior regions of the emission projection data. Analysis of difference images and profiles indicate that this method permits us to reconstruct SPECT images with no apparent noise or resolution degradation in comparison to those obtained via sequential emission and transmission scans.

Technological Development and Advances in Single-Photon Emission Computed Tomography/Computed Tomography

“…Moreover, the expense of HPGe made it difficult to envision how this detector technology could be implemented practically at a realistic size and cost. Second, the x-ray tube in the prototype system was operated at a very low power level (eg approximately 100-120 kV at 1 mA) but still produced x-ray data with sufficiently high count-rates to cause pulse pile-up that contaminated both the emission and transmission data [51]. The UCSF group designed and tested prototype electronics that allowed the HPGe detector array in the SPECT/CT system to be switched between photon-counting mode for radionuclide imaging and currentmode for x-ray imaging [52][53][54] for near-simultaneous x-ray and radionuclide imaging using a single detector array.…”