Technical note: comparing coherent and incoherent scatter effects for cone-beam CT

The intensity and distribution of scatter is of growing interest in the case of flat-detector CT (FD-CT) scanners. It is generally assumed that scatter is an offset distribution dominated by incoherent scatter (Compton effect). Coherent and incoherent scatter were computed with respect to acquisition parameters used in FD-CT. A hybrid method optimized the performance of the scatter simulation: a fast and exact analytical calculation of the single-scatter intensity combined with a coarse Monte Carlo (MC) estimate of multiple scatter to reduce overall computational expenses, while assuring an acceptable signal quality. The simulation package allowed for the separation of both coherent and incoherent single scatter. For cylindrical water phantoms varying from 40 mm to 320 mm diameter and for various z-collimations an investigation of the ratio of single-to-total scatter was performed. The intensity distribution of scatter (including coherent single scatter, incoherent single scatter and total scatter) was computed for phantoms of 160 mm and 320 mm diameter and energies varying from 40 keV to 120 keV. Additionally, voxel phantoms were generated from clinical head and thorax CT datasets and were employed in scatter simulations to demonstrate the effect of coherent and incoherent single scatter on the 2D projections. Single scatter determines a significant fraction of total scatter even for large body-sized phantoms and large field of measurement. Coherent scatter dominates the distribution of total scatter for most of the simulated cases, even for 120 keV. The Compton effect appeared to be less influential with respect to spatial distribution but dominated the overall magnitude.