Evaluation of the geometric, scatter and septal penetration components in fan beam collimators using Monte Carlo simulation

The quantitative analysis of SPECT data requires an accurate determination of the collimator point spread function (PSF). The aim of this work is to characterize the PSFs of fan beam and parallel collimators by using Monte Carlo simulation. Given a particular collimator configuration, a detailed hexagonal hole array is generated and information describing its geometry is stored in a look-up table. When a photon crosses the collimator front plane, a forty-hole array is placed around its impact position using this table. Each photon is then tracked up to the detector surface by using the Monte Carlo code PENELOPE and its associated geometry handling routines. Particle counters are defined that score the probability of impact on the detector as a function of the final photon position. Four sets of counters are employed so as to differentiate contributions to the geometric, septal penetration, coherent (Rayleigh) and incoherent (Compton) scatter components. Furthermore, sensitivity quantification and pulse-height energy spectra are calculated for different source locations. Monte Carlo results have been compared with sensitivity values obtained experimentally and good agreement was found. The authors´ results show that for ^99mTc imaging, the geometric component represents about 95% of the fan beam PSF, whereas the incoherent scattering component is negligible