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

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