SIMIND based pinhole imaging: Development and validation

Monte Carlo programs, like the SIMIND program, have become increasingly used to simulate imaging systems like the scintillation camera and SPECT systems. Up to now, it has not been able to simulate a pinhole-imaging device with SIMIND. The aim of this work was to develop a routine for pinhole-imaging consisting of a knife-edge collimator and a conical shielding. The routine tracks the path of each photon through the pinhole-collimator and scores if photons either i) pass geometrically through the pinhole ii) penetrate through the edges of the pinhole or iii) being scattered in the collimator. This allows for calculation of fractions of geometrical, penetrating and scattered photons that contribute to an image. Variance reduction is implementing by forcing the photon, emitted from the last interaction point (or from the initial decay location), into a direction towards the center of the pinhole. Characteristic x-ray emissions from photon interactions are included. Results from simulations were compared to results from experimental studies using a SPECT system with a physical pinhole-collimator. The parameters compared were here the sensitivity (cps/MBq) and the shape of line-spread functions as function of distance. Comparisons were also made with results from previously published Monte Carlo simulations of pinhole collimators for different radionuclides. Results from our simulations mostly showed a good agreement but for some cases we found differences especially in the values of the fraction of geometrical, penetrating and scattered photons when comparing to previously reported results. Our conclusion is, however, that the routine provides accurate pinhole collimator simulations.