Monte Carlo modeling of penetration effect for iodine-131 pinhole imaging

In our Monte Carlo simulation, we employed the variance reduction technique, forced detection, to improve simulation efficiency. For the forced detection, the minimal cone that covers the knife-edge region of a pinhole aperture was used to confine the direction of a photon emission with the vertex located at the emission point. A lead pinhole insert was used to validate our Monte Carlo model. For the validation, the responses of a point source at six different locations along the central ray of the pinhole aperture were measured to compare with simulated responses. The range of distances for the source locations was 3-18 cm from the aperture with the inter-location distance equal to 3 cm. The comparison demonstrated the accuracy of our Monte Carlo model. With the validated Monte Carlo program, we simulated point response functions for pinhole aperture with various aperture span angle, hole size, and materials. The point responses were parameterized using radially circularly symmetric two-dimensional exponential functions. The parameter describing the roll-off rate of an exponential function was expressed in terms of the span angle of the pinhole knife-edge opening and the material used to make the pinhole aperture. The parameterized penetration model can he incorporated into image reconstruction algorithms that compensate for the penetration effect.