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

Monte Carlo simulation is applied to model the penetration effect for iodine-131 pinhole imaging. In the authors' Monte Carlo simulation, they 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 is 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 the authors' Monte Carlo model. For the validation, the responses of a point source at 6 different locations along the central ray of the pinhole aperture were measured to compare with simulated responses. The distance of the source locations is 3-18 cm away from the aperture with the inter-location distance equal to 3 cm. The comparison demonstrates the accuracy of the authors' Monte Carlo model. With the validated Monte Carlo program, the authors simulated point response functions for pinhole aperture with various spanning angle, hole size, and materials. The point responses are parameterized using radially circularly symmetric two-dimensional exponential functions. The parameter describing the roll-off rate of an exponential function is 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 be incorporated into image reconstruction algorithms which compensate for the penetration effect.