1. A method of reconstructing a tomographic image of a subject imaged with a nuclear imaging detector having a non-parallel hole collimator comprising: acquiring first projection data of a radiation field within the subject at a first view angle of said detector; acquiring second projection data of said radiation field within the subject at a second view angle of said detector different than said first view angle; combining the first and second projection data to form a difference image; and determining a location of the radiation field relative to the collimator surface using the difference image based on Point Spread Function (PSF) which is non-stationary and dependent on a location of a gamma event with respect to the collimator surface of the non-parallel hole collimator.