A handheld gamma camera or PET system with a disposable detector head is provided. This system includes a configuration of a gamma camera or PET scanner in which optical fibers or bundles of optical fibers are coupled to a scintillator or array of scintillators and the other end of the optical fibers or bundles of optical fibers are coupled to a light-sensitive camera, such as a photomultiplier. The system may further include a mechanism to rapidly couple and/or decouple the optical fiber or fibers from the light sensitive camera or from the scintillator or array of scintillators so that the detector can be disposed of or sterilized without damaging the light-sensitive camera. A method for image reconstruction and image simulation is also provided. The method includes an application of deterministic sampling using Gaussian quadrature parameters to construct a transition matrix for purposes of image reconstruction.