Ability to peer into human body is an essential diagnostic tool in medicine. Out of the four major imaging modalities that include ultra-sound, magnetic resonance imaging (MRI), and positron emission tomography (PET), nuclear medicine is gaining more widespread attention. Nuclear medicine involves measurement of radiation from inside human body using in vivo pharmaceutical tracer. X-ray computed tomography (CT) and single photon emission computer tomography (SPECT) are three-dimensional (3D) extensions of planar nuclear imaging techniques. These 3D images provide a view of physiological processes that determined relative concentration of the pharmaceutical tracer.