- Recent advances have greatly enhanced the three-dimensional (3D) imaging of radioactive tracers in living animals. this article introduces the physics of imaging behind the imaging methods. The article first discusses the selection of the radiation emitted from the tracer and then the process of tomographic reconstruction or how 3D images are made from imaging around the outside of the animal. The technique of single photon emission computed tomography (SPECT) in which the detection of one X-ray or gamma ray at a time is employed for image formation is then described. Finally, positron emission tomography (PET) which relies on the simultaneous detection of the pair of gamma-rays formed when the positron annihilates is presented.