Positron emission tomography (PET), is a tool of major promise in biomedical research and clinical applications, which yields images representing the distribution of a systemically administered positron-emitting radionuclide in transverse tomographic sections of the body of human subjects or experimental animals. The usefulness of PET stems from the fact that some elements of fundamental importance in the investigation biological processes possess radionuclides which decay by the emission of positrons. Most of these radionuclides (11C, 13N, 15O, 18F) decay with half-lives of minutes and must be prepared in the vicinity of the site of utilization. Many molecules of physiological importance have been labeled with these radionuclides. Through the use of these molecules, PET permits the in vivo regional assessment of a number of biochemical processes essential to life. Most PET devices utilize scintillation detectors fitted either with sodium iodide, bismuth germanate or cesium fluoride crystals. A promising improvement in PET consists in the incorporation of photon time-of-flight information in the image reconstruction process. PET images are reconstructed from a large number of measurements and the storage and utilization of this information requires large computer memory capabilities and fast processing systems. State of the art PET devices yield images with the spatial resolution better than 1 cm with a contrast resolution of better than 10% in a period of time of less than 1 minute.