From methods of imaging human anatomy with X-rays, attention is now turned to imaging physiological function using radiation emitted by radioactive isotopes inside the human body. The techniques discussed in this chapter contrast with most other medical imaging techniques, which generally provide anatomical details of the body organs. The use of radioisotopes in tracer quantities for the clinical diagnosis of human disease grew rapidly in the 1970s and 1980s and became the recognised medical speciality known as nuclear medicine (Wagner 1975, Maisey 1980, Sharp et al. 1998, Ell and Gambhir 2004). In the 1980s, radioisotope imaging split into two major areas, the most common being single-photon imaging in which single gamma rays emitted from the radionuclide are detected (Figure 5.1). The less common alternative is positron emission tomography (PET) (Phelps 1986) in which the two annihilation photons emitted from a positron-emitting radionuclide are detected simultaneously (Figure 5.2). In both techniques, images of the biodistribution of radionuclide-labelled agents in the body are formed. These agents, known as radiopharmaceuticals, are designed to determine the physiological function of individual tissues or organs in the body. The distribution of these agents within the body is determined by route of administration and by such factors as blood ¦ow, blood volume and a variety of metabolic processes.