The radiopharmaceutical is generally made up of two parts: the pharmaceutical, which is targeted to a specific organ, and the radionuclide, which emits gamma rays (high-energy electromagnetic radiation; short wavelength) and allows the organ to be visualized by the gamma camera. Nuclear medicine imaging (NMI) can yield quantitative as well as qualitative data. A measurement of the ejection fraction of the heart is an example of quantitative data derived from a multigated acquisition (MUGA) scan or a myocardial stress procedure.

In general, nuclear medicine images visualize the distribution of a particular radiopharmaceutical, with hot, warm, or cold spots of activity indicating an abnormality. In a hot spot, there is an increased area of uptake of the radiopharmaceutical in diseased tissue compared with the distribution in normal tissue. Examples of this type of uptake can be seen on bone images. An example of a warm spot would be in a thyroid nodule. In a cold spot, there is an area of decreased uptake of the radiopharmaceutical compared with the distribution in normal tissue. Liver and lung imaging are examples of this type of uptake. Prompt uptake in transplanted organs correlates with (1) adequate perfusion, such as reperfusion of the transplanted lungs or pancreas; (2) excretory function, such as in kidney transplants; and (3) evidence of cardiac viability and reinnervation. Poor uptake and nonvisualization of the transplanted organ are evidence of rejection.