Since its inception, nuclear medicine imaging has always been known as a functional imaging technique rather than an anatomical imaging method. Three-dimensional (3D) images obtained from single-photon emission computed tomographic (SPECT) or positron emission tomographic (PET) devices depict the accumulation of radiopharmaceutical by specic cells, thus representing cellular function (see Figure 14.1a). As a result, these images lack the spatial information and instant recognition of other medical imaging techniques, such as X-ray computed tomography (CT) or magnetic resonance imaging (MRI). The underlying information conveyed to an educated reader, however, is substantial, since the distribution of radiopharmaceutical within the body provides physiological information about the body rather than strictly anatomical information. Nuclear medicine images often provide an early glimpse into disease progression, since physiological changes occur prior to structural changes appearing. Even so, nuclear medicine has long been described as “unclear” medicine because images typically lack any readily identiable landmark information, such as bones or lungs. In fact, an ideal nuclear medicine image should contain no anatomical information whatsoever but rather depict the highly specic uptake of radiotracer in certain cell types. Admittedly, the lack of structural information may introduce some difculties in localizing disease or framing the extent of disease. To provide some measure of anatomical structure and to correlate the radiopharmaceutical distribution with anatomy, numerous methods have been used.