Single photon emission computerized tomography (SPECT) and positron emission tomography (PET) are noninvasive medical imaging modalities that provide the three-dimensional distribution of a radiopharmaceutical in the human body. The radiopharmaceutical is a biological compound or drug that has been labeled with radioactive atoms. Other imaging studies using radiopharmaceuticals, such as dynamic and static studies, are based only on the projection of the three-dimensional distribution of radioactivity into two-dimensional planar images. In dynamic studies, a sequence of planar images displays the dynamic or kinetic behavior of the radiopharmaceutical; while in static studies, single planar images display the regional uptake of the radiopharmaceutical. All these imaging procedures are based on the external detection of gamma rays emitted from the patient and are commonly referred as emission imaging to be differentiated from those modalities based on the transmission of x-rays through the human body (e.g., radiographic imaging and x-ray computerized tomography). Since emission imaging is derived from the internal distribution of a radiopharmaceutical, the clinical information these images provide is related to those biochemical and physiologic processes in which the radiopharmaceutical is involved. The functional or metabolic information provided by emission imaging is the major characteristic that differentiates this modality from others that basically provide anatomical or structural information. This advantage has been used for physicians for earlier diagnosis and management of diseases based on metabolic and physiological changes that can occur before structural or anatomical modifications may be noted. The wide range of clinical and research applications of emission imaging have been determined by progress in three important fields. One has been the radiopharmaceutical labeling of different drugs, substrates, monoclonal antibodies, ligands, neurotransmitters and blood cells with certain radioactive atoms that can be safely administered to humans. The second has been the technical advances in radiation detectors, electronics and associated instruments for the efficient detection of gamma rays and correction of physical effects degrading image quality and quantitative accuracy. The third factor has been the increasing power of affordable computers and the development of software algorithms for image processing and biokinetic modeling.
