Currently, positron emission tomography (PET) and single photon emission computed tomography (SPECT) are the only noninvasive imaging modalities that can be used to image specific receptor molecules and to quantify their kinetics. By formally comparing the output of the model to the experimentally obtained PET or SPECT data, one can estimate values for the kinetic parameters and thus extract information on binding, or any hypothesized process. The simplest of compartmental models applied to receptor-ligand studies postulates two tissue compartments. These two tissue compartments along with a plasma compartment are arranged in series. The partial volume effect is widely recognized as a limiting factor in the ability to quantify the amount of radioactivity in tissue accurately. The pharmacokinetic approach requires the PET or SPECT data to be analyzed using a mathematical model of radioactivity uptake. PET and SPECT brain imaging studies are divided into two groups: receptor-ligand studies and tracer studies.