Introduction

Positron emission tomography (PET) and single photon emission computed tomography (SPECT) are the only noninvasive imaging modalities, currently, that can be used to image specific receptor molecules and to quantify their kinetics. The specificity of these methods results from the use of radiolabeled ligands that are themselves specific for a receptor or transporter molecule. In PET/SPECT studies, the images are a composite of various signals only one of which relates to ligand bound to the receptor site of interest. The component of the image that is from receptor binding is isolated using a mathematical model relating the dynamics of the ligand to the resultant PET image. The mathematical model typically derives from the three-compartment model in which a directly measured blood curve serves as the model's input function. (There are modifications of the model (reference region models) that do not use an independently measured blood curve as input. These models are addressed later in this chapter.) The model contains a number of parameters that are taken to be constants and reflective of inherent kinetic properties of the system and the particular ligand. By formally comparing the output of the model to the experimentally obtained PET or SPECT data,we can estimate values for the kinetic parameters and thus extract information on binding, or any hypothesized process, as distinct from all other processes contributing to the PET signal.