Quantification provides the link between the concentrations of radioactivity measured in tissue and the underlying physiologic processes occurring in the organ. It relates the rate at which radioactivity levels in the body change over time to quantitative parameters such as absolute rate glucose metabolism, regional blood flow, or concentrations of receptors or other binding sites. Absolute measurement of physiologic parameters generally requires accurate measurement of activity concentrations in arterial blood, which provides the input function to the kinetic model. Although absolute quantification can be a difficult process, simplifications of these invasive techniques, involving reference tissues or normalization approaches (eg, SUV), have been applied with some success. Any simplified model of tracer behavior must be validated against the full model to test for bias and systematic errors.