Compartment Models

A physiologic system is often described by decomposition into a number of interacting subsystems, called compartments. Compartments should not be understood as a physical volume, but rather as a mass of well-mixed, homogeneous material that behaves uniformly. Each compartment may exchange material with other compartments. Examples of frequently used compartments are

• Authentic (unchanged) tracer in the arterial plasma that can be extracted into the tissue. The concentration of authentic tracer as a function of time during the PET acquisition is called the "Arterial Input Function" (AIF), or simply the "Input Curve".
• Free tracer in tissue that can be bound or that may diffuse back into the blood.
• Tracer in tissue that has been specifically bound, for instance, at the target receptor site.
• Tracer in tissue that has been non-specifically bound to other than the targeted cell components.

Compartment models are visualized by diagrams wherein the rectangles symbolize compartments and the arrows represent material exchange, as illustrated for the different models below. Although the mechanisms of material transport between compartments may differ, models that can be reasonably analyzed with standard mathematical methods assume first-order processes. As a consequence, the change of tracer concentration in each compartment is a linear function of the concentrations in the other compartments. Because of the tiny amounts of tracer material applied in nuclear medicine it is usually assumed that the observed system is not disturbed by the tracer. It is furthermore assumed, that the physiologic conditions do not change during the study, so that the rate of the material exchange can be considered constant.