Tissue blood flow (Q), tissue tracer clearance (Cl) and tracer extraction fraction (E) are related in the following equation: Cl = E*Q. Clearance is fundamental to the understanding of quantitative nuclear medicine and has several variants: tissue, urinary, biliary and plasma, as well as clearance between compartments within a tissue. There are multiple methods for measuring blood flow, which may be expressed as whole organ blood flow (ml/min), tissue perfusion (ml/min/ml) or fraction of cardiac output.