The retention fraction is the fraction of the total tracer delivered to an organ that is extracted into and retained by the tissue. It is the residual after clearances of the vascular component and the portion of the tracer that rapidly back diffuses from tissue to blood. Usually this fraction of tracer is sequestered in more slowly turning over metabolic or membrane binding processes. Glossary 
Given a time-activity curve and an approximate value of tissue perfusion F, this auxiliary model allows calculating the retention fraction R by
dividing the tissue TAC at each sampling time by F times the plasma activity integrated from the beginning to frame mid-time.