### Tracer Retention Fraction

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. [1]

Operational Equation

Given a tissue TAC C_{T}(t) and the input curve C_{P}(t) this auxiliary model allows calculating the retention fraction R by [2]:

The denominator with the input curve integrated from the time of injection represents the available tracer, the numerator with the integral of the late tissue uptake the actually extracted tracer.

Implementation

The **Tracer Retention** auxiliary model has two input parameters **Start **and **End** for the specification of t_{s} and t_{e} in the integration formula above, respectively. The curve display shows the input curve interpolated at the frame mid-times.

References

1. *see *Glossary in Phelps ME: PET : molecular imaging and its biological applications. New York: Springer; 2004.

2. Hutchins GD, Chen T, Carlson KA, Fain RL, Winkle W, Vavrek T, Mock BH, Zipes DP: PET imaging of oxidative metabolism abnormalities in sympathetically denervated canine myocardium. J Nucl Med 1999, 40(5):846-853.