Modeling Tracer Kinetics in Dynamic GdlDTPA

Three major models (from Tofts, Larsson, and Brix) for collecting and analyzing dynamic MRI gadolinium-diethylenetriamine penta-acetic acid (GI-DTPA) data are examined. All models use compartments representing the blood plasma and the abnormal extravascular extracellular space (EES), and they are intercompatible. All measure combinations of three parameters: (1) kPSp is the influx volume transfer constant (min-’), or permeability surface area product per unit volume of tissue, between plasma and EES; (2) v, is the volume of EES space per unit volume of tissue (0 < v, < 1); and (3) kc,, the efflux rate constant (min-’), is the ratio of the 5rst two parameters (kep = kPsp/ve). The ratio k,, is the simplest to measure, requiring only signal linearity with Gd tracer concentration or, alternatively, a measurement of T1 before injection of Gd (TI,,). To measure the physiologic parameters kPsr and v, separately requires knowledge of T,, and of the tissue relaxivity R, (= in vitro value).