Modeling tracer kinetics in dynamic Gd-DTPA MR imaging

Three major models (from Tofts, Larsson, and Brix) for collecting and analyzing dynamic MRI gadolinium-diethylene-triamine penta-acetic acid (Gd-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) k(PS rho) is the influx volume transfer constant (min(-1)), or permeability surface area product per unit volume of tissue, between plasma and EES; (2) v(e) is the volume of EES space per unit volume of tissue (0 < v(e) < 1); and (3) k(ep), the efflux rate constant (min(-1), is the ratio of the first two parameters (k(ep) = k(PS rho)/v(e)). The ratio k(ep) is the simplest to measure, requiring only signal linearity with Gd tracer concentration or, alternatively, a measurement of T1 before injection of Gd (T-10), To measure the physiologic parameters k(PS rho) and v(e) separately requires knowledge of T-10 and of the tissue relaxivity R(1) (approximate to in vitro value).