Determination of the MRI contrast agent concentration time course in vivo following bolus injection: Effect of equilibrium transcytolemmal water exchange

For bolus-tracking studies, it is commonly assumed that CR concentration be ars a linear relationship with the measured (usually longitudinal) (H2O)-H- 1 relaxation rate constant, R-1* =(T-1*)(-1). This requires that equilibriu m transcytolemmal water exchange be in the fast exchange limit (FXL). Howev er, though systems remain in fast exchange, the FXL will not usually obtain . Here, the consequences are considered: 1) the measurement of R-1* itself can be affected, 2) the resultant non-linear [CR]-dependence causes signifi cant error by assuming FXL, 3) the thermodynamic [CR] (based on the space i n which CR is actually distributed) can be determined, 4) transcytolemmal w ater permeability may be estimated, and 5) the pharmacokinetic parameters c an be factored. For a 30-sec, 0.17 mmol/kg dose of GdDTPA(2-), the FXL assu mption underestimates the [CR] maximum in rat thigh muscle by a factor of a lmost two. Similar results are obtained for a rat brain GS-SL gliosarcoma t umor model. Magn Reson Med 44:563-574, 2000. published 2000 Wiley-Liss, Inc .(dagger)