On the use of R1 and R2* for measurement of contrast agent concentration in isolated perfused rat liver

We present experimental MRI protocols at 4.7 T for quantitative determination of the Dotarem distribution volume in isolated perfused rat liver. The procedures involved either constant contrast agent (CA) concentration or bolus administration conditions. R1 and R2* effects of the CA in liver and perfusate were measured using gradient echo fast imaging (GEFI) experiments by varying either the excitation angle or the echo time. CA concentrations in liver and perfusate were also measured after MRI by inductively coupled plasma atomic emission spectroscopy, in order to determine in situ relaxivities in the perfusate (r1=4.2 +/- 0.1 s(-1) mm(-1), r2*=17 +/- 2 s(-1) mm(-1)) and in the liver (r1=7.2 +/- 0.2 s(-1) mm(-1), r2*=99 +/- 5 s(-1) mm(-1)). When CA concentrations were estimated from R1 measurements and r1, the CA distribution volume estimations in liver resulting from bolus (0.31 +/- 0.01) and stationary (0.32 +/- 0.05) experiments were not significantly different. In contrast, after a bolus, CA concentrations derived from R2* and r2* were overestimated in liver and even more in perfusate. However, with R1 and R2* being measured before CA bolus administration, zero echo time signal intensities computed from multiple TE measurements during multiple boli yielded good estimations of R1 and thus correct CA concentrations in liver and in perfusate. Under these conditions, a single multi-echo GEFI acquisition should be sufficient to determine the concentration-time curves. Consequently, this protocol should be appropriate to rapidly estimate the distribution volume in vivo when multiple boli have to be avoided.