Flow versus permeability weighting in estimating the forward volumetric transfer constant (K trans ) obtained by DCE-MRI with contrast agents of differing molecular sizes.

PURPOSE: To quantify the differential plasma flow- (Fp -) and permeability surface area product per unit mass of tissue- (PS-) weighting in forward volumetric transfer constant (Ktrans ) estimates by using a low molecular (Gd-DTPA) versus high molecular (Gadomer) weight contrast agent in dynamic contrast enhanced (DCE) MRI.

MATERIALS AND METHODS: DCE MRI was performed using a 7T animal scanner in 14 C57BL/6J mice syngeneic for TRAMP tumors, by administering Gd-DTPA (0.9kD) in eight mice and Gadomer (35kD) in the remainder. The acquisition time was 10min with a sampling rate of one image every 2s. Pharmacokinetic modeling was performed to obtain Ktrans by using Extended Tofts model (ETM). In addition, the adiabatic approximation to the tissue homogeneity (AATH) model was employed to obtain the relative contributions of Fp and PS.

RESULTS: The Ktrans values derived from DCE-MRI with Gd-DTPA showed significant correlations with both PS (r2 =0.64, p=0.009) and Fp (r2 =0.57, p=0.016), whereas those with Gadomer were found only significantly correlated with PS (r2 =0.96, p=0.0003) but not with Fp (r2 =0.34, p=0.111). A voxel-based analysis showed that Ktrans approximated PS (<30% difference) in 78.3% of perfused tumor volume for Gadomer, but only 37.3% for Gd-DTPA.

CONCLUSIONS: The differential contributions of Fp and PS in estimating Ktrans values vary with the molecular weight of the contrast agent used. The macromolecular contrast agent resulted in Ktrans values that were much less dependent on flow. These findings support the use of macromolecular contrast agents for estimating tumor vessel permeability with DCE-MRI.