Correlative dynamic contrast MRI and microscopic assessments of tumor vascularity in RIP-Tag2 transgenic mice.

The purpose of this study was to define the feasibility of dynamic contrast-enhanced magnetic resonance imaging (MRI) to estimate the vascular density and leakiness of spontaneous islet cell tumors in RIP-Tag2 transgenic mice. Dynamic T(1)-weighted spoiled gradient echo (SPGR) imaging at 2.0 T was performed in 17 RIP-Tag2 mice using a prototype blood pool macromolecular contrast medium (MMCM), albumin-(Gd-DTPA)(35). Kinetic analysis of the dynamic enhancement responses based on a two-compartment model was used to estimate fractional plasma volume (fPV) and the coefficient of endothelial permeability (K(PS)) for each tumor. The MRI estimate of fPV was correlated on a tumor-by-tumor basis with corresponding microscopic measurements of vascular density. The fPV assays by MMCM-enhanced imaging ranged from 2.4%-14.1% of tissue volume. Individual tumor fPV values correlated significantly (r = 0.79, P < 0.001) with the corresponding microscopic estimates of vascularity consisting of the combined area densities of lectin-perfused microvessels plus erythrocyte-stained blood lakes. A biotinylated derivative of the albumin-based MMCM confirmed extravasation of the contrast agent from some tumor blood vessels and accumulation in 25% of blood lakes. The K(PS) values ranged from 0 (no detectable leak) to 0.356 mL/min/100 cm(3). Dynamic MMCM-enhanced MRI is feasible in RIP-Tag2 pancreatic tumors, yielding estimates of vascular permeability and microscopically validated measurements of vascular richness.