Evaluation of magnetic resonance vessel size imaging by two-photon laser scanning microscopy.

MR vessel size imaging (MR-VSI) is increasingly applied to noninvasively assess microvascular properties of tumors and to evaluate tumor response to antiangiogenic treatment. MR-VSI provides measures for the microvessel radius and fractional blood volume of tumor tissue. However, data have not yet been evaluated with three-dimensional microscopy techniques. Therefore, three-dimensional two-photon laser scanning microscopy (TPLSM) was performed to assess microvascular radius and fractional vessel volume in tumor and muscle tissue. TPLSM data displayed a mazelike architecture of the tumor microvasculature and mainly parallel oriented muscle microvessels. For both MR-VSI and TPLSM, a larger vessel radius and fractional blood volume were found in the tumor rim than in the core. The microvessel radius was approximately six times larger in tumor and muscle for MR-VSI than for TPLSM. The tumor blood volume was 4-fold lower with MR-VSI than with TPLSM, whereas muscle blood volume was comparable for both techniques. Differences between the tumor rim, core, and muscle tissue showed similar trends for both MR-VSI and TPLSM parameters. These results indicate that MR-VSI does not provide absolute measures of microvascular morphology; however, it does reflect heterogeneity in microvascular morphology. Hence, MR-VSI may be used to assess differences in microvascular morphology.