*E·F* derived by DCE-MRI *versus* microvascular density derived by intravital microscopy for eleven A-07-GFP tumors. (A) Tumor area measured by DCE-MRI *versus* tumor area measured by intravital microscopy (IVM). (B) Intravital microscopy images of the microvascular network (upper panels), *E·F* images (middle panels), and plots of *E·F* versus vascular area fraction (lower panels) for a heterogeneous and poorly vascularized tumor. The images illustrate how the tumors were divided into three concentric circular ROIs and quadratic ROIs. The circular ROIs are bounded by lines drawn at distances of *nR*/3 from the tumor center, where *R* is tumor radius and *n* is ROI number. The size of the quadratic ROIs corresponds to 4 x 4 *E·F* pixels (i.e., 1.24 x 1.24 mm^{2}). The *E·F* scale is given by the color bar. Scale bars, 1 mm. The plots show median *E·F versus* vascular area fraction for the three circular ROIs (left panel) and the sixteen quadratic ROIs (right panel). (C) Plots of median *E·F* versus microvascular density. The symbols represent individual tumors (left panels), individual circular ROIs (middle panels), and individual quadratic ROIs (right panels). Peripheral quadratic ROIs consisting of <75% tumor pixels were not included in the analysis. The parameters used for microvascular density are total vessel length per µm^{2} tumor area (first row), length of large vessels (i.e., vessels with diameter ≥23.8 µm, corresponding to 7 pixels) per µm^{2} tumor area (second row), vascular area fraction [i.e., # pixels (vascular mask)/# pixels (tumor); third row], and interstitial distance (i.e., the median of the distance from a tumor pixel outside the vascular mask to the nearest pixel within the vascular mask; fourth row). Solid lines represent significant correlations and were fitted to the data by linear regression analysis.

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