Evaluation of Rat Heart Microvasculature with High-Spatial-Resolution Susceptibility-weighted MR Imaging

Purpose: To demonstrate that strandlike hypointense signals seen in the myocardium of normal rat hearts correspond to myocardial microvessels with high-spatial-resolution susceptibility-weighted (SW) magnetic resonance (MR) imaging without injection of contrast medium. Materials and Methods: Animal experiments were performed with institutional animal care committee approval. Ex vivo cardiac MR imaging was performed in 10 normal Wistar rats with a 4.7-T imager and a cryogenic probe. The hypothesis that thin tubular hypointense signals in the myocardium of rat hearts at SW MR imaging sequences (group 1, n = 6; in-plane resolution, 39 mu m) represent intramyocardial microvessels was tested. A superparamagnetic intravascular contrast agent (ferumoxsil; Lumirem) was used to explore the distribution of the intramyocardial microvessels (group 2, n = 4; three-dimensional fast imaging with steady-state free precession sequences). Nonparametric Mann-Whitney tests were performed to compare groups 1 and 2 both for microvascular densities (MVD) on histologic sections and for MR imaging signal intensities (SIs). Wilcoxon signed rank tests were used for paired comparison of subepicardial and subendocardial MVD and SI within groups. Results: Ferumoxsil opacified the coronary microvasculature (group 2) on MR-matched histologic sections. No statistically significant difference was found between groups 1 and 2 for either MVD or MR imaging SI expressed as ratios between subendocardium and subepicardium (P = .40 and P = .46, respectively). The comparison of mean subendocardial and subepicardial SI within groups revealed significantly more microvessels in the subepicardium with MR (group 1: P = .01; group 2: P = .004). Conclusion: Myocardial microvessels appear as strandlike structures on high-spatial-resolution SW MR images without the aid of contrast medium injection. (C) RSNA, 2013