Imaging brain vasculature with BOLD microscopy: MR detection limits determined by in vivo two-photon microscopy.

Rat brain vasculature was imaged at 9.4T with blood oxygenation level-dependent (BOLD) microscopy. Data were acquired without exogenous contrast agent in <35 min using 3D gradient-echo imaging with 78-microm isotropic resolution. Detailed vascular patterns including intracortical veins and some branches were observed in simple magnitude-contrast data acquired at an experimentally optimized echo time. The venous origin of the dark patterns was confirmed by oxygenation-dependent studies, and when the systemic arterial oxygen saturation level was <80% BOLD microscopy revealed additional intracortical vessels presumed to be of arterial origin. Quantification shows a decrease of intracortical venous density with depth. The full width at half-minimum intensity was 90-190 microm for most intracortical venous vessels identifiable by BOLD venography. Since actual diameters are not directly quantifiable by BOLD, we also measured diameter-dependent intracortical venous density in vivo by two-photon excitation fluorescent microscopy. Density comparisons between the two modalities, along with computer simulations, show that venous vessels as small as approximately 16-30 microm diameter are detectable with 9.4T BOLD microscopy under our experimental conditions.