High-resolution MR imaging of mouse brain microvasculature using the relaxation rate shift index Q

Magnetic resonance imaging (MRI) is a powerful method for in vivo quantification of tissue properties. It has been previously proposed that the index Q identical with Delta R2/(Delta R2*)2/3, where Delta R2 and Delta R2* are the spin echo and gradient echo relaxation rate shifts caused by the injection of an intravascular contrast agent, may be useful for characterizing microvasculature. In particular, Q is expected to correlate well with the density of microvessels. This study presents high-resolution in vivo Q-maps of normal mouse brain obtained with a superparamagnetic iron oxide contrast agent at a field of level of 9.4 T. Normative Q values are derived for several regions of interest and significant interregional variations are observed. Microvessel densities estimated from the Q-maps are found to be in reasonable accord with histologically determined values. A possible application of Q-maps is the assessment of angiogenic activity in tumors.