Vessel size imaging

Vessel size imaging is a new method that is based on simultaneous measurement of the changes δR₂ and δR₂/^* in relaxation rate constants induced by the injection of an intravascular superparamagnetic contrast agent. Using the static dephasing approximation for δR₂/^* estimation and the slow-diffusion approximation for δR₂ estimation, it is shown that the ratio δR₂/ δR₂/^* can be expressed as a function of the susceptibility difference between vessels and brain tissue, the brain water diffusion coefficient, and a weighted mean of vessel sizes. Comparison of the results with 1) the Monte Carlo simulations used to quantity the relationship between tissue parameters and susceptibility contrast, 2) the experimental MRI data in the normal rat brain, and 3) the histologic data establishes the validity of this approach. This technique, which allows images of a weighted mean of the vessel size to be obtained, could be useful for in vivo studies of tumor vascularization.