Vessel size imaging is a new method that is based on simultaneous measurement of the changes ΔR2 and ΔR *2 in relaxation rate constants induced by the injection of an intravascular superparamagnetic contrast agent. Using the static dephasing approximation for ΔR *2 estimation and the slow‐diffusion approximation for ΔR2 estimation, it is shown that the ratio ΔR2/ΔR *2 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 quantify 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. Magn Reson Med 45:397–408, 2001. © 2001 Wiley‐Liss, Inc.

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Intermittent hypoxia induces sequential cardiovascular events suggesting increased chemoreflex and depressed baroreflex, resulting in sympathoadrenal hyperactivity, early hemodynamic alterations with proximal histologic remodeling, and delayed changes in peripheral vasoreactivity. Such early alterations before overt cardiovascular disease strengthen the need for identifying at-risk individuals for systematic treatment.