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In magnetic resonance imaging (MRI), cerebral blood volume (CBV) quantification is dependent on the MRI sequence and on the properties of the contrast agents (CAs). By using the rapid steady-state T1 method, we show the potential of gadolinium per (3,6-anhydro) α-cyclodextrin (Gd-ACX), a new MRI paramagnetic CA (inclusion complex of Gd3+ with per (3,6-anhydro)-α-cyclodextrin), for the CBV quantification in the presence of blood–brain barrier lesions. After biocompatibility and relaxivity experiments, in vivo experiments on rats were performed on a C6 tumor model with 0.05 mmol Gd-ACX/kg (<1/10 of the median lethal dose) injected at a 25 mmol/L concentration, inducing neither nephrotoxicity nor hemolysis. On T1-weighted images, a signal enhancement of 170% appeared in vessels after injection, but not in the tumor (during the 1 h of observation), in contrast to the 90% signal enhancement obtained with Gd-DOTA (a clinical MRI CA) injected at a T1 isoefficient dose. This result shows the absence of Gd-ACX extravasation into the tumor tissue and its confinement to the vascular space. Fractional CBV values were found similar to Gd-ACX and Gd-DOTA in healthy brain tissue and in the contralateral hemisphere of tumor-bearing rats, whereas only Gd-ACX was appropriate for CBV quantification in tumor regions.Journal of Cerebral Blood Flow & Metabolism (2008) 28, 1017–1029; doi:10.1038/sj.jcbfm.9600602; published online 9 January 2008 [ABSTRACT FROM AUTHOR]