Disclosed is an MRI contrast agent which enables direct detection and imaging of exfoliated vascular endothelial sites. The MRI contrast agent comprises an imaging unit which contains an unpaired electron-carrying atom and/or molecule and is capable of amplifying or reducing MRI signals, and a detection unit which is bonded to said imaging unit and is capable of selectively recognizing exfoliated vascular endothelial sites and binding thereto. The detection unit is exemplified by one having a chemical structure expressed by the following formula (I), wherein at least one of R1-R11 is, for example, sulfonic acid group, and X is, for example, a phenyl group which may be substituted: