Multiple sclerosis is a disease of the central nervous system that is associated with leukocyte recruitment and subsequent inflammation, demyelination and axonal loss. Endothelial vascular cell adhesion molecule-1 (VCAM-1) and its ligand, α₄β₁ integrin, are key mediators of leukocyte recruitment, and selective inhibitors that bind to the α₄ subunit of α₄β₁ substantially reduce clinical relapse in multiple sclerosis. Urgently needed is a molecular imaging technique to accelerate diagnosis, to quantify disease activity and to guide specific therapy. Here we report in vivo detection of VCAM-1 in acute brain inflammation, by magnetic resonance imaging in a mouse model, at a time when pathology is otherwise undetectable. Antibody-conjugated microparticles carrying a large amount of iron oxide provide potent, quantifiable contrast effects that delineate the architecture of activated cerebral blood vessels. Their rapid clearance from blood results in minimal background contrast. This technology is adaptable to monitor the expression of endovascular molecules in vivo in various pathologies.