Stroke is the third leading cause of death in the Western world. More than 140 000 people die each year from stroke in the US. Patients are treated by catheter based surgery. We developed a light-switchable MRI contrast agent to visualize blood-vessels. Switching is performed with an optical fiber that guides the light from the light source to the site of intervention. Stroke is the third leading cause of death in the Western world. More than 140 000 people die each year from stroke in the US. Patients are treated by catheter based surgery. The catheter is introduced through a large artery (usually upper leg) and pushed into the brain to remove blood clots, or to stop bleeding. All manipulations are performed under imaging control using X-ray. We developed a light-switchable MRI contrast agent to visualize blood-vessels. Switching is performed with an optical fiber that guides the light from the light source to the site of intervention. Potential advantages are i.a. no radiation exposure, lower concentrations and 3D-imaging. The contrast agent is based on a nickel-porphyrin to which a switchable ligand is attached. It is the first molecule whose spin state can be reversibly switched at room temperature. Further developments are aimed at MRI contrast agents that are responsive to pH, temperature, and other parameters for the early detection of inflammation and cancer.