MRI contrast agents work by shortening the T1 or T2 relaxation times of protons in tissues where the agents accumulate. The most commonly used contrast agents contain gadolinium, which has paramagnetic properties and shortens T1 relaxation times. Gadolinium agents are administered intravenously and do not cross the blood-brain barrier. Their effects enhance lesions and tumors where the agent leaks out of vessels. A rare but serious side effect of some gadolinium agents is nephrogenic systemic fibrosis, which can occur in patients with kidney disease who cannot clear the agent from their bodies.