The blood-brain barrier (BBB) is a functional concept to describe unique features of intracranial blood vessels that prevent many substances in the systemic circulation from entering the brain. In the setting of acute ischemic stroke, loss of blood-brain barrier (BBB) integrity is believed to be a precursor to hemorrhagic transformation. CT and MR imaging may evaluate BBB integrity by detecting leakage of intravenously administered contrast media into the extravascular space. In its simplest form, BBB integrity is assessed qualitatively, by determining the presence or absence of contrast enhancement on structural images of the brain. When dynamic contrast-enhanced (DCE) MRI or CT is combined with a suitable pharmacokinetic model, one can quantify and spatially map BBB integrity throughout the brain.