An active MS lesion has perivenous inflammatory changes and abnormalities in the blood-brain barrier. Enhancement following contrast administration distinguishes between lesions with an abnormal blood-brain barrier and those with a sufficiently intact blood-brain barrier to prevent passage of the gadolinium DTPA agent into the brain parenchyma. A minority of MS patients will have enhancing plaques when studied with Gd-DTPA. However, MR with Gd-DTPA is more sensitive than double- dose CT or the physical examination for detecting active disease. This is partially true because many active MS plaques are located in clinically silent areas of the brain which the physical exam cannot accurately detect. On the other hand, when there is a definite clinically detectable active plaque, enhanced MR has excellent correlation. A recent study has shown that 3 minutes following injection of Gd-DTPA appears to be the most efficient time for demonstrating enhancement of active MS plaques; delayed