The hypodense artery sign.

An acute intracranial embolus may be associated with a hyperdense artery sign on CT, related to acute thrombus within the affected artery, a phenomenon that is well known and has been extensively documented. We present an unusual case of a middle cerebral artery territory acute infarct with a hypodense artery on CT. The Hounsfield unit attenuation of the embolic lesion was fat density. CT angiography and MR imaging confirmed the fatty lesion to be within the middle cerebral artery.