Emergency magnetic resonance of the brain.

We will review the role of magnetic resonance (MR) in assessment of patients with acute neurological abnormalities. The major stumbling block to the use of MR in these patients is the belief that MR is insensitive to hyperacute (<12 h) intracranial hemorrhage and acute subarachnoid hemorrhage (SAH). Hyperacute hemorrhage has characteristic features on MR. Hematomas are iso-to hyperintense on T1-weighted and hyperintense on T2-weighted images. Gradient-echo scans that reveal characteristic peripheral hypointensity are critical to the detection and delineation of hyperacute hematomas. Use of fast fluid-attenuated inversion recovery (FLAIR) sequences has made it possible to detect SAH on MR with a sensitivity that is equal to or greater than computed tomography (CT). SAH produces dramatic hyperintensity in the normally hypointense cerebrospinal fluid on FLAIR. MR has proven useful in the detection of hypertensive encephalopathy and venous thrombosis. These entities can be difficult to diagnose on CT, and in both, early treatment can dramatically improve prognosis. The same is true for acute intracranial infections such as pyogenic abscess, subdural empyema, and herpes simplex encephalitis. MR improves diagnostic accuracy, resulting in more rapid institution of appropriate treatment and improved outcome.