Aneurysms and vascular malformations.

Intracranial aneurysms and vascular malformations are frequently detected following intracranial hemorrhages. A CT scan is the most sensitive method of detecting acute subarachnoid, parenchymal, and intraventricular hemorrhages. Small aneurysms are inconsistently visualized on MR scans. Angiography remains the standard for complete and accurate depiction of patent aneurysms, as well as of arteriovenous and venous malformations that have not thrombosed. Giant and thrombosed aneurysms present as mass lesions and are frequently detected when MR is used as a screening examination. Often MRI characterizes these lesions better than CT or angiography. Patients with vascular malformations who have focal neurologic symptoms without hemorrhage are best evaluated with MRI. Patent vascular malformations demonstrating flow void and other flow-related phenomena are readily demonstrated. Occult vascular malformations, including thrombosed arteriovenous, venous, and cavernous malformations and telangiectasia, are also best detected by MRI and are not visible on angiography.