Various radiologic patterns of subarachnoid hemorrhage on noncontrast computed tomography (CT) of the head

(A) Obvious large SAH: hyperdense blood in all the basal cisterns, with some dilatation of the temporal horns of the lateral ventricles, suggesting early hydrocephalus.
(B) More subtle, smaller SAH: small hyperdense collection of blood in the basal cistern adjacent to the left pons and suprasellar cistern (short solid arrow).
(C) Perimesencephalic SAH: the long solid arrows indicate a perimesencephalic (sometimes called a pretruncal) SAH. These hemorrhages represent approximately 10% of nontraumatic SAHs. They are thought to be caused by venous bleeding, will have a negative CTA result, and usually have an excellent outcome. However, the radiographic pattern is also observed with posterior circulation aneurysms, so all of these patients require neurosurgical consultation and vascular imaging.
(D) Convexal SAH: the arrowheads indicate a high convexal SAH. This pattern is observed in two groups of patients. In younger patients, it is usually due to RCVS, but in older ones, it often indicates amyloid angiopathy. In a patient presenting with a severe rapid-onset headache, RCVS would be the likely diagnosis.
(E) Traumatic SAH: the history usually suggests a traumatic SAH (the most common cause). However, if this pattern (dashed arrows indicate small amounts of SAH abutting bone, often in the anterior frontal and temporal bones) is observed in a patient without a clear history of trauma, the likely cause is a traumatic SAH.