Specific Imaging Findings

On CT, subarachnoid hemorrhage (SAH) characteristically presents as hyperdense material filling the basal cisterns and/or fissures and cortical sulci. The density and extent depend on the volume of blood. If sufficiently diluted by the CSF, a small SAH may not be seen on CT. Dilution and redistribution may lead to intraventricular extension and the hyperdensity gradually fades away. Diluted SAH can appear as effacement of the cortical sulci. Traumatic SAH may be associated with other injuries such as parenchymal and extra-axial hematomas. The most common cause of nontraumatic SAH is aneurysmal rupture, usually presenting with diffuse SAH, while a filling defect within the hyper-dense clot may indicate the aneurysm location. An associated parenchymal hematoma may also be present. Nonaneurysmal SAH (NASAH) is most commonly perimesencephalic, located almost exclusively in the basal cisterns with possible minimal extension into the interhemispheric and sylvian fissures. Other types of NASAH tend to be located along the convexity - apart from trauma, vasculitis, cortical vein thrombosis, Moyamoya, and cerebral amyloid angiopathy may present this way. On MRI, SAH is best seen with FLAIR sequence, which is more sensitive than CT. T2*WI tend to show hypointensity, but this is variable. Hyperacute SAH (within the first few hours), similar to hyperacute hematoma, is extremely T2 hyperintense, brighter than the CSF; it becomes hypointense in the acute phase. T1 signal varies but is always hyperintense compared to the CSF. Leptomeningeal enhancement may be present. In patients with nontraumatic SAH and either the perimesencephalic pattern or no blood on CT, negative CTA is reliable in ruling out aneurysms. DSA is indicated for diffuse SAH with negative CTA.