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Background
Subarachnoid hemorrhages (SAH) is a frequently encountered entity by radiologists that is characterized by the presence of blood in the subarachnoid space. The typical presentation is characterized by a thunderclap headache, or alternatively, a preceding history of trauma. A newer but less commonly thought about etiology for SAH is Reversible Cerebral Vasoconstriction Syndromes (RCVS), an umbrella term for a series of conditions characterized by reversible vasoconstriction of the cerebral arteries, as the name suggests. RCVS typically presents as a thunderclap headache in young to middle aged adults with a predilection for females as compared to males. Given the possible overlapping presentation of RCVS and other more serious causes of SAH, such as trauma or ruptured aneurysms, it is important for radiologists to recognize, and suggest, the possibility of RCVS when reviewing imaging in the setting of SAH to prevent patients from undergoing further testing or intervention; notably, RCVS typically has a benign course and spontaneously resolves.

Educational Goals / Teaching Points
Our educational exhibit aims to review the imaging and clinical presentation of RCVS, in addition to relevant etiologies and possible management recommendations from a radiological standpoint. More importantly, we will provide a comparative review between RCVS and posterior reversible encephalopathy syndrome (PRES), an entity with similar imaging findings. Additionally, an institutional case-based review of different etiologies of SAH will be provided, including demonstration of SAH in the setting of cortical vein thrombosis, cerebral amyloid angiopathy, dural arteriovenous fistulas, endocarditis, cortical tumors, migrating basal SAH, traumatic SAH, and mycotic SAH. Imaging for cases will be presented via a multimodality approach, including presentation on CT, MR, angiography, and cinematic rendering.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
In general, subarachnoid hemorrhages depicts the presence of blood within the subarachnoid space, which presents as linear hyperdensities following the gyral and sulcul grooves of the brain on non-contrast enhanced CT of the head. Similarly, MRI of the brain demonstrates hyperintense signal in the subarachnoid space as early as 12 hours on fluid-attenuated inversion recovery (FLAIR) sequences. SAH is the most common presentation of RCVS, present in approximately 22 - 34% of cases. Specifically, RCVS is well evaluated on angiographic imaging, where short segments of the medium-sized cerebral arteries will demonstrate narrowing, this narrowing will reverse on follow up imaging and is an important distinction from other causes of SAH. Using maximal intensity projection (MIP) of CT angiography, in addition to cinematic rendering of the arteries, RCVS can be more easily recognized and reported.

Conclusion
SAH is most often thought to be seen in the setting of aneurysmal rupture or trauma, but it is important for radiologists to suggest the possibility of a more benign cause of SAH such as RCVS to prevent further unnecessary radiation or possible intervention.