Editing Subarachnoid hemorrhage (section)

===Imaging===
The modality of choice is [[computed tomography]] (CT scan), without [[Contrast CT|contrast]], of the brain. This has a high [[sensitivity (tests)|sensitivity]] and will correctly identify 98.7% of cases within six hours of the onset of symptoms.<ref>{{cite journal | vauthors = Dubosh NM, Bellolio MF, Rabinstein AA, Edlow JA | title = Sensitivity of Early Brain Computed Tomography to Exclude Aneurysmal Subarachnoid Hemorrhage: A Systematic Review and Meta-Analysis | journal = Stroke | volume = 47 | issue = 3 | pages = 750–5 | date = March 2016 | pmid = 26797666 | doi = 10.1161/STROKEAHA.115.011386 | s2cid = 7268382 | doi-access = free }}</ref> A CT scan can rule out the diagnosis in someone with a normal neurological exam if done within six hours.<ref name=ACEP2019>{{cite journal | vauthors = Godwin SA, Cherkas DS, Panagos PD, Shih RD, Byyny R, Wolf SJ | title = Clinical Policy: Critical Issues in the Evaluation and Management of Adult Patients Presenting to the Emergency Department With Acute Headache | journal = Annals of Emergency Medicine | volume = 74 | issue = 4 | pages = e41–e74 | date = October 2019 | pmid = 31543134 | doi = 10.1016/j.annemergmed.2019.07.009 | doi-access = free }}</ref> Its efficacy declines thereafter,<ref name=EM2016/> and [[magnetic resonance imaging]] (MRI) is more sensitive than CT after several days.<ref name=VanGijn2007/>

====Angiography====
After a subarachnoid hemorrhage is confirmed, its origin needs to be determined. If the bleeding is likely to have originated from an aneurysm (as determined by the CT scan appearance), the choice is between [[cerebral angiography]] (injecting radiocontrast through a [[catheter]] to the brain arteries) and [[computed tomography angiography|CT angiography]] (visualizing [[blood vessel]]s with [[radiocontrast]] on a CT scan) to identify aneurysms. Catheter angiography also offers the possibility of coiling an aneurysm (see below).<ref name=VanGijn2007/><ref name=Suarez/>

In emergency department patients complaining of acute-onset headache without significant risk factors for SAH, evidence suggests that CT scanning of the head followed by CT angiography can reliably exclude SAH without the need for a lumbar puncture.<ref name="AEM-CTA">{{cite journal |vauthors=McCormack RF, Hutson A |title=Can computed tomography angiography of the brain replace lumbar puncture in the evaluation of acute-onset headache after a negative noncontrast cranial computed tomography scan? |journal=Acad Emerg Med |volume=17 |issue=4 |pages=444–51 |date=April 2010 |pmid=20370785 |doi=10.1111/j.1553-2712.2010.00694.x }}</ref> The risk of missing an aneurysmal bleed as the cause of SAH with this approach is less than 1%.<ref name="AEM-CTA"/>