Editing Thunderclap headache (section)

==Diagnosis==
The most important initial investigation is [[computed tomography]] of the brain, which is very sensitive for subarachnoid hemorrhage. If this is normal, a [[lumbar puncture]] is performed, as a small proportion of SAH is missed on CT and can still be detected as [[xanthochromia]].<ref name=Schwedt/><ref name=ACEP2008/>

If both investigations are normal, the specific description of the headache and the presence of other abnormalities may prompt further tests, usually involving [[magnetic resonance imaging]] (MRI). [[Magnetic resonance angiography]] (MRA) may be useful in identifying problems with the arteries (such as dissection), and magnetic resonance venography (MRV) identifies venous thrombosis. It is not usually necessary to proceed to [[cerebral angiography]], a more precise but invasive investigation of the brain's blood vessels, if MRA and MRV are normal.<ref name=Schwedt/>