If the scan was normal, does that rule out a SAH?
Unfortunately no. In a case like this with a typical history of thunderclap headache, the patient should have a lumbar puncture to test for xanthochromia. This looks for breakdown products of blood (bilirubin, xantho=yellow) and needs to be performed at least 12 hours after onset of symptoms. As a useful tip, if you are taking CSF to the labs for this, it needs to be protected from sunlight which can degrade the results. Other tests could include angiography to look for aneurysms in a neuro-radiology centre.
Diffuse hyperdensity throughout the basal cistern, interhemispheric fissure and Sylvian fissures.
Also small left subdural collection, also likely representing blood.
Incidental calcification left basal ganglia.
SUMMARY
Subarachnoid haemorrhage. Suggest further imaging to identify possible underlying aneurysm.