This is an article published in the last year in the British Medical Journal that looked at the sensitivity of modern CT scanners in detecting subarachnoid hemorrhage. This article made a lot of waves because it suggested that a head CT within 6 hours of headache onset is 100% sensitive for subarachnoid hemorrhage.  Some have called it a practice changer that allows us to avoid doing a lumbar puncture so its important to read it for yourself and decide if it should change your practice.