However, advances in magnetic resonance imaging of the brain, particularly the use of fluid-attenuated inversion recovery, proton density, diffusion-weighted imag- ing, and gradient echo sequences,101 103 can often allow the diagnosis of aSAH to be made when a head CT scan is negative and there is clinical suspicion of aSAH, possibly avoiding the need for lumbar puncture. The role of magnetic resonance imaging in perimesencephalic aSAH is controver- sial.104 Indications for magnetic resonance angiography in aSAH are still few because of limitations with routine availability, logistics (including difficulty in scanning acutely ill patients), predisposition to motion artifact, patient compli- ance, longer study time, and cost. Aneurysms 3 mm in size continue to be unreliably demonstrated on computed tomo- graphic angiography (CTA),105,106 and this generates contin- ued controversy in the case of CTA-negative aSAH.107 In cases of perimesencephalic subarachnoid hemorrhage (SAH), some authors claim that a negative CTA result is sufficient to rule out aneurysmal hemorrhage and that cerebral angiogra- phy is not required, but this is controversial. In 1 study, the overall interobserver and intraobserver agreement for nonan- eurysmal perimesencephalic hemorrhage was good, but there was still a level of disagreement among observers, which suggests that clinicians should be cautious when deciding whether to pursue follow-up imaging.108 In another study,109 a negative CTA result reliably excluded aneurysms when head CT showed the classic perimesencephalic SAH pattern or no blood. Digital subtraction angiography (DSA) was indicated if there was a diffuse aneurysmal pattern of aSAH, and repeat delayed DSA was required if the initial DSA findings were negative, which led to the detection of a small aneurysm in 14% of cases. 