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Angiography reveals an aneurysm in 85% of cases of subarachnoid haemorrhage. In the other 15% of cases, a repeat angiography is indicated if the pattern of haemorrhage on CT suggests an aneurysmal origin, or if there is arterial spasm which might reduce the flow sufficiently for the aneurysm not to be discernible.
Three views are required - antero-posterior, lateral and oblique. This avoids mistaking any vessel loop seen end-on for an aneurysm. Aneurysms occur preferentially at vessel bifurcations, particularly acute angle junctions, and should be sought around the circle of Willis, on the middle cerebral and pericallosal vessels, and on the vertebral artery at the origin of the posterior inferior cerebellar artery.
Arterio-venous malformations are evident by an abnormal release of blood in the arterial phase.
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