Subarachnoid blood will be seen on cranial CT (Fig 5) in 90 per cent of cases if performed within 3 days of aneurysmal hemorrhage, but in less than 40 per cent after
5 days. In contrast, MRI may detect aneurysmal hemorrhage in about 50 per cent of cases within 3 days, but in over 80 per cent after 3 days and perhaps more when non-routine protocols are used. Nevertheless the specificity of MRI remains questionable; CT is the preferred initial investigation and should precede lumbar puncture. MR or CT angiography have not yet replaced invasive angiography for the detection of small aneurysms that have bled. Subarachnoid clot confined to the midline basal cisterns is very unlikely to be due to an angiographically definable cause and is benign. The development of delayed cerebral infarction after subarachnoid hemhorrage is associated with the overall extent of the blood shown by CT, not its location.
Fig. 5 (a) Plain CT of a patient's head 12 h after the onset of severe headache showing extensive subarachnoid blood. A localized partly intracerebral hematoma in the right temporal lobe suggests that the cause of hemorrhage is a middle cerebral artery aneurysm. (b) Cerebral digital subtraction angiography: selective injection into the right internal carotid artery demonstrates a right middle cerebral artery aneurysm.
Acute dissection of the wall of the internal carotid artery, which can simulate subarachnoid hemhorrage clinically, is best diagnosed by axial MRI which shows changes in the vessel wall where the hemorrhage is located.
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