Present or absent

necessary. Magnetic resonance (MR) and CT angiography are not presently sufficiently sensitive to replace conventional angiography, but the techniques are rapidly advancing (9,10). CT and MR angiograms may serve as an additional tool with which to plan surgery (11).

In 15% to 20% of patients, the cause of nontraumatic SAH remains undetermined even after angiography. In a few instances, this may be due to vasospasm or inadequate views incapable of detecting smaller aneurysms. Repeat angiography is therefore often recommended in a few days to two weeks. In patients whose bleeding is confined to the perimesencephalic and ambient cisterns, a source of bleeding might not be evident despite high-quality angiograms. Such hemorrhages are referred to as "perimesencephalic SAH" and have been attributed to venous rather than arterial bleeding (12). However, detecting the typical perimesencephalic pattern of SAH on CT should not prevent one from pursuing angiography, because this pattern may be seen with aneurysmal rupture (13,14), especially in the posterior circulation (15 ).

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