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A 46 year old female with left sided frontal headache and a partial left third nerve palsy underwent magnetic resonance angiography (MRA) followed by digital subtraction angiography (DSA). MRA identified a left posterior communicating artery aneurysm (fig 1). DSA confirmed a 7 mm aneurysm on the left and also revealed a 3 mm right posterior communicating aneurysm (fig 2), which was not evident using MRA.
Detection rates using non-invasive imaging methods are significantly poorer for smaller (<5 mm) aneurysms.1 Aneurysm size is an important factor in aneurysm detection, with studies of MRA consistently indicating sensitivity of >95% for aneurysms >6 mm diameter. For aneurysms <5 mm detection rates as low as 56% have been reported.2 With standard time of flight MRA technique flow related artefacts may obscure some of the anatomical detail.1
Despite the known limitations clinicians often request MRA to screen for intracranial aneurysms, aware that smaller aneurysms will not be demonstrated.3 This case supports the view that clinically significant aneurysms will be detected by MRA, but smaller asymptomatic lesions will be missed. It gives a convenient comparison of MRA visible and invisible aneurysm dimensions.