Intracranial aneurysm (IA)

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Diagnosis Definition

Intracranial aneurysms (IA) are found in 1-14% of the population and rupture of an IA is the most common atraumatic cause of subarachnoid hemorrhage (SAH)
Unruptured IAs are usually asymptomatic; treatment (surgical clipping or endovascular coiling) is often recommended because morbidity and mortality from SAH reaches 25% and 60%
IAs are classified by morphology (saccular, fusiform), size, location (intradural, extradural), and etiology (congenital, traumatic, neoplastic, dissecting, mycotic, atherosclerotic)
Most IAs are saccular without any underlying predisposing factor and occur most commonly at vessel bifurcations
Giant aneurysms exceed 2.5 cm; as the size of an IA increases, so does the risk of rupture

Imaging Findings

On CTA, IAs appear as well-defined, round to ovoid, slightly hyperattenuating lesions, best seen on maximum intensity projection reformatted images (MIPs); enhancement is uniform when the IA is patent whereas a filling defect with rim enhancement is seen with thrombosis
On MRI/MRA most of the aneurysm appears as a flow void on T1 and hypointense on T2; unless thrombosed, the IA will enhance homogeneously; thrombosed IAs will vary in appearance depending on the age of the clot
Assessment should include size (maximum in three dimensions), neck width, shape, orientation, branches arising from the IA, the presence of other IAs, and any variant arterial anatomy that may complicate endovascular treatment

Pearls

CTA better detects aneurysmal wall or neck calcification compared with MRI/MRA and is often the initial test of choice because of the logistical ease of CTA compared with MRI
IAs are multiple in 20-25% of cases
3D time of flight (TOF) MRA and contrast-enhanced MRA are comparable with digital subtraction angiography in revealing aneurysmal remnants after coiling; however, MRA cannot be used in patients with older ferromagnetic clips, which can move on exposure to magnetic fields

References

Holtas S, Olsson M, Romner B. Comparison of MR imaging and CT in patients with intracranial aneurysm clips. AJNR Am J Neuroradiol 1988; 9:891-897
Kwee TC, Kwee RM. MR angiography in the follow-up of intracranial aneurysms treated with Guglielmi detachable coils: systematic review and meta-analysis. Neuroradiology 2007; 49(9):703-713

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