Vignette: A 42 year old woman with autosomal dominant polycystic kidney disease and family history of subarachnoid hemorrhage underwent CT angiography to screen for intracranial aneurysms.
Image Findings: Demonstrated above is a CT angiogram with MIP (maximal intensity projection) images illustrating a focal outpouching at the anterior communicating artery consistent with an intracranial aneurysm. The right anterior cerebral artery is not entirely seen due to artifact of volume averaging.

Anatomy of Cerebral arteries: The circle of Willis forms the major arterial supply to the brain and is a common site for cerebral aneurysms. It is comprised of anterior, middle and posterior cerebral arteries which are linked by the anterior and posterior communicating arteries (see Figure) forming a circular anastamosis. Major sites for aneurysms are indicated by *.

Intracranial Aneurysm:
The prevalence of intracranial aneurysms is estimated at about 2% of adults; the majority (85%) of intracranial aneurysms are in the anterior circulation, commonly in the circle of Willis. Thirty percent of cerebral aneurysms are in the anterior communicating artery.

In patients with autosomal dominant polycystic kidney disease, there is an increased prevalence of cerebral aneurysms, with estimates ranging from 4-40% of patients; the increased prevalence may be related to increased rates of hypertension in this population.

Most aneurysms do not rupture and the incidence of subarachnoid hemorrhage due to rupture is roughly 1 case per 10,000 people annually. Aneurysms smaller than 7mm have a very low risk of rupture (~0.05% per yr); aneurysms that cause symptoms and/or are over 10mm have a higher likelihood of rupture (~6% per yr), and may require surgical management.

Unruptured aneurysms rarely produce symptoms. If present, symptoms may include:
· Headache (“worst headache of life”)
· Loss of visual acuity
· Third nerve palsy or other cranial neuropathy
· Pyramidal tract dysfunction
· Facial pain

Radiographic Findings:
Saccular aneurysms are visualized as focal outpouchings in the vasculature, usually at branch points or arterial junctions. Both CT angiography (CTA) and MR angiography (MRA) are useful for screening for intracranial aneurysms with over 95% sensitivity for aneurysms > 6mm. 3D CTA is useful for improved visualization of unruptured aneurysms and in some cases, may replace arteriography for pre-operative planning.

Intra-arterial digital subtraction arteriography is more expensive and invasive, but with greater resolution for detecting intracranial aneurysms. This technique is used to determine the exact size and spatial orientation of intracranial aneurysms for surgical intervention or to screen for aneurysms following a negative CTA or MRA when clinical symptoms are present.

Rinkel GJ. Natural history, epidemiology and screening of unruptured intracranial aneurysms. J Neuroradiology. 2008; 35: 99-103.

Brisman JL et al. Cerebral Aneurysms. NEJM. 2006; 355:928-39.

Case Vignette authored and created by C. Westra (radRounds Radiology Network) and peer-reviewed by medical education board of radRounds.