Specific Imaging Findings

Perisellar aneurysms are rarely visible on non-enhanced CT if not calcified or large, when isodense outpouching or lobulation may be present along the course of the vessels, usually cavernous or paraclinoid internal carotid artery (ICA) segments. Sometimes very large aneurysms from other intracranial vessels, primarily the basilar or the anterior communicating arteries, may present as perisellar masses. A small aneurysm in a perisellar location can manifest as a cavernous sinus asymmetry, while large ones may erode into the sphenoid sinus and sella turcica. The aneurysm wall may be calcified, and intraluminal thrombus is hyperdense. Patent aneurysmal lumen enhances the same as the other vessels on post-contrast CT. On T2WI a flow-void outpouching may be seen adjacent to the parent vessel, while the presence of a thrombus leads to T1 hyperintensity. In larger aneurysms a laminated “onion skin” appearance may be seen, due to clot apposition within the lumen, and a pulsatility artifact can be noted along the phase-encoding direction. Postcontrast MRI findings are variable and unpredictable – from flow-void to hyperintensity, due to thrombosis and turbulent flow. CTA, 3D TOF MRA, and contrast-enhanced MRA have very high sensitivity and accuracy for aneurysms. CTA may be limited for aneurysms within the enhancing cavernous sinus and at the skull base. Large aneurysms may be underestimated or even completely absent on TOF MRA due to saturation artifacts caused by turbulent flow. Important diagnostic questions in intracranial aneurysms are patency versus thrombosis, and extra- versus intra-dural location.