Less than 5% of aneurysms occur in the posterior fossa. They are located at the junctions of the cerebellar arteries with the basilar artery. Giant aneurysms can simulate other mass lesions in the posterior fossa, but the MR appearance is usually characteristic. Giant aneurysms have mixed signal intensity on MR due to a combination of subacute and chronic hemorrhage and calcification. The patent lumen of the aneurysm may be high or low signal depending on the rate of flow. 

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