MR of CSF flow phenomenon mimicking basilar artery aneurysm

CSF motion from transmitted vascular pulsations can result in focal areas of CSF hypointensity on MR images because of signal mismapping from phase shift. When this CSF flow void occurs adjacent to the basilar artery in the prepontine cistern, it may be mistaken for a basilar artery aneurysm on proton-density and T2-weighted spin-echo images. Head MR scans of 50 consecutive patients referred for various indications were reviewed, and this phenomenon was noted in the prepontine cistern in 38 patients. Fifteen of these images had an appearance mimicking an aneurysm. When doubt exists, dynamic high-resolution CT scans should be sufficient for clarification.