Benign enhancing foramen magnum lesions, also described as high signal lesions, have been anecdotally seen by radiologists for years but only recently described as an incidental finding on 3D FLAIR MRI in a typical location in the foramen magnum just posterior to the intradural vertebral artery.
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Epidemiology
The prevalence of this finding was 3.4% in a retrospective review of 3,717 patients who had a 3D FLAIR sequence on a 3T MR unit 2.
Pathology
The precise nature of this finding has not been entirely elucidated, but it has been postulated to represent a venous varix, a ganglion/pseudoganglion related to the spinal accessory nerve, or an ectopic glial nest or heterotopia in the leptomeninges 1-4. A single resected lesion was shown to have arachnoid tissue with a dense fibrotic nodule with collagenous fibers on pathology7.
Radiographic features
MRI
The lesion is a small nodular structure, typically <10 mm with a mean diameter of 3-4 mm 2. They are usually solitary lesions, while 10% have multiple 2. They are located posterior to the intradural vertebral artery just superior to its dural entrance.
3D FLAIR: hyperintense (main feature)
3D T1 C+: enhancing (90%)
3D T1 C-: occult due to isointensity to CSF
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T2
usually occult on 2D spin-echo images due to isointensity to CSF
may be distinguishable on balanced fast field echo (similar to CISS or FIESTA) imaging 3
2D FLAIR: usually occult due to small size and CSF flow artifacts
CT
The finding is also visible on post-contrast CT as an enhancing nodule, with attenuation following close to venous structures 5,6.
Differential diagnosis
The differential includes schwannoma and meningioma 3.
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