Benign enhancing foramen magnum lesions, also described as high signal lesions, are 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% in a retrospective review of 3717 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 fibres on pathology 7.
Radiographic features
CT
The finding is also visible on post-contrast CT as an enhancing nodule, with attenuation following close to venous structures 5,6.
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
Differential diagnosis
The differential includes schwannoma and meningioma 3.
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