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Normal vertebral body height, alignment and marrow signal. Prominent S1 intervertebral disc.
A rounded lesion in the left L5 lateral recess is T1 isointense to muscle/disc and T2 hyperintense with a hypointense rim. This contacts the exiting left and contributes to severe foraminal stenosis. The descending left S1 nerve root is displaced medially.
Right foraminal zone disc protrusion causes moderate foraminal stenosis at L4/5. No other significant spinal canal or foraminal stenosis.
Conclusion: Large (16 mm) lesion in the left L5 lateral recess has signal characteristics of a synovial cyst, arising from the left L5/S1 facet joint. This contacts and displaces the left L5 and S1 nerve roots.