The empty thecal sac sign or empty sac sign is when the thecal sac appears empty on MRI of the lumbar spine, best seen on T2-weighted images. If the empty thecal sac sign is present, a diagnosis of adhesive arachnoiditis can be made.
There is usually no gadolinium contrast enhancement. Delamarter et al. 1 described three categories of MRI appearances consistent with chronic adhesive arachnoiditis. Groups 1 and 2 were mild and group 3 severe. Groups 1 and 2 were consistent with adhesions developing between nerve roots and the parietal arachnoid.
The adherent nerve roots were seen centrally within the CSF in group 1. Whilst in group 2 the nerve roots were adherent peripherally to the theca, which leads to the appearance of an empty thecal sac.
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