Empty thecal sac sign

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.​

Radiographic features


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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Article information

rID: 14052
Section: Signs
Synonyms or Alternate Spellings:
  • Empty sac sign
  • Empty theca sign

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Cases and figures

  •  Case 1
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  • Case 2
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