Thecal Sac and CSF Spaces¶
Definition¶
The thecal sac (dural sac) is the tubular membrane formed by the spinal dura mater and arachnoid mater that contains the spinal cord, cauda equina, and cerebrospinal fluid (CSF). It extends from the foramen magnum to approximately the S2 vertebral level, where it terminates as the dural cul-de-sac.
Anatomy¶
Meningeal Layers¶
The thecal sac is formed by two meningeal layers:
- Dura mater — the outermost, toughest layer; forms the external wall of the thecal sac
- Arachnoid mater — a thin, avascular membrane lining the inner surface of the dura; the subarachnoid space between the arachnoid and pia mater contains CSF
The pia mater is the innermost layer, intimately adherent to the spinal cord surface, and is not part of the thecal sac wall.
Extent and Dimensions¶
- Superior: continuous with the cranial dura at the foramen magnum
- Inferior: terminates at the S2 level (range S1–S3) as the dural cul-de-sac
- AP diameter: varies by level; normally >12 mm in the cervical and lumbar regions on axial imaging
- Cross-sectional area: a useful measurement for grading central stenosis; <100 mm² is considered severe stenosis in the lumbar spine
Spaces¶
- Epidural space — between the dura and the vertebral canal walls; contains fat, veins (internal vertebral venous plexus), and connective tissue; this is the target for epidural injections
- Subdural space — a potential space between the dura and arachnoid; may accumulate fluid in subdural hygromas or hematomas
- Subarachnoid space — between the arachnoid and pia mater; contains CSF, nerve roots, and blood vessels; widens below the conus medullaris to form the lumbar cistern
Dural Nerve Root Sleeves¶
At each level, the thecal sac gives off paired lateral extensions — the nerve root sleeves — which surround the exiting nerve roots as they pass through the neural foramina. These sleeves carry a small amount of CSF around the proximal nerve root.
Clinical Pearl
The thecal sac terminates at S2, but the spinal cord ends at the conus medullaris (L1–L2 in adults). The space between L2 and S2 — the lumbar cistern — contains only the cauda equina nerve roots floating in CSF. This is why lumbar puncture is safely performed at the L3–L4 or L4–L5 interspaces: the needle enters the subarachnoid space below the cord.
Imaging Findings¶
CT¶
- CSF within the thecal sac appears as low-density fluid on non-contrast CT
- CT myelography (after intrathecal contrast injection) opacifies the subarachnoid space, allowing detailed evaluation of the thecal sac contour, nerve root sleeves, and sites of compression
- Useful when MRI is contraindicated
MRI¶
MRI is the primary modality for evaluating the thecal sac and CSF spaces:
| Sequence | CSF Appearance | Key Utility |
|---|---|---|
| T1-weighted | Low signal (dark) | Anatomy; nerve roots visible against dark CSF background |
| T2-weighted | High signal (bright) | "Myelographic effect" — highlights compression and stenosis |
| STIR | High signal | Evaluates epidural pathology |
| Post-contrast T1 | Low signal (normal CSF doesn't enhance) | Leptomeningeal disease if CSF enhances |
Key MRI Finding
On axial T2-weighted images, the thecal sac normally appears as a bright ring of CSF surrounding the nerve roots of the cauda equina (which appear as dark dots). Effacement of the CSF signal indicates compression — the degree of effacement correlates with stenosis severity. Complete obliteration of the CSF column on sagittal T2 indicates severe stenosis.
Key Points¶
- The thecal sac extends from the foramen magnum to the S2 level
- It contains the spinal cord (to L1–L2) and cauda equina in CSF
- The lumbar cistern (L2–S2) is the target for lumbar puncture and myelography
- The epidural space lies between the dura and the spinal canal; it is the target for epidural injections
- CSF is dark on T1 and bright on T2 — T2 provides a natural "myelographic effect"
- Thecal sac cross-sectional area <100 mm² indicates severe lumbar stenosis
References¶
- Schizas C, Theumann N, Burn A, et al. Qualitative grading of severity of lumbar spinal stenosis based on the morphology of the dural sac on magnetic resonance images. Spine (Phila Pa 1976). 2010;35(21):1919-1924. PMID: 20671589.
- Lurie JD, Tosteson AN, Tosteson TD, et al. Reliability of readings of magnetic resonance imaging features of lumbar spinal stenosis. Spine (Phila Pa 1976). 2008;33(14):1605-1610. PMID: 18552677.
- Khasawneh AH, Garling RJ, Harris CA. Cerebrospinal fluid circulation: What do we know and how do we know it? Brain Circ. 2018;4(1):14-18. PMID: 30276331.
- Margetis K, Baker S. Physiology, Cerebral Spinal Fluid. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025. Available from: https://www.ncbi.nlm.nih.gov/books/NBK519007/
- Margetis K, Weisbrod LJ, Launico MV. Neuroanatomy, Cerebrospinal Fluid. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470578/
- Phongkitkarun S, Jaovisidha S, Dhanachai M. Determination of the thecal sac ending using magnetic resonance imaging: clinical applications in craniospinal irradiation. J Med Assoc Thai. 2004;87(11):1368-1373. PMID: 15825715.