Spinal Canal and Neural Foramina¶
Definition¶
The spinal canal (vertebral canal) is the bony tunnel formed by the sequential alignment of the vertebral foramina, housing the spinal cord, meninges, and epidural contents. The neural foramina (intervertebral foramina) are paired lateral openings between adjacent vertebrae through which the spinal nerve roots exit the canal.
Anatomy¶
Spinal Canal¶
The spinal canal is bounded by:
- Anterior: posterior surface of the vertebral bodies and intervertebral discs, covered by the posterior longitudinal ligament
- Posterior: laminae and ligamentum flavum
- Lateral: pedicles and facet joints
Canal dimensions vary by region:
| Region | AP Diameter (Normal) | Stenosis Threshold |
|---|---|---|
| Cervical | 14–23 mm | <13 mm (relative), <10 mm (absolute) |
| Thoracic | 12–14 mm | Smallest normal canal |
| Lumbar | 15–27 mm | <12 mm (relative), <10 mm (absolute) |
Canal shapes by region:
- Cervical: triangular, relatively large to accommodate the cervical enlargement
- Thoracic: circular, smallest diameter
- Lumbar: triangular to trefoil, widest overall
Neural Foramina¶
Each neural foramen is bounded by:
- Superior: pedicle of the vertebra above
- Inferior: pedicle of the vertebra below
- Anterior: posterior aspect of the vertebral body, disc, and uncinate process (cervical)
- Posterior: facet joint and ligamentum flavum
Contents of the neural foramen:
- Spinal nerve root (dorsal and ventral rootlets)
- Dorsal root ganglion
- Radicular arteries and veins
- Recurrent meningeal (sinuvertebral) nerve
- Epidural fat and connective tissue
Clinical Pearl
In the cervical spine, nerve roots exit above their corresponding vertebra (C6 nerve exits the C5–C6 foramen). In the lumbar spine, nerve roots exit below their corresponding vertebra (L4 nerve exits the L4–L5 foramen). The transition occurs at C7–T1, where the C8 nerve root exits (there is no C8 vertebra). This numbering distinction is critical for correlating imaging findings with clinical symptoms.
Imaging Findings¶
Radiography¶
- AP view: interpedicular distance provides an estimate of canal width; narrowing suggests central stenosis
- Lateral view: AP canal diameter measured from the posterior vertebral body line to the spinolaminar line
- Oblique views: best for visualizing the neural foramina in the lumbar spine
CT¶
- Gold standard for measuring bony canal dimensions
- Axial images demonstrate canal shape and area
- Sagittal reformats show foraminal dimensions and any bony encroachment
- Foraminal stenosis grading: normal (fat surrounds the nerve root), mild (fat reduced), moderate (nerve root contacts foramen borders), severe (nerve root compressed and deformed)
MRI¶
| Finding | Best Sequence |
|---|---|
| Central canal stenosis | Axial and sagittal T2 — CSF effacement around the cord or cauda equina |
| Foraminal stenosis | Sagittal T1 — loss of normal high-signal perineural fat |
| Lateral recess stenosis | Axial T2 — nerve root compression in the lateral recess (<3 mm) |
| Cord compression | Sagittal T2 — cord deformation, intramedullary signal change |
Key MRI Finding
On sagittal T1-weighted images, the normal neural foramen contains bright epidural fat surrounding the dark nerve root. Foraminal stenosis is diagnosed when this fat signal is obliterated by disc, osteophyte, or facet hypertrophy. Sagittal T1 is the single most useful sequence for evaluating foraminal narrowing.
Key Points¶
- The spinal canal houses the spinal cord/cauda equina; neural foramina transmit the exiting nerve roots
- Canal dimensions are smallest in the thoracic region
- Cervical nerve roots exit above their numbered vertebra; lumbar roots exit below
- Stenosis results from disc bulging, facet hypertrophy, ligamentum flavum thickening, or congenital narrowing
- CT is best for bony canal measurements; MRI is best for evaluating soft tissue stenosis and neural compression
- Sagittal T1 MRI is the key sequence for foraminal stenosis evaluation
References¶
- Lee S, Lee JW, Yeom JS, et al. A practical MRI grading system for lumbar foraminal stenosis. AJR Am J Roentgenol. 2010;194(4):1095-1098. doi:10.2214/AJR.09.2772. PubMed
- Seo J, Lee JW. Magnetic Resonance Imaging Grading Systems for Central Canal and Neural Foraminal Stenoses of the Lumbar and Cervical Spines With a Focus on the Lee Grading System. Korean J Radiol. 2023;24(3):224-234. PubMed
- Peabody T, Black AC, Das JM. Anatomy, Back, Vertebral Canal. In: StatPearls. Treasure Island (FL): StatPearls Publishing; updated November 16, 2023. NCBI Bookshelf
- Ulbrich EJ, Schraner C, Boesch C, et al. Normative MR cervical spinal canal dimensions. Radiology. 2014;271(1):172-182. doi:10.1148/radiol.13120370. PubMed
- Sioutas G, Kapetanakis S. Clinical anatomy and clinical significance of the cervical intervertebral foramen: a review. Folia Morphol (Warsz). 2016;75(2):143-148. PubMed
- Demondion X, Lefebvre G, Fisch O, Vandenbussche L, Cepparo J, Balbi V. Radiographic anatomy of the intervertebral cervical and lumbar foramina (vessels and variants). Diagn Interv Imaging. 2012;93(9):690-697. PubMed