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Topic

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Sacral canal

The sacral canal is the continuation of the vertebral canal within the sacrum. It begins at the sacral base and terminates at the sacral hiatus, located at the inferior end of the dorsal sacrum. The canal transmits the terminal portion of the dural sac, cauda equina nerve roots, filum terminale, and epidural fat/venous plexus.

The canal is formed by the fusion of the vertebral arches of the sacral vertebrae. It tapers downward, with its posterior wall incomplete due to the sacral hiatus. The sacral foramina open laterally, allowing passage of sacral spinal nerves.

Clinically, the sacral canal is important for caudal epidural anesthesia, surgical access to sacral nerve roots, and evaluation of congenital anomalies such as spina bifida occulta or tethered cord. Pathological processes such as sacral tumors, meningeal cysts, trauma, and infections may also involve the canal.

Synonyms

  • Canalis sacralis

  • Terminal spinal canal

  • Sacral vertebral canal

Function

  • Protects and houses the cauda equina nerve roots and filum terminale

  • Provides passage for the sacral venous plexus

  • Forms an access route for caudal epidural injections

  • Contributes to neurological and pelvic function by protecting sacral nerves

Contents

  • Terminal portion of the dural sac (usually ends at S2)

  • Cauda equina nerve roots (sacral spinal nerves S1–S5, coccygeal nerve)

  • Filum terminale

  • Internal vertebral venous plexus and epidural fat

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MRI Appearance

T1-weighted images:

  • Canal shows low-signal cortical bone walls

  • Epidural fat appears bright (hyperintense), highlighting nerve roots and filum terminale as intermediate-to-low signal structures

T2-weighted images:

  • Cerebrospinal fluid (if extending into canal) appears bright hyperintense

  • Nerve roots appear as low-to-intermediate signal threads within CSF and fat

STIR:

  • Suppresses fat signal, enhancing visibility of nerve roots, dural sac, and pathology

  • Highlights inflammation, tumor infiltration, or edema

T1 Fat-Saturated (Pre-contrast):

  • Epidural fat suppressed, making nerve roots and vascular structures stand out more clearly

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Normal epidural veins enhance

  • Pathologies (meningeal cysts, tumors, infection, inflammation) show variable enhancement

  • Helps delineate nerve sheath tumors, metastases, and epidural abscesses

MRI Non-Contrast 3D Imaging:

  • Allows volumetric reconstruction of sacral canal anatomy

  • Useful in congenital anomalies (spina bifida occulta, tethered cord), surgical planning, and caudal block guidance

CT Appearance

CT Pre-Contrast:

  • Sacral canal visible as a bony cavity within sacrum

  • Demonstrates sacral laminae fusion, fractures, or spina bifida occulta

CT Post-Contrast:

  • Enhances visualization of vascular structures or infiltrative lesions

  • Useful for identifying epidural masses, infections, or metastases

CT Myelography (if performed):

  • Outlines nerve roots and dural sac in cases of stenosis, tethering, or CSF leak

CT image

sacral canal ct axial image

CT image

sacral canal ct sag image

MRI image

sacral canal MRI sag anatomy image -img-00000-00000

MRI image

sacral canal MRI axial anatomy image -img-00000-00000