Back pain and radiculopathy affect hundreds of millions of people worldwide, representing one of the leading causes of disability and lost productivity globally. CT of the spine — particularly the lumbar region — is one of the most commonly performed CT examinations in clinical practice. While MRI with its superior soft-tissue contrast has largely supplanted CT as the first-line modality for disc and neural element assessment, CT retains an essential and irreplaceable role in spinal imaging — particularly for evaluating bony anatomy, spondylosis, fractures, post-operative hardware, and as a problem-solving tool when MRI is contraindicated or technically limited.

Normal Disc Anatomy on CT

On CT, the intervertebral disc appears as a slightly hypodense (soft tissue density, typically 60–80 HU) structure between adjacent vertebral bodies. The nucleus pulposus cannot be reliably distinguished from the annulus fibrosus on standard CT due to their similar attenuation — this is where MRI's T2 signal advantage is most significant. Normal disc margins are smooth and confined within the posterior vertebral body line. The thecal sac appears as a low-density (CSF-density) structure within the spinal canal, visible on CT myelography (CTM) or inferred by the fat surrounding it on non-contrast CT. The nerve roots exit through the intervertebral foramina bounded anteriorly by the disc, superiorly/inferiorly by the pedicles, and posteriorly by the facet joint.

Classification of Disc Herniation

The North American Spine Society (NASS) provides a standardized nomenclature for disc pathology that is used in structured radiology reporting:

  • Disc Bulge: Diffuse, symmetric extension of disc tissue beyond the vertebral endplates by 25–100% of the disc circumference. Not a true herniation — typically represents degenerative disc disease rather than a discrete anatomical lesion
  • Disc Protrusion: Focal or broad-based displacement of disc material where the base (width of disc touching the parent disc) is wider than the apex (the displaced portion). This is the most common true herniation morphology
  • Disc Extrusion: Displaced disc material in which any dimension of the displaced fragment is greater than the base dimension — indicating that the nucleus has breached the annulus fibrosus completely
  • Sequestered Disc (Free Fragment): An extruded fragment that has lost all continuity with the parent disc. Can migrate superiorly or inferiorly within the canal, sometimes to an unexpected level

Directional Classification: Where Is the Herniation?

Along with morphological type, disc herniation location in the axial plane determines the specific neural structures at risk: Central herniation — into the spinal canal, potentially compressing the thecal sac and cauda equina at lumbar levels; Paracentral (Subarticular) — the most common location, impinging on the traversing nerve root in the lateral recess; Foraminal — compressing the exiting nerve root within the foramen (clinically produces same-level radiculopathy); Extra-foraminal (Far lateral) — extending beyond the foramen, compressing the dorsal root ganglion.

CT Assessment of Spinal Canal and Foraminal Stenosis

CT provides excellent assessment of the bony dimensions of the spinal canal and intervertebral foramina. Central canal stenosis at the lumbar level is defined as an anteroposterior diameter below 10 mm (severe) or 10–12 mm (relative stenosis). Contributors to central stenosis in the lumbar spine include: disc herniation and bulging, ligamentum flavum hypertrophy and buckling (particularly in extension), facet joint hypertrophy with osteophyte formation, and spondylolisthesis (anterior translation of one vertebral body relative to the next). Foraminal stenosis is graded by the degree of nerve root fat effacement within the foramen — from grade 0 (normal fat surrounding the root) to grade 3 (complete absence of fat and nerve root compression visible).

CT Myelography: When to Use It

CT myelography (CTM) involves intrathecal injection of water-soluble iodinated contrast (via lumbar puncture) followed by CT imaging. It provides superb delineation of the nerve roots and thecal sac — equivalent to or even superior to MRI in some complex post-operative and hardware-related cases. CTM is particularly valuable when MRI is contraindicated (non-MRI-compatible pacemakers, cochlear implants, severe claustrophobia) or when metallic implants create prohibitive susceptibility artifacts that degrade MRI quality in the region of interest. It remains an essential technique in complex spinal radiology.