CT Thoracolumbar Spine

Computed tomography of the thoracolumbar spine covers the thoracic and lumbar vertebrae and sacrum with a bone algorithm volume, usually reconstructed from the chest and abdomen and pelvis data in trauma, or acquired separately for spine specific questions. Thin slices (0.6 to 1.25 mm) with sagittal and coronal reformats show fractures, canal compromise and alignment.

Effective dose is about 6 to 10 mSv for the whole thoracolumbar spine, so targeted ranges are used where the clinical question allows.

Thoracolumbar fractures occur in about 5 to 15% of major blunt trauma patients, most at the thoracolumbar junction (T11 to L2), and about 20% are multilevel or non-contiguous. Osteoporotic compression fractures affect about 1 in 4 women over 50.

CT shows bony injury with a sensitivity of over 95%, far better than plain radiographs (about 60 to 70%), but shows cord, disc and ligament injury poorly.

Indications. Blunt trauma with back pain, midline tenderness, deformity, neurological deficit, high energy mechanism, fall from height, or distracting injury. Unconscious patients. Part of whole body trauma CT.

Abnormal radiographs or suspected fracture not defined by radiographs. Pre-operative planning for fusion, pedicle screw placement and kyphoplasty. Assessment of fusion and complications after instrumentation.

Spinal tumour, pathological fracture and infection (with contrast). Scoliosis and congenital anomalies (three dimensional reconstructions). Lumbar canal stenosis and spondylolysis when MR is not possible.

Image directed biopsy or injection. Not for uncomplicated low back pain without red flags.

Contraindications and precautions. No absolute contraindications. Spine precautions (log roll, flat bed) continue until the report. Pregnancy: avoid unless essential, limit the range. Children: use age specific low dose settings. Contrast has the usual precautions for renal function and allergy, and is used only for infection, tumour or vascular question.

Preparation and technique. No preparation. The patient lies supine with arms raised over the head or folded on the chest, to avoid arm artefact. A scout sets the range.

Data are acquired in a volume, reconstructed in a bone algorithm at 1 to 2 mm in axial, sagittal and coronal planes, and in a soft tissue algorithm for the canal, discs and paraspinal tissues. For trauma, reformats are made from the chest, abdomen and pelvis acquisition.

Curved reformats follow the spine in scoliosis, and three dimensional volume rendering is added for surgical planning.

Normal appearances. Twelve thoracic vertebrae with a kyphosis of 20 to 40 degrees and five lumbar vertebrae with a lordosis. Vertebral heights are even with smooth endplates; the posterior vertebral line is smooth and the canal round to triangular, with the thecal sac and cauda equina visible on soft tissue windows.

Pedicle thickness and interpedicular distance are symmetrical and widen gradually towards the lumbar spine. Facet joints are congruent, intervertebral discs have normal height with no calcification, and paraspinal tissues are symmetrical. Sacroiliac joints are symmetrical with smooth margins.

Interpretation.

• Compression fracture: loss of anterior vertebral height with an intact posterior wall (stable), wedge or crush; osteoporotic fractures show endplate depression and cleft, acute fractures show a sharp lucent line and surrounding oedema (on MR).
• Burst fracture: fracture of anterior and posterior vertebral body walls with retropulsion of fragments into the canal, interpedicular widening and a vertical laminar fracture; canal compromise over 50% and neurological deficit favour surgery.
• Flexion distraction (seat belt) injury: horizontal fracture through the vertebral body, pedicles and posterior elements, or through the disc and ligaments, with widening of the interspinous distance and perched facets; associated with bowel and mesenteric injury.
• Fracture dislocation: translation of one vertebra on another, with facet and pedicle fractures and canal narrowing; highly unstable.
• Transverse process and spinous process fractures: stable but often accompany renal and retroperitoneal injury.
• Pathological fracture: lytic or sclerotic lesions in the body or pedicle (metastasis, myeloma), posterior wall bulging, soft tissue mass; the "winking owl" sign (missing pedicle) on radiographs.
• Infection: endplate erosion, disc space narrowing, vertebral destruction, paravertebral and psoas abscess (with rim enhancement and gas), and tuberculous spondylitis (anterior wedging, subligamentous spread, calcified abscess, gibbus).
• Degenerative: disc narrowing, vacuum phenomenon, osteophytes, facet arthropathy, spondylolisthesis, spondylolysis (pars defect), calcified disc herniation, canal stenosis (sagittal diameter under 10 to 12 mm in the lumbar spine), and ossified ligamentum flavum.
• Spinal fusion and instrumentation: screw and rod position, loosening (lucent halo around screws), non-union (persistent lucent line at the graft), adjacent segment disease, and cement leakage after vertebroplasty.
• Deformity: scoliosis (curve angle measured on coronal reformats), vertebral anomalies (hemivertebra, block vertebra), and kyphosis.
• Sacral and sacroiliac fractures and sacroiliitis (erosions and sclerosis on both sides in spondyloarthritis).
• Incidental findings: bone islands, haemangioma (coarse vertical trabeculae, "polka dot"), Schmorl nodes, aortic aneurysm, renal and adrenal lesions.

Critical values and findings needing immediate action.

• Unstable fracture with canal compromise or neurological deficit: spinal immobilisation, urgent spinal surgical review, and MR to examine the cord and ligaments.
• Cauda equina compression (fragment or abscess in the lumbar canal): emergency MR and decompression.
• Flexion distraction injury: search for abdominal visceral injury on the same CT.
• Spinal infection with abscess and deficit: antibiotics, culture, and urgent surgical drainage.
• Pathological fracture with cord or cauda equina compression: urgent oncological and surgical review.
• Aortic injury adjacent to a thoracic spine fracture: CT angiography and vascular review.
• Fracture of an ankylosed spine: regard as unstable even with minor displacement.

Limitations and artefacts.

• CT does not reliably show cord injury, epidural haematoma, disc herniation, ligament rupture or bone marrow oedema; MR is needed when there is deficit or discordant findings.
• Osteoporotic bone and degenerative change reduce contrast between fracture and background; acute versus chronic fracture is often impossible on CT alone.
• Streak artefact from metalwork, arms and bowel contents limits the canal and screw assessment; metal artefact reduction software helps.
• Sagittal reformats are needed: axial slices alone miss horizontal fractures and mild compression.
• Non-contiguous fractures are found in about 20%, so the whole spine is reviewed when one fracture is found.
• Scoliosis measurement depends on patient position; standing radiographs reflect load bearing alignment.
• Radiation dose is high for a long range; limit it in children and young people.

Special groups.

• Older adults and those on steroids: fragility fractures after minimal trauma, with a risk of further fractures and need for bone health treatment.
• Children: transitional anomalies, apophyseal injuries, and non-accidental injury (fractures of different ages); use low dose settings.
• Pregnancy: avoid unless essential; MR if possible.
• Patients with cancer: pathological fracture and cord compression need prompt assessment.
• Patients with ankylosing spondylitis: transverse fractures through fused segments.
• Regions: spinal tuberculosis is common, often with large paravertebral abscesses and kyphosis; falls from trees and roofs, mining and road injuries give burst fractures; MR availability is limited, so CT supports the surgical decision.

Follow up tests. MR of the spine (cord, discs, ligaments, marrow), CT angiography for vascular injury, bone density measurement and fracture risk assessment, serum calcium, vitamin D, parathyroid hormone and thyroid function in fragility fractures, serum protein electrophoresis, prostate specific antigen and imaging of the primary in suspected metastasis, blood cultures,

inflammatory markers and biopsy in suspected infection, tuberculosis tests (sputum, tissue, interferon gamma release assay), standing radiographs for alignment, repeat CT after fixation, and referral to spinal surgery, orthopaedics, oncology or rheumatology.