MRI Spine

MRI of the spine is the best test for the spinal cord, nerve roots, intervertebral discs, vertebral marrow, epidural space and paraspinal soft tissues. The study can cover the cervical, thoracic or lumbar spine, or the whole spine, depending on the question.

It uses a 1.5 or 3 tesla scanner with a phased array spine coil. Slices are 3 to 4 mm thick. A typical study takes 20 to 40 minutes.

Routine sequences are sagittal T1-weighted, sagittal T2-weighted and fat suppressed T2 (or short tau inversion recovery), and axial T2-weighted images through the levels of interest. Gadolinium (0.1 mmol each kg) is added for infection, tumour, inflammation, post-operative assessment and cord disease. Diffusion weighted imaging, MR angiography and myelographic sequences are added when needed.

Back pain affects up to 80% of people at some time. Most cases are non-specific and need no imaging, but MRI is essential when there are red flags. Degenerative disc disease is common, with disc bulges in over 50% of asymptomatic people over 40.

MRI has a sensitivity of over 90% for cord compression, epidural abscess, discitis and marrow metastases. It is far better than CT for the soft tissues and the bone marrow, and it involves no radiation.

Indications.

• Suspected spinal cord compression or cauda equina syndrome (emergency).
• Progressive or severe neurological deficit, myelopathy, or radiculopathy not responding to treatment.
• Back pain with red flags: cancer history, fever or infection risk, weight loss, trauma with deficit, age over 50 with new pain, or night pain.
• Suspected spinal infection (discitis, osteomyelitis, epidural abscess, tuberculosis).
• Primary and metastatic spinal tumours, and marrow disease (myeloma, lymphoma).
• Demyelinating disease and cord lesions (multiple sclerosis, transverse myelitis, neuromyelitis optica).
• Spinal trauma with neurological signs, ligament injury and cord injury.
• Disc herniation and spinal stenosis with surgical or injection planning.
• Congenital anomalies (tethered cord, diastematomyelia, Chiari malformation, syrinx).
• Vascular lesions (dural arteriovenous fistula, cord infarction, haematoma).
• Inflammatory spondyloarthritis (sacroiliac joints and spine).
• Post-operative back pain, and assessment of fusion complications.

Not indicated for acute uncomplicated back pain under 6 weeks without red flags.

Contraindications and precautions.

• The MRI safety screen for pacemakers, implants and metallic foreign bodies. Most spinal fusion hardware is safe, but it causes artefact.
• Gadolinium: avoid or limit in severe renal impairment. Use macrocyclic agents.
• Pregnancy: non-contrast MRI is acceptable if needed.
• Claustrophobia or severe pain preventing lying flat: sedation, analgesia, or a wide bore scanner.
• Patients with an implanted spinal stimulator or intrathecal pump need device checks.

Preparation and technique.

No preparation is needed. The patient lies supine, with a pillow under the knees for lumbar studies to reduce pain.

Sagittal sequences cover the whole segment first. Axial images are taken at the disc levels or through the lesion. Whole spine screening (sagittal T1 and T2 in sections) is used for metastases and for multilevel disease.

Post-contrast T1 images are taken with fat suppression in sagittal and axial planes. Short tau inversion recovery sequences show marrow oedema.

For suspected cord ischaemia, diffusion weighted imaging of the cord is used. In the neck, motion from swallowing and pulsation is reduced with saturation bands.

Standing or flexion extension (upright) MRI can show dynamic instability in some centres.

Normal appearances.

The vertebral bodies have bright fatty marrow on T1, and the marrow is lower in signal in children and in anaemia. The cortex is dark.

Intervertebral discs are intermediate on T1 and bright on T2 in the nucleus pulposus (water content), with a dark outer annulus. Disc height and signal decrease with age.

The spinal cord is intermediate signal and lies centrally in the CSF-filled thecal sac. It ends at the conus medullaris at L1 to L2 in adults (up to L3 in newborns). Distal to this, the cauda equina nerve roots lie in the sac.

The canal is at least 12 mm (sagittal diameter) in the cervical spine, and over 10 to 12 mm in the lumbar spine. Epidural fat is bright on T1.

The facet joints are symmetrical with a thin cartilage layer. The ligaments (anterior and posterior longitudinal, ligamentum flavum, interspinous) are dark and continuous.

Interpretation.

• Disc degeneration: loss of T2 signal, height loss, annular tears (a high intensity zone on T2), vacuum phenomenon (dark on all sequences), and endplate marrow changes. The first type is oedema (low T1, high T2), the second is fatty (high T1 and T2), the third is sclerotic (low on both).
• Disc herniation: bulge (symmetric extension beyond the margin), protrusion (focal, base wider than the neck), extrusion (neck narrower than the displaced fragment), and sequestration (free fragment). Describe the level, the side (central, paracentral, foraminal, extraforaminal) and the nerve root affected.
• Spinal stenosis: central canal narrowing (ligamentum flavum thickening, facet hypertrophy, disc bulge, spondylolisthesis), lateral recess narrowing, and foraminal narrowing with obliteration of foraminal fat. Cord compression shows cord flattening, T2 hyperintensity (oedema or myelomalacia) and loss of CSF around the cord.
• Spondylolysis and spondylolisthesis: a pars defect, with forward slip of the vertebra (graded by percentage of slip) and foraminal narrowing.
• Fractures: acute fractures show marrow oedema (low T1, high short tau inversion recovery) and a fracture line. Benign osteoporotic fractures keep some normal marrow signal with a retropulsed fragment or band like pattern. Malignant fractures show convex posterior cortex bulging, pedicle involvement and a soft tissue mass.
• Ligament injury: disruption of the anterior or posterior longitudinal ligament, interspinous oedema and facet capsule injury, which signal instability.
• Spinal cord injury: oedema (high T2), haemorrhage (dark on gradient echo), and cord swelling. Prognosis is related to haemorrhage and length of oedema.
• Infection: discitis and osteomyelitis show high T2 signal in the disc, loss of the endplate outline, marrow oedema, enhancement, and paravertebral or epidural collections. Epidural abscess shows a rim enhancing collection compressing the cord. Tuberculosis causes anterior vertebral destruction, subligamentous spread, large paravertebral abscess, relative disc sparing in early disease and kyphosis.
• Tumours: metastases (low T1 marrow replacement, enhancing, with or without a soft tissue mass and epidural extension), myeloma (diffuse or focal "salt and pepper" marrow), lymphoma, haemangioma (bright on T1 and T2, benign), chordoma (sacrum, midline), and primary tumours.
• Intradural tumours: extramedullary (meningioma, schwannoma, neurofibroma, with dumbbell shape through the foramen) and intramedullary (ependymoma, astrocytoma, haemangioblastoma), which expand the cord, with enhancement, cysts and syrinx.
• Cord lesions: multiple sclerosis (short segment, peripheral, dorsal lesions under two vertebral segments), neuromyelitis optica (long segment over three vertebral segments), transverse myelitis, acute disseminated encephalomyelitis, cord infarction (anterior "owl's eye" T2 hyperintensity with restricted diffusion), subacute combined degeneration (dorsal column T2 hyperintensity in B12 deficiency), and radiation myelopathy.
• Syrinx: a CSF-signal cavity in the cord, associated with Chiari malformation, trauma, tumour or tethering.
• Congenital: tethered cord (low conus distal to L2 with a thickened filum), diastematomyelia, lipomyelomeningocele, spina bifida, caudal regression, and scoliosis with vertebral anomalies.
• Vascular: dural arteriovenous fistula (dilated perimedullary veins with cord oedema), epidural haematoma, and cavernoma.
• Sacroiliitis and spondyloarthritis: bone marrow oedema at the sacroiliac joints (subchondral, on fat suppressed sequences), erosions, fat metaplasia, and ankylosis. Vertebral corner inflammatory lesions are seen.
• Post-operative: epidural fibrosis (enhances homogeneously) versus recurrent disc (does not enhance centrally), arachnoiditis (clumped nerve roots), pseudomeningocele, infection, and adjacent segment disease.
• Incidental: perineural cysts, haemangiomas, endplate changes, and renal or pelvic findings in the field.

Critical values and findings needing immediate action.

• Spinal cord compression (metastatic or otherwise): high dose dexamethasone and urgent spinal surgical and oncology review for decompression or radiotherapy.
• Cauda equina compression (large central disc, tumour, abscess, haematoma): emergency surgical decompression.
• Epidural abscess or vertebral osteomyelitis with deficit: antibiotics, blood cultures and surgical drainage.
• Unstable fracture or ligament disruption with cord injury: spinal precautions and surgical review.
• Spinal epidural haematoma: emergency decompression, especially on anticoagulants.
• Cord infarction or acute myelopathy: urgent neurological assessment and exclusion of compression.
• Intradural tumour with progressive deficit: neurosurgical referral.
• Tuberculous spondylitis with neurological deficit or instability: anti-tuberculous therapy and surgery.

Limitations and artefacts.

• Imaging findings correlate poorly with pain. Disc bulges, annular tears and degenerative changes are found in large numbers of asymptomatic people, so findings must match the symptoms and examination.
• Motion (breathing, swallowing, pain), CSF pulsation and metal artefact from fusion hardware can obscure the canal. Titanium causes less artefact than stainless steel.
• Cord signal changes are not specific: oedema, demyelination, ischaemia, infection and tumour all cause T2 hyperintensity.
• MRI shows fractures less well than CT. Cortical detail, small fragments, and calcified lesions (ossification of the posterior longitudinal ligament) are better seen with CT.
• Marrow oedema in the acute phase of infection, trauma and tumour overlap. Enhancement patterns and clinical context help.
• Radiographic instability (dynamic) can be missed with a supine MRI.
• Gadolinium may not show leptomeningeal and intradural disease without delayed images.
• Early infection (the first 1 to 2 weeks) may not appear, and inflammatory markers are needed.
• Over reading degenerative change and incidental findings can lead to unnecessary treatment and surgery.
• Scoliosis, small size and body habitus limit coverage.

Special groups.

• Children: tethered cord, scoliosis, discitis, tumours, and non-accidental injury. Sedation may be needed.
• Older adults: osteoporotic fractures, stenosis and metastases.
• Patients with cancer: whole spine imaging for multiple metastases and sites of compression.
• Patients on anticoagulants: epidural haematoma risk.
• Pregnant patients: non-contrast MRI for suspected cord compression or infection.
• Patients with fusion hardware: expect artefact. Use metal artefact reduction sequences.
• Regions: spinal tuberculosis, pyogenic infection, trauma, cord compression from neglected tumours, HIV-related myelopathy and noma like vertebral infections are frequent. MRI is scarce, and radiographs and CT with clinical examination direct care. Laminectomy and anti-tuberculous therapy are the mainstays.

Follow up tests.

• CT of the spine for bone detail and surgical planning.
• Plain radiographs, standing and flexion extension views.
• Blood tests: full blood count, erythrocyte sedimentation rate, C-reactive protein, blood cultures, calcium, alkaline phosphatase, prostate specific antigen, serum protein electrophoresis, vitamin B12, HLA-B27.
• Image directed biopsy for suspected infection or tumour.
• Tuberculosis tests (interferon gamma release assay, tissue culture and histology).
• Nerve conduction studies and electromyography for radiculopathy.
• Bone density scan.
• CSF analysis for cord inflammation (oligoclonal bands, aquaporin-4 antibodies).
• Referral to spinal surgery, neurosurgery, oncology, rheumatology or pain services.