🔬 MRI Spine

MRI Spine provides detailed imaging of the vertebrae, intervertebral discs, spinal cord, and nerve roots. It is the gold standard for diagnosing cord compression, cauda equina syndrome, and spinal infections. Its use is reserved for patients with red flags or persistent radiculopathy due to the high prevalence of incidental findings in the general population.

Primary indications include 'red flag' back pain (suspected cauda equina, malignancy, or infection), radiculopathy that hasn't responded to conservative management, and myelopathy (signs of cord compression like hyperreflexia or gait ataxia). It is also used to evaluate spinal trauma and congenital spinal anomalies. Routine back pain without red flags is NOT generally an indication for MRI in the UK.

The patient lies supine in the MRI scanner. High-resolution sagittal and axial images are acquired. 'Stir' sequences (fat suppression) are often added to highlight bone marrow oedema, which is useful for detecting acute fractures or active inflammation. Gadolinium contrast is rarely used but can help in cases of suspected tumour or post-operative scarring vs. recurrent disc.

A normal scan shows well-aligned vertebral bodies with normal height and signal. Intervertebral discs should show high T2 signal (well-hydrated) and not bulge into the canal. The spinal cord should have a uniform signal and end at approximately L1/L2. The cauda equina nerve roots should be free-floating in the CSF with no evidence of compression in the central canal or exit foramina.

Interpretation involves assessing the alignment of the vertebrae, the height and hydration of the discs, and the patency of the spinal canal and exit foramina. T2-weighted sagittal images are most useful for a quick overview of the cord and CSF space. Any narrowing must be correlated with the patient's dermatomal symptoms (e.g., L5/S1 disc heralds S1 nerve root compression).

Findings include intervertebral disc herniations (protrusion/sequestration) causing nerve root compression, and spinal stenosis (narrowing of the canal). Critical findings include 'conus medullaris' or 'cauda equina' compression, vertebral fractures, and spinal cord signal changes (oedema or myelomalacia). Bony metastases often show as low T1 signal replacing normal fatty marrow, while discitis/osteomyelitis shows endplate destruction and fluid collections.

MRI Spine is the definitive investigation for suspected Cauda Equina Syndrome (CES)—a surgical emergency. It is also the primary tool for planning spinal surgery or injections. In oncology, it is used to investigate suspected metastatic spinal cord compression (MSCC). Identifying the exact level and nature of a lesion allows for targeted physiotherapy, neurosurgery, or radiotherapy.

The main pitfall is failing to correlate clinical signs with radiological findings; treating the 'scan' rather than the 'patient'. Also, missing 'skip lesions' in the context of MSCC—the whole spine should be imaged if malignancy is suspected. Technical pitfalls include 'aliasing' or 'truncation' artefacts which can mimic cord cavities (syrinx).

The primary limitation is the 'over-diagnosis' of age-related degenerative changes (e.g., disc bulges) which are present in many asymptomatic individuals. This can lead to unnecessary patient anxiety or procedures. Similar to MRI Brain, it is limited by claustrophobia, patient size, and the presence of metallic implants. Physical spinal deformity (scoliosis) can make acquiring standard planes difficult.

Crucial: Any patient with new-onset bladder/bowel dysfunction and back pain must have an MRI 'within 4 to 6 hours' of the request to rule out Cauda Equina Syndrome. Know that 'disc desiccation' (dark discs on T2) is a normal part of ageing and often asymptomatic.