How can I determine when to refer a patient for an MRI versus a CT scan for suspected neurological conditions?

When determining whether to refer a patient for an MRI versus a CT scan for suspected neurological conditions, the choice often depends on the specific condition suspected, the urgency, and whether MRI is contraindicated ¹,²,³,⁵.

Conditions where MRI is generally preferred or indicated:

• For suspected spinal metastases or metastatic spinal cord compression (MSCC), an MRI scan should be offered as soon as possible, ideally within 24 hours for MSCC, or within 1 week for spinal metastases without MSCC ¹. MRI of the spine should include specific sequences to identify metastases and assess cord compression ¹.
• In adults with progressive, sub-acute loss of central neurological function, an urgent direct access MRI scan of the brain should be considered within 2 weeks to assess for brain or central nervous system cancer ²,³.
• For people over 16 who have had a suspected transient ischaemic attack (TIA) or acute non-disabling stroke, after specialist assessment in a TIA clinic, MRI (including diffusion-weighted and blood-sensitive sequences) should be considered on the same day to determine the territory of ischaemia, detect haemorrhage, or identify alternative pathologies ⁷.
• For trigeminal neuralgia, an MRI scan is indicated in younger people (advised under 40 years), those with atypical symptoms, individuals who do not respond to initial therapy, or anyone for whom neurosurgery is being considered ⁶. Referral to a specialist for MRI assessment is recommended ⁶.
• If there is a neurological abnormality that could be attributed to spinal cord injury, an MRI should be performed after a CT scan, regardless of whether the abnormality is evident on the CT ⁸.

Conditions where CT is generally preferred or indicated:

• For detecting an acute clinically important traumatic brain injury, CT imaging of the head is the current primary investigation of choice ⁵. MRI is generally not used as the primary investigation for acute traumatic brain injury due to safety, logistic, and resource reasons ⁵.
• In adults (16 or over) with spinal injury, perform CT if imaging for cervical spine injury is indicated by the Canadian C-spine rule, or if there is a strong suspicion of thoracic or lumbosacral spine injury associated with abnormal neurological signs or symptoms ⁸. Whole-body CT is used in adults with blunt major trauma and suspected multiple injuries ⁸.
• If MRI is contraindicated for suspected spinal metastases or MSCC, a CT scan should be carried out ¹. Similarly, for suspected brain or central nervous system cancer in adults, a CT scan can be considered if MRI is contraindicated ²,³.
• CT scans can also be considered for multiplanar viewing or 3-plane reconstruction in people with spinal metastases or MSCC to assess spinal stability and plan procedures ¹.

Other considerations:

• Plain X-rays of the spine should not be performed to diagnose or rule out spinal metastases or MSCC ¹. Similarly, plain X-rays of the skull should not be used to diagnose important traumatic brain injury without discussion with a neuroscience unit ⁵.
• For suspected functional neurological disorder (FND), structural and functional neuroimaging can help differentiate FND from other neurological disorders, but a neurology consultant with appropriate expertise should ideally make the diagnosis to reduce unnecessary investigations ⁴.