MRI vs X-ray for back pain

The majority of back pain is non-specific: it comes from muscles, ligaments and joints, and settles over weeks. Scans rarely identify a specific cause and do not predict recovery. NICE guidance recommends assessing the pattern of pain, offering advice to stay active, and using a risk-stratification approach rather than automatic imaging.

A scan taken too early can also cause harm. Findings such as disc bulges or "degeneration" may be labelled as the problem when they are common age-related changes, which can increase worry and lead to unnecessary treatment.

A lumbar X-ray shows the vertebrae, their alignment, fractures, narrowing of disc spaces and bone spurs. It can help when a fracture is suspected, for example after an injury or in someone with osteoporosis, or to check spinal curvature.

It cannot show a disc, nerve, spinal cord or most causes of sciatica. A "normal" X-ray does not exclude a slipped disc, and an "abnormal" X-ray often shows only wear that does not correspond to pain.

MRI shows soft tissue: discs, nerve roots, the cord, ligaments, fat and any collection or tumour. It is the test for suspected nerve compression, infection, cancer involvement, inflammatory spinal conditions and for planning surgery or targeted injections. It uses no radiation.

Its drawback is over-sensitivity. Many people without back pain have disc bulges or bright discs on MRI, so a finding must be matched to symptoms and examination before it counts. An MRI is also not usually offered for pain without nerve symptoms unless it is persistent and a procedure is being considered.

NICE NG59 recommends that imaging is considered in non-specific low back pain only if the result is likely to change management, and that MRI be reserved for situations such as considering spinal fusion, or suspected serious pathology. The logic is that treatment for most back pain is movement, exercise and time, none of which depends on a scan.

For sciatica that persists beyond several weeks despite conservative care, MRI becomes useful because it can show whether a disc is pressing on a nerve and guide the choice between injections and surgery.

• Back pain for less than a few weeks with no red flags: no scan; keep moving, use simple analgesia as advised, and seek review if not improving.

• Back pain after a significant injury or with osteoporosis: X-ray or CT for fracture, as chosen by the clinician.

• Leg pain, tingling or weakness persisting after weeks: MRI is reasonable, usually via a GP or specialist.

• Weight loss, fever, a history of cancer, or night pain that does not ease: urgent assessment and MRI.

• Loss of bladder or bowel control or numbness in the saddle area: emergency; go to A&E or call 999.

An MRI showing a "slipped disc" does not prove the disc is the cause of pain; the term is misleading, since discs do not slip out of place. Nor does a normal scan mean the pain is imaginary.

It is also a misconception that more scans produce better outcomes. Evidence shows early imaging in simple back pain does not improve recovery.