By Mr Ahmed-Ramadan Sadek, Neurosurgeon · Last reviewed October 2026 · 6 min read

What is an MRI scan?

MRI stands for magnetic resonance imaging. You lie inside a large, tube-shaped magnet while pulses of radio waves are sent into your body. Tiny particles in your tissues respond by giving off faint signals of their own, and a computer turns these into very clear images of the bones, discs, nerves and spinal cord. MRI has been in medical use since the early 1980s.

Because MRI does not use X-rays or any other ionising radiation, it can be repeated when needed without building up a radiation dose. the British Association of Spine Surgeons (BASS) notes that there are no known harmful side effects from the scan itself.

When is a scan useful?

MRI is very good at showing structure. Spinal specialists find it most valuable in these situations:

• Suspected serious disease. MRI is highly sensitive for infection and tumours, so if these are present, it will almost always pick them up.
• Nerve or spinal cord compression. If you have sciatica, weakness, altered reflexes or numbness that suggests a trapped nerve, the scan shows where and why. Pressure on a nerve usually produces signs that a clinician can find on examination.
• Spinal stenosis. In older adults with leg symptoms on walking, MRI confirms narrowing of the spinal canal and helps plan treatment.
• Planning a procedure. If your symptoms are bad enough that you are considering surgery or an injection, the scan acts as a map that guides exactly where treatment is directed.

Doctors also use a list of “red flags” to decide who needs a scan urgently, such as a past history of cancer, unexplained weight loss or new weakness in a limb.

When a scan may not help

It is very common to ask for a scan, and very common for one not to be necessary. For straightforward back pain, a careful conversation and examination tell your doctor far more than pictures do.

BASS estimates that in roughly 85% of people with back pain, no specific source can be identified. This is known as non-specific back pain. It may come from muscles, ligaments, joints or discs, but a scan cannot pinpoint which, and most national and international guidelines do not recommend MRI in this situation. If your symptoms are mild or already improving, a scan is unlikely to change your treatment.

Scans also pick up many findings that are entirely normal for your age. BASS cites research in which 87% of scans of people with no back pain showed some abnormality. Reported “disc degeneration” is present, according to the same source, in about 20% of people in their twenties, rising to 40% by age 40 and 80% by age 60. Blaming symptoms on these normal changes can lead to the wrong treatment, which is why spinal surgeons like to treat the person, not the scan.

Surgery for back pain alone, such as spinal fusion, is reserved for people who have already tried other treatments. BASS notes that some studies suggest fewer than 3% of people with back pain are suitable for an operation, and that intensive exercise and education programmes can work as well as fusion with less risk. A scan for back pain is therefore usually reserved for those in whom surgery is seriously being considered.

For the same reasons, scans for simple back pain have a lower priority on waiting lists than scans for suspected cancer, infection or nerve compression.

Before your scan

The strong magnet means safety checks are essential. You will complete a questionnaire about metal in or on your body. Tell the team if you have:

• A pacemaker or implanted defibrillator, or any other electronic implant. Some devices make MRI unsafe.
• Metal fragments (for example from welding or an injury), surgical clips, implants or a cochlear implant.
• A possible pregnancy. BASS advises that scans are avoided in the first 12 weeks because of a small theoretical risk to the baby.
• A fear of enclosed spaces (claustrophobia).

On the day

1. You will be asked to remove jewellery, watches, hairpins, bank cards and anything else metallic, and may change into a gown.
2. You lie on your back on a padded table that slides into the scanner. For spinal scans, usually only part of your body is inside, but the tunnel can still feel snug.
3. The machine makes loud knocking and buzzing sounds while it takes pictures. You will be given earplugs or headphones.
4. You can talk to the radiographer through an intercom and will hold a call button in case you need to stop.
5. Keeping as still as you can gives the sharpest images. The scan is made up of several short sequences, and the radiographer will tell you roughly how long yours will take, as this depends on how many areas are being imaged.

The scan is completely painless; you will not feel the magnet or the radio waves.

Risks and side effects

MRI is one of the safest imaging tests available. According to BASS, the scan itself has no recognised harmful effects, and no radiation is involved. The main issues are practical:

• Metal and devices: pacemakers and certain metal implants can cause serious problems, which is why screening is so thorough.
• Claustrophobia: some people find lying in the tunnel distressing.
• Noise: the banging can be unpleasant, though ear protection helps.
• Early pregnancy: scans are generally postponed in the first trimester as a precaution.

Understanding your results

MRI reports can contain dramatic-sounding words. A disc that has lost some water looks darker and may be called “degenerate”, yet this is part of normal ageing, rather like greying hair. A disc “bulge” is common and not the same as a slipped (prolapsed) disc, and even a prolapse can be painless.

Your specialist should look at the images with you and explain what is relevant to your symptoms and what is not. Remember too that scans have a shelf life: if surgery is planned, for example for sciatica, a recent scan is usually needed.

• BASS patient information: The spine and MRI scanning
• NHS: MRI scan
• Spinemathema guide: Acute back pain
• Spinemathema guide: Degenerative disc disease

This guide is general information and is not a substitute for advice about your own situation. Please speak to your GP or specialist.