Do I need an MRI or X-ray for back pain?

What guidelines say about routine imaging

The NICE guideline NG59 on low back pain and sciatica advises against routine imaging in a non-specialist setting for people with low back pain, with or without sciatica (pain traveling down the leg from an irritated nerve). It suggests considering imaging in specialist settings only if the result is likely to change management, meaning the treatment plan. The guideline also says people referred for a specialist opinion may not need imaging.

NIAMS, a US National Institutes of Health institute, makes a similar point: most people do not need additional testing, although doctors sometimes order tests to rule out or confirm a specific cause. MedlinePlus says a provider usually will not order tests of the spine at the first visit, or for 4 to 6 weeks, unless certain symptoms are present. Many episodes settle in that time, as described on How long does back pain usually last?

What each test can and cannot show

Each imaging test answers a different question, according to NIAMS. An X-ray shows only bones and can reveal fractures, changes due to aging and changes in the alignment of the spine. An MRI (magnetic resonance imaging, which uses a strong magnet) shows soft tissues such as discs, ligaments and nerve roots. A CT scan (computerized tomography, which uses a scanner to take images from several angles) shows the spinal canal and the tissues around it.

MedlinePlus explains why more detail is not always better. An MRI can pick up most injuries and the changes that come with aging, including small changes that are not the cause of the current pain. These findings rarely change how a provider first treats the problem, and they can lead to extra tests, more worry, and treatment that is not needed, especially for changes that occur naturally with age. See Are disc changes on an MRI normal? for more.

MedlinePlus also lists some risks of MRI. In rare cases the contrast dye can cause severe allergic reactions or kidney damage, and the strong magnet can affect pacemakers and other implants or move a piece of metal in the body. People are asked to tell the technologist about any implants or metal.

When a scan is more likely

MedlinePlus says a person will likely need an MRI right away if there are warning signs of a more serious cause. Its list includes being unable to pass or control urine or stool, difficulty walking and loss of balance, fever, a history of cancer or other signs of cancer, a recent serious fall or injury, back pain so severe that even prescribed pain pills do not help, and a numb or weak leg that is getting worse. It says that without such signs an MRI is not expected to lead to better treatment, better pain relief or a quicker return to activities. If pain does not improve or gets worse, the provider may order one later.

Several of these signs overlap with the emergency symptoms described on When is back pain an emergency? For the common causes that scans often cannot pin down, see What causes most low back pain? and the wider Back pain basics collection.

When to get help

Whether a scan is appropriate is a decision for a clinician who has examined you, and practice varies by country and by clinician. The NHS advises seeing a GP if back pain does not improve after a few weeks of home care, stops you doing daily activities, comes with unexplained weight loss, or is worse at night. It advises an urgent GP appointment or a call to 111 for back pain with a fever, or for severe pain that starts suddenly or worsens quickly. Pain, tingling, weakness or numbness in both legs, loss of feeling around the genitals or anus, bladder or bowel changes, or back pain after a serious accident are reasons for the NHS to advise calling 999 or going to A&E. This page is general education and is not personal medical advice.