In most cases, there isn't a clear cause. But some common causes include injury, overuse or muscle strain, disc problems, or arthritis. No matter what caused it, low back pain often goes away after 4 to 6 weeks of non-surgical treatment. An MRI usually isn't helpful in these cases.

Pressure on a spinal nerve can cause sciatica symptoms, which usually include a shooting pain down the back or side of the leg. Your leg may also feel weak, tingly, or numb. This pressure on the nerve can be caused by:

• A bulging or ruptured disc in your spine. This is also called a herniated disc or protrusion. Your symptoms may get better on their own in a few weeks or months.
• Arthritis.
• Spinal stenosis.

Other serious causes of back pain, such as infection, a tumour, or cancer, are rare.

Very few cases of low back pain are caused by serious disease or nerve-related problems. Imaging, including MRI, is helpful for diagnosing and planning treatment only for these types of conditions.

An MRI is a test that uses a magnetic field and pulses of radio wave energy to make pictures of the spine. MRI stands for "magnetic resonance imaging."

For this test, your body is placed inside a special machine that contains a strong magnet.

In some cases, a contrast material is used during the MRI scan. This means that you have a dye injected into your bloodstream, through an I.V.. The dye makes certain areas show up better on the MRI pictures.

The MRI can find problems such as infection or a tumour. It may also find things that aren't normal but that aren't causing any problems.

MRI issues

An MRI isn't painful, but there are certain details about having an MRI that you should know about:

• Loud noises. The machine makes loud tapping or snapping noises. Earplugs or headphones with music are usually offered to help block the noise.
• Being confined. Some people feel nervous (claustrophobic) about being confined in that small space. If this worries you, you may be given medicine (a sedative) to help you relax. Some MRI machines, called "open MRI," are now made so that the machine doesn't enclose your entire body. Open MRI machines may be helpful if you are claustrophobic, but they aren't available everywhere. The images from an open MRI may not be as good as those from a standard MRI machine.
• Lying very still. You must lie very still for quite a while so that the machine can get clear pictures. For people with back pain, this can be hard. Talk to your doctor if you're worried about having to lie on your back.
• Time. An MRI usually takes 30 to 60 minutes but can take as long as 2 hours.
• False-positive results. A false-positive test result is one that appears to find a problem when in fact there isn't one. This can lead to more tests that really aren't needed. And it may cause needless worry that something is wrong.

An MRI is not a standard test for finding the cause of low back pain. A complete exam that includes questions about your medical history is enough to diagnose and treat most low back pain.

Your doctor may suggest getting an MRI if they think your pain is caused by something other than a common cause like an injury. An MRI may be helpful if you might need to decide about surgery.

An MRI usually isn't done when simple muscle strain is suspected, because:

• An MRI will not show muscle strains or other problems with soft tissues.
• The pain usually will improve on its own within several months.
• An MRI won't change your treatment plan.

An MRI can be loud, but the test itself isn't painful.

There are no known harmful effects from the magnet used in MRI machines, but it is very strong:

• It can affect pacemakers, artificial limbs, and other medical devices that contain iron.
• Any loose metal object in the body—for example, a surgical clip or a bullet fragment—could cause damage or injury if it gets pulled toward the strong magnet.
• Metal parts in the eyes can damage the retina. If you may have metal fragments in your eye, you may need an X-ray of your eyes before you can have an MRI. If metal is found, then you won't be able to have the MRI.
• Iron pigments in tattoos or tattooed eyeliner can cause skin or eye irritation.

There is a slight risk of an allergic reaction if contrast material is used during the MRI. But most reactions are mild and can be treated with medicine. There also is a slight risk of an infection at the I.V. site.

Contrast material that contains gadolinium may be used in this test. Be sure to tell your doctor if:

• You are pregnant or think you may be pregnant.
• You have kidney problems.
• You've had more than one test that used gadolinium.

Health Canada has safety warnings about gadolinium. But for most people, the benefit of its use in this test outweighs the risk.

An MRI can find tumours, damage to bones and nerves, and narrowing of the spinal canal that pushes on the spinal cord. The test can also show if a herniated disc is putting pressure on nerves. Your doctor might recommend an MRI if:

• Your history and physical exam show signs of a serious problem, such as a fracture, tumours, infection, or nerve damage.
• You are older than 70. Your doctor may also recommend testing if you are older than 50 and also have osteoporosis or a history of compression fracture.
• You have had osteoporosis for a long time or you have diabetes.
• Your body's defence system (immune system) is not able to fight infection.
• You have a history of long-term steroid use or a history of substance use disorder.
• You have a history of a previous spine injury or back surgery.
• You have symptoms related to compression on a certain nerve root or roots.
• Back pain has not improved after at least 6 weeks of home treatment that may include pain relievers, heat or ice, and exercises.