Magnetic resonance imaging (MRI) is very safe and most people are able to have the procedure.

But in some instances an MRI scan may not be recommended.

When your doctor refers you for an MRI scan, tell them if:

• you think you have any metal in or on your body
• you're pregnant or breastfeeding
• you've ever had an allergic reaction to a contrast agent
• you have kidney problems

The strong magnets used during the scan can affect any metal implants or fragments in your body.

There's no evidence that MRI scans are a risk for pregnant women, but whether you'll be offered one depends on why you need it.

A doctor can discuss with you whether an MRI scan or contrast agent (dye) is suitable for you. If you do have an MRI scan, the radiographers will make sure it's as safe as possible for you and your baby.

Having something metallic in your body doesn't necessarily mean you can't have an MRI scan, but it's important for medical staff carrying out the scan to be aware of it.

They can decide on a case-by-case basis if there are any risks, or if further measures need to be taken to ensure the scan is as safe as possible.

For example, it may be possible to make a pacemaker or defibrillator MRI-safe, or to monitor your heart rhythm during the procedure.

You may need to have an X-ray if you're unsure about any metal fragments in your body.

Examples of metal implants or fragments include:

• a pacemaker – a small electrical device used to control an irregular heartbeat
• an implantable cardioverter-defibrillator (ICD) – a similar device to a pacemaker that uses electrical shocks to regulate heartbeats
• metal plates, wires, screws or rods – used during surgery for bone fractures
• a nerve stimulator – an electrical implant used to treat long-term nerve pain
• a cochlear implant – a device similar to a hearing aid that's surgically implanted inside the ear
• a drug pump implant – used to treat long-term pain by delivering painkilling medication directly to an area of the body, such as the lower back
• brain aneurysm clips – small metal clips used to seal blood vessels in the brain that would otherwise be at risk of rupturing (bursting)
• some types of shunts - devices used to treat too much fluid in your brain or spine
• stents, filters and vascular coils – small metal devices that are surgically implanted in blood vessels
• metallic fragments in or near your eyes or blood vessels (common in people who do welding or metalwork for a living)
• prosthetic (artificial) metal heart valves
• penile implants – used to treat erectile dysfunction (impotence)
• eye implants – such as small metal clips used to hold the retina in place
• an intrauterine device (IUD) or coil – a contraceptive device made of plastic and copper that fits inside the womb
• artificial joints – such as those used for a hip replacement or knee replacement
• dental fillings and bridges
• tubal ligation clips – used in female sterilisation
• surgical clips or staples – used to close wounds after an operation
• tattoos or permanent make-up
• objects such as bullets or shrapnel
• breast expanders – used after a mastectomy
• insulin pumps and glucose monitors – electronic devices used by some people with diabetes

Some tattoo ink contains traces of metal, but most tattoos are safe in an MRI scanner.

Tell the radiographer immediately if you feel any discomfort or heat in your tattoo during the scan.