What are the risks and benefits of MRI?

Your doctor should only request an MRI when there is likely to be a clear benefit, and the benefit outweighs the risk or possible disadvantages. As long as sensible precautions are followed, MRI is considered a low-risk procedure. Apart from any possible harm from the procedure, other possible risks include the discovery of incidental findings that won’t help your diagnosis or treatment, but may cause you unnecessary concern and lead to further tests or medical consultations.

It’s important to discuss any concerns you have with your doctor.

Benefits of an MRI scan

MRI scans are an important tool that doctors use to investigate the cause of your symptoms. They can help confirm the presence or absence of a disease or injury. However, the diagnosis of a condition usually requires more than a single examination or test. An MRI scan should always be used to supplement — not replace — your doctor’s history-taking and examination.

MRI scans:

  • provide very detailed diagnostic pictures of most of the important organs and tissues in your body
  • are sometimes able to show unique information that other tests are unable to show
  • are generally painless
  • do not use radiation and are therefore suitable for use in children and pregnant women.

Safety studies have found no long-term negative effects from MRI scans.

Disadvantages of an MRI scan

Risks from metal objects

MRI scans are considered to be a safe procedure providing you do not have any implants or objects on you that must not go in the scanner.

The powerful magnetic fields generated by the MRI scanner will attract metal objects, often with great force. For this reason, you’ll be instructed to remove all metallic belongings, such as watches, keys and jewellery.

The magnetic field of the MRI scanner can also pull on any metal-containing object in your body, such as medicine pumps and aneurysm clips. In other cases, (older-style) medical implants may heat up during the scan as a result of the technology (radiofrequency energy) that is used for the procedure.

MRI scans can cause heart pacemakers, defibrillation devices and cochlear implants to malfunction.

Every MRI facility will have a comprehensive screening procedure that, when carefully followed, will ensure that MRI is only used on people for whom it is safe. Many newer medical implants are now manufactured to be MRI-compatible, so once the doctors know the exact nature of your implant they’ll be able to tell you if it’s safe for you to have an MRI. If you do have an implant that could make an MRI unsafe, the radiologist may recommend you have a different type of scan.

Pregnancy risks

MRI is safer for the unborn child (foetus) than imaging with X-rays or CT scans. However, MRI scans can cause slight warming of the body, so as a precaution most clinics avoid MRI scanning during the first 3 months of pregnancy, unless the scan is considered essential. Beyond that, MRI scans are usually considered safe in pregnancy and are occasionally used to check on the baby’s development, although non-urgent scans are generally delayed until after the baby is delivered. In some situations other scans, such as an ultrasound, may be used instead of MRI.

Gadolinium-based contrast dyes are usually avoided in pregnancy.

Risks associated with contrast media

Unlike contrast agents used in X-rays, the contrast dye used in MRI scans (gadolinium chelate) does not contain iodine and rarely causes allergic reactions (such as rashes, hives, nausea, flushing, and dizziness). Severe reactions, such as difficulty breathing and swelling of the lips and mouth, are even rarer, occurring in only about 1 in 10,000 people given gadolinium.   

Nevertheless, it’s essential you tell the doctor and radiology practice about any previous allergic reactions you’ve had, especially if you’ve had a previous reaction to contrast media. You should also tell the radiology practice before the procedure about all medicines you are taking.  

In very rare cases — in people with poor kidney function — gadolinium chelate injections can cause a serious condition called nephrogenic systemic fibrosis, which involves the build-up of fibrous tissue in the skin, joints, muscles and internal organs. If there is any chance you may have kidney problems, your doctor may organise a blood test before the scan to assess whether the gadolinium contrast dye is safe to use.  

Risks associated with general anaesthesia or sedation

Most young children (under 6 years of age) and babies will need to be given a light general anaesthetic before the MRI. This is because young children find it difficult to stay still long enough for the radiographer to get good-quality pictures from the scan. If your child needs an anaesthetic before their MRI, a doctor at the radiology practice will discuss the procedure and the risks and benefits of anaesthesia prior to the scan. Read about ways to help your child during imaging.

Occasionally adults — for example a person who is highly agitated or in pain so severe that it makes it hard for them to lie quietly — may require general anaesthesia for an MRI scan.

You can request a sedative, especially if you suffer from claustrophobia or are very anxious. A sedative will help to calm you and make you drowsy, but not put you completely to sleep like a general anaesthetic does.

While sedatives are generally safe, if you are given a sedative during the scan you may feel drowsy for the rest of the day and will not be able to drive or operate machinery afterwards. You will need to arrange for a relative or friend to take you home after the procedure and stay with you for a period of time as recommended by the radiology practice.

Risks associated with incidental findings

Sometimes the sensitivity of MRI scans can create problems by picking up incidental findings — that is, apparent 'abnormalities' on the scan that have no actual relationship to the illness or injury being investigated. These incidental findings can cause anxiety and sometimes lead people to pursue unnecessary treatment (e.g. spinal surgery), which may have their own risks and costs.

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