Magnetic Resonance Imaging (MRI) is one of the most commonly used medical imaging tools today, with over four million scans performed annually in the UK. MRI scans provide highly detailed images that allow medical professionals to assess soft tissues, organs, and blood vessels. These scans can detect abnormalities in the torso and brain, and help diagnose a wide range of diseases and conditions, including cancer and neurological disorders, at an early stage.
If your doctor requires a detailed examination of an internal part of your body, they may recommend an MRI scan. For patients who feel anxious about the procedure, it’s reassuring to know that scans are painless, highly accurate, non-invasive, and generally very safe. While, like any medical procedure, there are some risks, they are minimal and rare. Healthcare facilities follow strict safety protocols to ensure patient well-being.
Below, we outline the main MRI risks and side effects, along with guidance on how to minimise any associated concerns.
1. Magnetic Field Risk
Unlike X-rays, Computed Tomography (CT) scans, or Positron Emission Tomography (PET) scans, MRI does not use ionising radiation. Instead, it relies on powerful magnets and radio wave bursts to create detailed images by mapping water molecules inside the body.
Standard MRI machines generate a magnetic field up to 30,000 times stronger than Earth’s. While harmless to the human body, this requires careful precautions to prevent interference.
Before your scan, you must:
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Removing all metal objects (jewellery, piercings, etc.).
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Ensure no loose metal or electronic devices are in the scan room, as they may be pulled toward the machine.
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Inform the facility of any implants, such as pacemakers or cochlear implants, as alternative imaging may be necessary.
MRI centres conduct thorough screenings of your medical history and a pre-scan check to ensure a safe and effective procedure.
2. Allergic Reaction To Contrast Dye
Some MRI scans require the use of a contrast dye to enhance image clarity. This typically involves administering a gadolinium-based contrast agent through an injection or intravenous (IV) drip. While most people tolerate it well, some may experience temporary mild side effects, such as:
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Headache
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Dizziness
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Nausea
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Mild rash
Additionally, you may notice a metallic taste in your mouth shortly after the injection or experience mild bruising at the injection site, similar to any routine injection. Severe allergic reactions are extremely rare. For individuals with severe kidney disease, there is a small risk of tissue or organ damage.
Before administering a contrast dye, the imaging centre will review your medical history and may also perform blood tests to assess kidney function. If you are pregnant or taking certain medications, contrast dye may be avoided as a precaution.
3. Claustrophobia
While MRI scans are safe and painless, some patients may experience claustrophobia. The procedure requires you to lie still inside a narrow 60 cm-wide tube for 30 to 90 minutes. If this is a concern, you can speak with the staff beforehand, as they may offer a sedative to help you relax. For individuals who cannot remain still, such as young children or those with certain medical conditions, anaesthesia may be an option.
To enhance comfort, various accessories are available, including cushions, foam pads, and headphones playing soothing music. Some imaging centres also offer larger, 70cm wide-bore machines or open MRI machines as alternatives.
MRI scanners are fitted with a two-way intercom system, allowing patients to communicate with the radiographer if they experience any discomfort or anxiety during the procedure.
4. Burns From Radio Waves and Magnetic Fields
This is one of the most common MRI risks, accounting for approximately 70% of scan complications. However, it’s also one of the easiest to prevent.
The radiofrequency (RF) energy and strong magnetic fields used in MRI scans can cause second-degree burns (resulting in blistering or swelling) if they interact with conductive metal materials.
These include the metal implants and removable metal objects (see above section), as well as metallic fibres found in some tattoos and synthetic clothing.
In addition to removing all detachable metal items, patients change into a hospital gown to eliminate clothing-related hazards.
Tattoos can be covered with protective cold packs, and MRI scanners are equipped with foam mattresses and pads to prevent direct contact between skin and the machine. If you experience any unusual warmth or discomfort during the scan, use the two-way intercom to alert the radiographer immediately.
5. Pregnancy Risks
There is no evidence that MRI scans are harmful during pregnancy. That said, doctors generally avoid recommending an MRI during the first trimester as a precautionary measure to minimise any potential impact from the heat and noise generated by the scanning equipment on the developing foetus.
If an MRI scan is needed during pregnancy, it is typically performed without contrast dye. This is also a precaution, as studies have found little evidence that gadolinium poses a risk to either the mother or the unborn baby. Research indicates that less than 0.04% of the gadolinium dose reaches breast milk, and only 0.8% of that is consumed by the infant. Breastfeeding mothers with concerns are advised to temporarily stop breastfeeding for 24 hours after the scan.
If you are pregnant and require an MRI, discuss any concerns with your GP or radiologist, who can guide you on the safest course of action.
6. Interference From Implanted Metal Devices
Due to the risks associated with magnetic fields, patients with metal-containing medical implants are generally unable to undergo an MRI scan. Instead, they are typically offered an alternative imaging method, such as a CT scan.
Metal or electronic implants can shift, overheat, or malfunction when the MRI scanner is activated. Additionally, they may interfere with the scanner’s function and degrade image quality.
The following implants and devices are generally considered unsuitable for MRI:
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Pacemakers and other electronic implants
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Cochlear implants
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Aneurysm clips
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Metal plates and screws
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Artificial joints with metal components
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Metal dental implants (although simple fillings are safe)
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Stents, shunts, valves, filters, and vascular coils
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Intrauterine devices (IUDs)
If you have any bullets, shrapnel, or other metal fragments in your body, inform the scanning facility before your appointment.
For a detailed list of safe and unsafe surgical implants for 3T MRI scanners, check here.
7. Anxiety
Although MRI scans are very safe, it’s natural to feel some anxiety before the procedure. If you have any concerns, you can share them with the staff beforehand - they will explain what to expect and discuss available options to help you feel more comfortable. As mentioned earlier, sedatives are available for those with claustrophobia, and some facilities offer larger or more open MRI machines.
Common concerns around MRI scans include:
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Scanner noise - MRI machines produce loud sounds due to the interaction of strong magnets with radio wave pulses. While this noise isn’t harmful, you will be provided with earplugs or music-playing headphones to reduce discomfort.
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Scan duration - MRI scans typically last between 30 and 90 minutes, depending on the type of scan. You will need to remain still for most of the procedure and may be asked to hold your breath for short intervals of 15-20 seconds. Cushions and pads are available for comfort, and you can communicate with the radiographer through a two-way intercom if needed.
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Side effect concerns - unlike X-rays or CT scans, MRI scans do not involve radiation exposure, despite common misconceptions. The magnetic fields and radio waves used in MRI scans cause no known side effects. Any effects from contrast dyes or sedatives are usually mild and short-lived.
8. Risk Of Nephrogenic Systemic Fibrosis (NSF)
For individuals with severe kidney disease, the gadolinium-based contrast agents used in some MRI scans can lead to a rare condition called Nephrogenic Systemic Fibrosis (NSF). This condition causes the skin, tissues, organs, and joints to become thickened and hardened. Due to this risk, contrast dyes are generally avoided in patients with significant kidney impairment.
Before any MRI scan that may require a contrast agent to boost image clarity, medical professionals will review your medical history and may perform blood tests to assess kidney function. If there is any risk of NSF, contrast dye will not be used.
Get An MRI Scan
If you have a health concern and need a quick and effective assessment of any internal part of your body, you can book a private MRI scan without a GP referral. Simply complete a short assessment form and select a convenient location. One of our expert clinicians will guide you through the process, and after your scan, they will review the results with you and recommend any necessary next steps.
