MRI scan: what to expect, safety checks, metal implants and noise

1. Introduction

Magnetic resonance imaging, or MRI, is a scan that makes detailed pictures of the inside of your body. The NHS explains that doctors use it to find the cause of symptoms, to look at bones and organs and to check how well a treatment is working ^[1]. Unlike X-rays and CT scans, MRI does not use radiation ^{[2, 3]}. This article explains how an MRI works, what to tell staff, what happens during the scan and how to cope with the noise and the enclosed space. Your doctor decides if an MRI is right for you.

2. How an MRI works

An MRI machine uses a very strong magnet, radio waves and a computer. The magnet lines up tiny particles called protons in the water in your body. Radio pulses knock them out of line, and as they settle back, sensors detect the energy they release. Different tissues settle at different speeds, which lets doctors tell them apart ^[3]. MRI shows organs and soft tissues clearly ^{[2, 3]}.

3. What MRI is used for

MedlinePlus and NIBIB list these common uses ^{[2, 3]}:

• The brain, spinal cord and nerves.
• Muscles, ligaments and tendons.
• Joints, such as the knee or shoulder.
• Brain scans, which can also tell grey matter from white matter and show tumours ^[3].

The NHS adds that MRI can monitor treatment, for example following the size of a tumour during and after cancer treatment ^[1]. Because it uses no radiation, MRI may be chosen when scans need to be repeated ^{[2, 3]}. A drawback is that it usually costs more than X-ray or CT ^[3].

4. Safety checks before your scan

The MRI magnet is very strong and can pull hard on objects that contain iron ^[3]. That is why you will fill in a safety form and why staff ask about metal in your body. The NHS asks you to tell staff about ^[1]:

• Pregnancy or possible pregnancy, and breastfeeding.
• Previous surgery.
• Kidney problems.
• Metal in the body, such as surgical clips, a pacemaker, or dental fillings or bridges.
• Allergies, including to contrast dye.

NIBIB states that people with some implants, including pacemakers, defibrillators, cochlear implants, deep brain stimulators and insulin pumps, should not enter the scanner ^[3]. MedlinePlus also lists metal fragments, an artificial joint and some darker tattoo or permanent makeup inks that may contain metal ^[2]. Never assume an implant is safe. Always tell the staff about anything in your body.

In pregnancy, NIBIB says no effects on the baby have been shown, but scans are generally avoided as a precaution, especially in the first trimester, the early part of pregnancy ^[3].

5. Getting ready

The hospital will tell you what to do. You will need to remove metal items such as jewellery, watches, hair clips, hearing aids and bras, and take coins and keys out of your pockets. You may change into a gown if your clothing has metal parts such as zips. Avoid wearing make-up ^{[1, 2]}.

6. What happens during the scan

A radiographer does the scan. It usually lasts between 15 minutes and an hour, and sometimes longer ^[1]. You lie on a bed that slides into a tunnel-shaped machine ^{[1, 2]}. Some centres use an open MRI, which is less enclosed ^[2].

• The machine is loud, so you will be given earplugs or headphones ^{[1, 2]}.
• You must keep very still, and you may be asked to hold your breath briefly ^[1].
• The radiographer works the scanner from another room, but can hear and speak to you ^[1].
• Some people feel a twitching sensation, which can be caused by rapidly changing magnetic fields ^[3].

Contrast dye

You may be given contrast, usually through a vein, so that some body parts show up more clearly ^{[1, 2]}. Common mild effects include a small bruise at the injection site, nausea, a rash, headache, a metallic taste or dizziness. Rarely, there is an allergic reaction, with coughing, wheezing, fainting, breathing or swallowing difficulty, or a rash or hives. Most reactions happen soon after the injection, so tell the radiographer if you feel unwell, and expect to wait up to 30 minutes afterwards ^[1]. People with severe kidney failure need special care with one kind of contrast, because of a rare but serious condition, so tell staff about any kidney problem ^[3].

If you feel anxious or afraid of small spaces

Speak to a healthcare professional before your appointment. You may be offered medicine to help you relax, and you can ask to bring someone for support ^[1]. Music and relaxation methods can also help, and an open scanner may suit some people ^[3]. If you were given a relaxing medicine, you will need someone to take you home and should not drive for 24 hours ^[1].

7. Results

A radiologist reviews the pictures and sends a report to the doctor who referred you ^{[1, 2]}. The NHS says results usually take 1 to 2 weeks, and you may need a follow-up appointment. If you have heard nothing after a few weeks, contact your referring doctor, and ask if anything is unclear ^[1].

8. When to see a doctor

Talk to a doctor about persistent symptoms such as ongoing headaches, joint pain after an injury, numbness or weakness, or anything else that your doctor may want to look into with a scan. Tell your doctor about all implants and devices before any MRI booking. If you feel unwell after a scan, especially with a rash, wheezing or trouble breathing after contrast dye, seek help ^[1].

In an emergency, call your local emergency number (112 in India, 911 in the United States).

9. Conclusion

MRI is a detailed, radiation-free scan that is especially useful for soft tissues such as the brain, spine and joints. Its main safety issues involve metal, implants and contrast dye, so complete the safety form honestly and tell the staff about everything in or on your body. If noise or small spaces worry you, say so early, because there are ways to help. Your doctor decides what the results mean for you.