An MRI (magnetic resonance imaging) is a medical scan that uses powerful magnets and radio waves to create detailed pictures of the inside of your body, without any radiation. It’s especially good at imaging soft tissues like the brain, spinal cord, muscles, ligaments, and organs, making it one of the most versatile diagnostic tools in medicine. Most scans take between 15 and 90 minutes depending on the body part being examined.
How an MRI Creates Images
Your body is mostly water, and water contains hydrogen atoms. Each hydrogen atom has a single proton at its center that behaves like a tiny bar magnet, spinning on its own axis. Under normal conditions, these protons point in random directions throughout your body.
When you lie inside an MRI scanner, its powerful magnet causes all those protons to line up in the same direction. The scanner then sends a pulse of radio waves into your body, which knocks those aligned protons out of position. When the radio wave switches off, the protons snap back into alignment and release a faint radio signal as they do. Sensors in the scanner detect these signals and use them to build a picture. Different types of tissue (fat, muscle, fluid, bone marrow) release signals at slightly different rates, which is what gives MRI images their remarkable detail and contrast.
What MRI Is Used For
MRI produces sharper images of soft tissue than CT scans, with better contrast and fewer distortions near bone. That makes it the preferred choice for examining the brain and spinal cord, detecting tumors (including very small ones that might be missed otherwise), evaluating joint injuries, and diagnosing conditions like epilepsy, herniated discs, and torn ligaments. It’s also widely used for heart imaging, breast screening, and abdominal conditions.
Because MRI doesn’t use ionizing radiation the way X-rays and CT scans do, it can be repeated more safely over time. This is particularly useful for monitoring conditions that require regular follow-up imaging.
What the Scan Feels Like
You’ll lie on a padded table that slides into a large tube (the scanner bore). The machine is loud. At 3 Tesla, the most common high-field strength, sound levels regularly exceed 95 decibels and can peak above 105 dB. That’s comparable to standing near a running lawnmower or a loud concert. You’ll be given earplugs or headphones, which reduce noise by 10 to 30 dB and bring levels within safe limits.
During the scan you need to stay as still as possible. You’ll hear a series of loud knocking, buzzing, and thumping sounds as different imaging sequences run. Some facilities pipe music through headphones to help pass the time. A technologist watches you through a window and communicates through an intercom, and you’ll typically have a squeeze ball or button to signal if you need to stop.
How Long Common Scans Take
- Brain or head: 30 to 60 minutes
- Knee: 30 to 60 minutes
- Lumbar spine: 30 to 60 minutes
- Cervical spine: 20 to 45 minutes
- Shoulder: 15 to 45 minutes
- Hip: 30 to 60 minutes
- Heart: approximately 90 minutes
- Abdomen: 30 to 90 minutes, sometimes up to two hours
If contrast dye is needed, add roughly 30 minutes to those estimates.
Contrast Dye
Some MRI scans use a contrast agent injected through an IV to make certain tissues or blood vessels show up more clearly. The most common type is gadolinium-based. It works by changing how nearby hydrogen protons behave, which brightens specific areas on the image. This helps doctors distinguish between healthy tissue and areas of inflammation, infection, or cancer.
About 98% of the contrast agent is filtered out through your kidneys without being chemically altered. Severe allergic reactions are rare, occurring in roughly 0.01% of cases. However, people with significant kidney problems face a higher risk of complications because their bodies clear the agent more slowly, allowing it to linger and potentially deposit in tissues. Your imaging team will typically check kidney function before administering contrast if there’s any concern.
Who Cannot Have an MRI
The MRI magnet is extraordinarily strong, and certain metal objects inside the body can shift, heat up, or malfunction during a scan. Some items are strictly incompatible:
- Pacemakers and implantable defibrillators (unless specifically labeled MRI-safe)
- Cochlear implants
- Metal fragments from shrapnel, bullets, or welding accidents, especially near the eyes
- Certain brain aneurysm clips
- Implanted drug infusion pumps
- Neurostimulation devices
Other items require case-by-case evaluation: joint replacements, IUDs, surgical clips, wire sutures, and some dental implants. If you have any implanted device, the MRI team will research its specific make and model before clearing you. Tattoos less than six weeks old in the area being scanned may need to be rescheduled, and medication patches must be removed before the scan because they can contain metallic components that heat up.
How to Prepare
For most MRI scans, preparation is minimal. You’ll change into a hospital gown and remove all jewelry, watches, eyeglasses, and anything else metallic. Bobby pins, belt buckles, and even some clothing with hidden metal threads or clasps need to come off.
Fasting requirements depend on the type of scan. Most MRIs don’t require fasting at all. Exceptions include some abdominal, rectal, and prostate scans, which may require you to stop eating after midnight the night before. Cardiac stress MRIs require avoiding caffeine for 24 hours beforehand, including coffee, tea, chocolate, energy drinks, soda, and nicotine.
Dealing With Claustrophobia
The standard MRI scanner is a narrow tube about 60 centimeters (roughly two feet) in diameter, and many people feel uneasy inside it. If you know you’re claustrophobic, let your doctor’s office know when scheduling. Mild anxiety can often be managed with a light sedative taken before the appointment. For children, some facilities use preparation techniques like MRI simulation sessions, child life specialists, or even animal-assisted therapy to reduce fear without sedation.
Open MRI scanners exist as an alternative. Instead of a full tube, they have open sides, which makes them far more comfortable for people who are claustrophobic or physically larger. The trade-off is image quality: open scanners use weaker magnets, which means lower resolution. In some cases, a scan done on an open machine may need to be repeated on a standard closed scanner if the images aren’t detailed enough for diagnosis.
Closed vs. Open vs. High-Field Scanners
Most hospital MRI machines operate at 1.5 Tesla or 3 Tesla. Tesla is the unit measuring magnetic field strength. For context, the Earth’s natural magnetic field is about 0.00005 Tesla, so even a 1.5T scanner is roughly 30,000 times stronger. A 3T machine produces sharper images and can capture thinner “slices” of tissue, which helps when examining small structures. Some research and specialized clinical centers now use 7T scanners, though these are less common and significantly louder.
Open MRI machines typically operate at lower field strengths (often below 1.5T), which is why their images tend to be less detailed. Closed, high-field scanners remain the standard for most diagnostic purposes because they scan faster and produce the clearest images.