MRI stands for magnetic resonance imaging. It’s a medical imaging technology that uses powerful magnets and radio waves to create detailed pictures of the inside of your body, without any radiation. Unlike X-rays or CT scans, MRI is particularly good at showing soft tissues like muscles, ligaments, the brain, and spinal cord, making it one of the most versatile diagnostic tools in medicine.
How MRI Works
Your body is mostly water, and water molecules contain hydrogen atoms. Each hydrogen atom has a proton at its center that behaves like a tiny magnet. When you lie inside an MRI machine, its powerful magnetic field forces these protons to line up in the same direction.
The machine then sends pulses of radio waves through your body. These pulses knock the protons out of alignment. When the radio waves stop, the protons snap back into position and release small amounts of energy as they do. Sensors inside the machine detect that released energy. Because different types of tissue (fat, muscle, bone, fluid) contain different amounts of water, their protons release energy at slightly different rates. The MRI computer translates these differences into a highly detailed cross-sectional image of whatever body part is being scanned.
This is why MRI excels at distinguishing between types of soft tissue in ways that CT scans can’t. A CT scan uses X-rays and is better for quickly imaging bone fractures or bleeding, but MRI picks up subtle differences between tissues that would otherwise look similar.
What MRI Is Used For
MRI is the go-to imaging method for a wide range of conditions. It’s especially useful for musculoskeletal injuries: torn ligaments, meniscal tears in the knee, rotator cuff damage, Achilles tendon ruptures, cartilage loss, and nerve compression. If you’ve had a sports injury or joint pain that isn’t explained by an X-ray, an MRI is often the next step.
Beyond joints and muscles, MRI is critical for examining the brain and spinal cord. It can detect early signs of stroke, identify multiple sclerosis plaques, evaluate spinal injuries, and map tumors before surgery. Specialized MRI techniques also allow imaging of the breast, prostate, heart, and blood vessels. In cancer care, MRI helps with diagnosis, staging, and tracking how well treatment is working.
What the Scan Feels Like
During an MRI, you lie on a flat table that slides into a large tube-shaped magnet. The machine is loud, producing rhythmic thumping and buzzing sounds as the magnets cycle on and off. You’ll typically be given earplugs or headphones. The most important thing you need to do is stay still, since even small movements can blur the images.
Brain and spine scans take about 45 minutes on average. Joint scans for areas like the knee, ankle, or wrist run between 25 and 45 minutes. If your scan requires a contrast agent (more on that below), add about 15 minutes. You won’t feel the magnetic field or the radio waves. The experience is painless, though lying still in a confined space for that long can be uncomfortable.
Open MRI vs. Closed MRI
Traditional “closed” MRI machines are tube-shaped tunnels with magnetic field strengths between 1.0 and 3.0 Tesla. They produce the sharpest images and can perform advanced scans like breast MRI, prostate MRI, and early stroke detection. The tradeoff is that the enclosed space triggers claustrophobia for some people.
“Open” MRI machines replace the tunnel with two flat magnetic discs above and below you, leaving the sides completely open. This design is much more comfortable if enclosed spaces bother you or if your body size makes a standard machine a tight fit. The downside is significantly lower image quality, since open machines operate at only 0.2 to 0.3 Tesla. In some cases, a scan done on an open machine may need to be repeated on a closed machine to get a clear enough picture for an accurate diagnosis. Open machines also tend to take longer to complete a scan.
Contrast Dye
Some MRI scans require an injection of a contrast agent, a liquid based on a metal called gadolinium, to make certain structures show up more clearly. This is common when imaging tumors, blood vessels, or areas of inflammation.
Allergic-type reactions to gadolinium contrast are uncommon and far less frequent than reactions to the contrast agents used in CT scans. The most significant risk historically was a rare but serious skin-thickening condition that occurred in people with kidney failure. Newer formulations of gadolinium have largely eliminated that risk. Trace amounts of gadolinium can remain in the brain and other organs after injection, a finding that’s been recognized for over a decade. To date, no studies have linked these trace deposits to any adverse health effects.
Safety and Metal
Because MRI uses an extremely strong magnet, metal objects near the machine can become dangerous projectiles or can heat up inside the body. Before your scan, you’ll be screened for anything metallic. Certain implants, like some older pacemakers, metal heart valves, or metal fragments from previous injuries, may disqualify you from having an MRI. Many modern implants are designed to be MRI-compatible, but each device needs to be verified individually.
You’ll also be asked to remove jewelry, watches, belt buckles, hairpins, and any clothing with metal snaps or zippers. Credit cards and electronic devices can be damaged by the magnetic field. Anyone who enters the MRI room, not just the patient, goes through the same screening process.
Getting Your Results
After your scan, a radiologist (a doctor who specializes in reading medical images) reviews the pictures and writes a report. Some facilities have radiologists on-site, while others send images electronically to radiologists who may be in a different location entirely. The radiologist’s report then goes to the doctor who ordered your MRI, who reviews both the images and the interpretation before discussing the findings with you.
For routine scans, expect to wait one to two weeks for results. Emergency MRIs are prioritized and results come faster. If the radiologist spots anything life-threatening, they notify the treating doctor immediately. Cancer-related MRIs can sometimes take longer because multiple specialists may need to weigh in on the findings. Your results will typically be shared either by phone or at a follow-up appointment.