An MRI (magnetic resonance imaging) scan is a medical imaging test that uses powerful magnets and radio waves to create detailed pictures of the inside of your body. Unlike X-rays or CT scans, it involves no ionizing radiation. MRI is especially good at showing soft tissues like the brain, spinal cord, muscles, ligaments, and joints, producing images that are significantly clearer than what X-rays or CT scans can achieve for those structures.
How MRI Works
Your body is mostly water, and water contains hydrogen atoms. Each hydrogen atom behaves like a tiny bar magnet with its own north and south pole. Normally, these miniature magnets spin with their axes pointing in random directions. When you lie inside an MRI scanner, its powerful magnetic field forces all those hydrogen atoms to line up in the same direction, creating a uniform magnetic signal throughout your body.
The scanner then sends pulses of radio waves into your body. These pulses knock the aligned hydrogen atoms out of position. When the radio waves stop, the atoms snap back into alignment and release a faint radio signal as they do. Different types of tissue (fat, muscle, bone marrow, fluid) release that signal at slightly different speeds, and the scanner’s sensors pick up those differences. A computer translates the signals into cross-sectional images, essentially building a map of your internal structures slice by slice. The scanner can also adjust its magnetic field in small increments from head to toe, allowing it to target specific slices of the body at different depths.
What MRI Is Used For
MRI excels at imaging soft tissue, which makes it the go-to test for a wide range of conditions. For the brain and nervous system, it helps diagnose brain tumors, aneurysms, stroke, multiple sclerosis, spinal cord compression, pinched nerves, and injuries from trauma. Because there’s no radiation exposure, MRI is the preferred choice when someone needs repeated imaging over time to monitor a condition.
In orthopedics, MRI is used to evaluate cartilage tears, ligament and tendon injuries, herniated spinal discs, bone infections, bone tumors, and joint deterioration from arthritis. Doctors also use it to check the results of corrective surgeries. Beyond bones and joints, MRI can image the heart, blood vessels, abdomen, and pelvic organs, making it one of the most versatile diagnostic tools available.
MRI vs. CT Scans
The biggest difference is radiation. CT scans use X-rays, a form of ionizing radiation that can damage DNA in cells. While the risk from a single CT scan is very low, there is concern that even standard doses below 100 to 200 milligray could contribute to cancer risk over a lifetime, and significant overdoses (particularly from brain perfusion CT) have caused skin burns and hair loss. MRI uses only magnetic fields and radio waves, which are non-ionizing. Research suggests that ionizing radiation causes more complex DNA damage that is harder for the body to repair, which is why MRI is generally preferred when frequent imaging is needed.
That said, CT scans are faster, often taking just a few minutes, and are better for imaging bones, detecting bleeding, and guiding emergency decisions. MRI provides superior detail for soft tissues. The two tests complement each other rather than competing.
What to Expect During the Scan
Most MRI exams take between 15 and 45 minutes, though some can run up to 60 minutes or longer depending on the body part and how many image sequences are needed. You’ll lie on a padded table that slides into a large, tube-shaped magnet. The machine makes loud tapping, knocking, and thumping noises while it runs. These sounds come from the rapidly changing magnetic fields inside the scanner. You’ll typically be given earplugs or headphones to reduce the noise.
Staying still is important. Movement blurs the images, so technologists will ask you to hold as steady as possible. You can usually breathe normally, though for certain abdominal scans you may be asked to briefly hold your breath. The scan itself is painless. Some people feel a slight warming sensation in the area being scanned, which is normal.
How to Prepare
For most MRI exams, you can eat, drink, and take your medications as usual. Some specialty scans require fasting or other restrictions, and you’ll be told about those when you schedule the appointment. At the facility, you’ll be asked to remove all clothing (including underwear) and change into a hospital gown. Jewelry, piercings, eyeglasses, hearing aids, hairpins, and removable dental work all need to come off. Anything metal can interfere with the magnetic field or become a safety hazard, so it’s simplest to leave valuables and jewelry at home.
Contrast Dye
Some MRI scans require an injection of contrast dye, a gadolinium-based liquid given through an IV in your arm. The contrast makes certain tissues and blood vessels show up more clearly, helping doctors spot abnormalities they might otherwise miss. Gadolinium-based contrast agents have been used safely in millions of patients worldwide since 1988.
The most common side effects are mild: discomfort at the injection site, nausea, itching, rash, headache, or dizziness. Serious reactions are rare but more likely in people with significant kidney problems, because the kidneys are responsible for filtering gadolinium out of the body. If your kidneys can’t clear it efficiently, the contrast can accumulate and, in rare cases, cause a condition called nephrogenic systemic fibrosis. Your imaging team will typically check your kidney function before giving contrast and ask about any known allergies.
Metal Implants and Safety Concerns
Because MRI uses an extremely powerful magnet, metal inside your body is a serious safety consideration. Traditional pacemakers and implantable defibrillators have long been considered incompatible with MRI. The magnetic fields can interfere with the device’s electronics, potentially causing it to malfunction or heat up.
Newer pacemaker systems, available since around 2008, have been specifically designed to be MRI-compatible under certain conditions, such as limiting the scanner to a specific field strength. Multiple manufacturers now offer these MRI-conditional devices. If you have any type of metal implant, whether it’s a pacemaker, joint replacement, surgical clips, or even shrapnel, you’ll fill out a detailed screening questionnaire before your scan. The MRI team uses this to determine whether it’s safe to proceed.
Open MRI Options
Standard “closed” MRI machines are tube-shaped and can feel confining. If you experience claustrophobia or are larger than the bore of a standard scanner can accommodate, an open MRI may be an option. Open scanners have wider openings or are open on the sides, which makes the experience more comfortable. The trade-off is that open MRI systems sometimes produce lower-resolution images than their closed counterparts, though the technology continues to improve. Your doctor can help decide whether an open scanner will provide adequate image quality for your specific situation.
Getting Your Results
After your scan, a radiologist reviews the images and writes a report. If anything looks immediately life-threatening, such as a brain bleed, the radiologist notifies your care team right away. In non-emergency situations, the radiologist sends their findings to the doctor who ordered the MRI. That doctor then reviews the report, sometimes alongside the images themselves, and discusses the results with you. In a typical outpatient setting, expect to wait one to two weeks from the time of your scan to the appointment where you discuss results. Emergency MRIs are read as quickly as possible, often within hours.