An MRI (magnetic resonance imaging) is a diagnostic scan that creates detailed pictures of organs, bones, and soft tissues inside your body without using radiation. Doctors order MRIs to investigate a wide range of problems, from torn knee ligaments to brain tumors to heart disease. The scan is particularly valuable for distinguishing between different types of soft tissue, something X-rays and CT scans do less effectively.
How an MRI Creates Images
Your body is mostly water, and water molecules contain hydrogen atoms. An MRI machine uses a powerful magnet to force the hydrogen atoms in your body into alignment. The machine then sends pulses of radio waves through you, knocking those atoms out of alignment. When the radio waves stop, the atoms snap back into position and release energy as they do. Sensors inside the machine detect that released energy and use it to build an image.
The key detail: different tissues release energy at different rates. Fat behaves differently from muscle, which behaves differently from fluid, which behaves differently from a tumor. The MRI translates these differences into remarkably detailed pictures that let doctors tell healthy tissue from damaged or abnormal tissue.
Brain and Spinal Cord Conditions
MRI is the go-to imaging tool for the brain and spinal cord. It can reveal tumors, the damage caused by a stroke, traumatic brain injuries, and the characteristic lesions of multiple sclerosis. It’s also used to evaluate Alzheimer’s disease, infections of the brain or spinal cord, and structural abnormalities that may be causing symptoms like headaches, seizures, or vision changes. A specialized version called magnetic resonance spectroscopy can even assess chemical abnormalities in brain tissue, helping doctors evaluate conditions that might not show visible structural damage.
Joint and Muscle Injuries
If you’ve hurt a knee, shoulder, ankle, or wrist, there’s a good chance your doctor will order an MRI. The scan excels at detecting injuries in soft tissues that X-rays simply can’t show: torn meniscus cartilage in the knee, ACL tears, rotator cuff damage in the shoulder, labral tears in the hip or shoulder, Achilles tendon ruptures, and cartilage loss in joints. For shoulder labral tears specifically, MRI detects them with up to 100% sensitivity in some studies.
MRI is also used for less obvious musculoskeletal problems. It can identify bone injuries that don’t appear on X-rays (called occult fractures), cartilage damage on weight-bearing joint surfaces, nerve compression causing pain or numbness, and soft tissue masses like cysts or tumors. For sports injuries in particular, MRI has become the primary imaging tool because it shows the full picture of what’s damaged and how severely.
Heart and Blood Vessel Assessment
Cardiac MRI gives doctors a detailed look at the heart muscle itself. It can distinguish between healthy heart tissue and areas damaged by a heart attack, detect fibrosis or scarring, and identify infiltration by abnormal materials. This is especially useful because it avoids the sampling errors and risks of a heart biopsy. The scan can also evaluate heart valve function with precise measurements of blood flow, and it’s considered the best technique for determining whether damaged heart muscle is still viable or permanently scarred.
For people born with heart defects, cardiac MRI provides complete anatomical and functional imaging, including accurate measurements of chamber volumes and blood flow through vessels.
Liver, Pancreas, and Other Organs
MRI plays a growing role in evaluating abdominal organs. In the liver, it can detect and characterize small lesions, distinguish benign growths (like hemangiomas) from malignant ones (like metastases), and measure fat and iron accumulation. It’s also useful for visualizing bile ducts and diagnosing conditions like gallstones, bile duct inflammation, and bile duct cancers.
For the pancreas, MRI provides clear visualization of the pancreatic duct and cystic lesions, and newer techniques may even detect early-stage chronic pancreatitis before it becomes obvious on other imaging. The scan is also used to examine breast tissue, the spine, and blood vessels throughout the body.
When Contrast Dye Is Used
Some MRI scans require an injection of a contrast agent, typically a gadolinium-based solution delivered through an IV in your arm. The contrast helps highlight specific structures, including blood vessels, tumors, and areas of inflammation. It’s commonly used for brain, spine, heart, liver, and breast MRIs. The contrast material makes abnormal tissues “light up” differently from surrounding healthy tissue, improving the scan’s ability to detect and characterize problems.
Why MRI Over a CT Scan or X-Ray
X-rays are fast and good for obvious fractures, but many problems are too subtle to show up on them. CT scans create more detailed images than X-rays and are better for emergencies because they’re much faster. However, CT scans use radiation and aren’t as effective at revealing subtle differences between tissue types.
MRI’s advantage is its superior contrast resolution. It shows soft tissue detail that CT simply can’t match, making it the better choice for evaluating ligament and tendon injuries, cartilage damage, joint inflammation, nerve compression, and spinal injuries. A 3-Tesla MRI (the higher-powered version of the standard 1.5-Tesla machine) offers even better image quality. Studies have found improved detection rates with 3T machines for conditions including multiple sclerosis lesions, coronary artery disease, liver metastases, and small joint injuries in the wrist. The tradeoff: MRI takes longer and costs more than CT.
What the Scan Is Like
You’ll remove all jewelry, watches, and metal objects and change into a hospital gown. You then lie on a table that slides into a large tube-shaped magnet. The average scan takes about 45 minutes, though this varies depending on what body part is being imaged and whether contrast is needed. The machine makes loud banging and clicking noises throughout the scan, and you’ll be offered earplugs or headphones. Because MRI is sensitive to motion, you need to stay still during the scan.
If your exam requires contrast, an IV line will be placed before or during the scan. Some specialized abdominal MRIs require fasting for eight hours beforehand and drinking a barium preparation in the hour before the scan, but most MRIs need no special preparation at all.
Claustrophobia and Size Concerns
The enclosed tube of a standard MRI machine is a real challenge for people with claustrophobia or larger body types. Wide-bore MRI machines address this with a wider tunnel opening, more headroom, and a lower exam table that’s easier to get onto. They accommodate patients weighing up to 550 pounds and people with wide shoulders. For many exams, the wider design allows your head to stay outside the bore entirely, which significantly reduces the feeling of being enclosed. Wide-bore machines still produce high-quality diagnostic images.
Who Should Not Have an MRI
The MRI’s powerful magnet makes it dangerous for people with certain metal implants or devices. Pacemakers, implantable defibrillators, and other cardiac electronic devices pose serious risks, including device heating, displacement, and triggering abnormal heart rhythms. Newer “MRI-conditional” versions of these devices exist and are increasingly common, but scans still require special precautions and dedicated scheduling.
Other items that typically rule out an MRI: metallic foreign bodies in the eyes (common in welders or people with facial metal injuries), cochlear implants, neurostimulation devices, insulin or chemotherapy infusion pumps, cerebral aneurysm clips, and metallic fragments like bullets or shrapnel. People with a history of metal exposure to the face typically need an orbital X-ray before they can be cleared for scanning.
Some implants fall into a gray area. Coronary stents, joint replacements, surgical clips, IUDs, and spinal rods often can be scanned safely, but each case requires checking the specific make and model of the device. If you have any implanted metal or electronic device, your MRI team will evaluate it before proceeding.