An MRI scan is a painless imaging procedure that typically takes 30 to 60 minutes, during which you lie still inside a large tube-shaped machine while it uses powerful magnets and radio waves to create detailed pictures of your body’s internal structures. No radiation is involved, unlike X-rays or CT scans. Here’s what actually happens before, during, and after the scan.
How the Machine Creates Images
Your body is mostly water, and water molecules contain hydrogen atoms. When you’re placed inside the MRI scanner, its powerful magnetic field (typically 1.5 Tesla, roughly 30,000 times stronger than Earth’s magnetic field) causes the hydrogen atoms in your tissues to align in one direction, similar to how a compass needle points north. The scanner then sends short bursts of radio waves at a specific frequency that knock these atoms out of alignment. When the radio waves stop, the atoms snap back into position and release tiny signals as they do.
Different tissues (fat, muscle, bone, fluid) release these signals at different rates. The scanner’s receivers pick up those differences and a computer translates them into highly detailed cross-sectional images. This is why MRI excels at showing soft tissues like the brain, spinal cord, ligaments, and organs in a way that X-rays simply cannot.
Preparing for Your Scan
Preparation depends on what part of your body is being scanned and whether contrast dye is needed. For many routine MRIs, you won’t need to do anything special beforehand. If your scan requires an intravenous contrast agent or sedation, you may be asked to avoid solid food for four to eight hours before the exam. Certain cardiac MRIs have stricter rules: no food for two hours prior and no coffee, tea, or chocolate for 24 hours.
Before you enter the scanning room, you’ll complete a safety screening questionnaire. The magnetic field is always active, so any metal in or on your body is a potential hazard. You’ll be asked to remove all jewelry, watches, body piercings, hair clips, belt buckles, and metallic drug patches. Most facilities provide a gown and a locker for your belongings.
Metal Implants and Devices
Certain implants can be dangerous inside an MRI scanner or can distort the images. People with implanted cardiac pacemakers, defibrillators, or other battery-powered medical devices are generally not eligible for MRI unless the device has been specifically certified as MRI-compatible. Aneurysm clips in the brain require documentation of the exact manufacturer and model before a scan can proceed. If you’ve ever had metal fragments in your eyes (from grinding or welding, for example), you may need a CT scan of your eye sockets beforehand to confirm no metal remains.
Many modern joint replacements, screws, and plates are made of MRI-safe materials, but the technologist will want to know exactly what you have and when it was implanted. Bring any implant cards or surgical records you have.
What Happens During the Scan
You’ll lie down on a motorized table that slides into the scanner’s tunnel. Depending on the body part being imaged, the technologist may place a specialized coil (a device that looks like a frame or cage) around or near the area. For a brain scan, this coil fits snugly over your head. For a knee, it wraps around the joint. The coil acts like an antenna, improving the signal quality from that specific region. The technologist may tuck cushions around you to help you stay still and comfortable, and sometimes place a cushion under your knees to reduce lower back strain.
Once you’re positioned, the table slides into the bore of the machine. For a head or upper body scan, your head will be inside the tunnel. For a knee or foot scan, only your lower body enters, and your head may remain outside.
The machine then begins acquiring images in sequences, each lasting anywhere from two to six minutes. During each sequence, you’ll hear loud knocking, buzzing, or thumping sounds. These are completely normal. They come from the rapid switching of magnetic field gradients inside the machine. You’ll be given earplugs or headphones (sometimes with music) to reduce the noise. Between sequences, the machine goes quiet for a few seconds before starting the next round.
The single most important thing you can do is stay still. Even small movements can blur the images and may require repeating a sequence. Brain and spine exams typically take about 45 minutes. Body scans (abdomen, pelvis, chest) can run 45 to 60 minutes. If contrast is needed, add roughly 15 minutes.
If Your Scan Requires Contrast
Not every MRI uses contrast, but when your doctor needs to see blood vessels, inflammation, or tumors more clearly, you’ll receive a gadolinium-based contrast agent through an IV line in your arm. The technologist usually starts the scan without contrast, then pauses partway through to inject the dye before continuing with additional image sequences.
Gadolinium works by altering the magnetic properties of nearby water molecules, which makes certain tissues light up more brightly on the images. Most people feel nothing from the injection itself, though some notice a brief cool sensation in the arm. Allergic reactions are uncommon but possible, so the technologist will ask about any prior reactions to contrast agents.
Staying Calm in the Scanner
The standard MRI tunnel is roughly 60 centimeters (about two feet) wide, and the scan requires you to remain inside for an extended period. For people who experience claustrophobia, this can be the hardest part. A few things help. You’ll have a call button or squeeze ball to alert the technologist at any time. The technologist can talk to you through an intercom and will often tell you how much time is left in each sequence. Keeping your eyes closed from the moment you enter the tunnel prevents you from focusing on the confined space. Some people find it helpful to focus on slow, steady breathing.
If anxiety is a significant concern, your doctor may prescribe a mild sedative to take before the appointment. In that case, you’ll need someone to drive you home afterward.
Open MRI as an Alternative
Open MRI machines were developed in the 1990s and use two large horizontal discs above and below you instead of a surrounding tube. There are no walls on the sides, which dramatically reduces the enclosed feeling. The tradeoff is image quality: open scanners operate at 0.2 to 0.3 Tesla compared to the 1.5 Tesla (or higher) of standard closed scanners. This lower field strength means less resolution, thinner detail, and fewer advanced imaging techniques. Specialized exams like breast MRI, prostate MRI, and early stroke detection are only available on high-field closed scanners. In some cases, a scan done on an open machine may need to be repeated on a closed one for an accurate diagnosis.
A middle ground exists in newer “short bore” closed scanners. These have a wider, shorter tunnel with flared ends, which reduces the claustrophobic feeling while still delivering high-quality images.
After the Scan
When the scan is finished, the table slides out and the technologist removes the coil and any IV line. There’s no recovery period for a standard MRI. You can eat, drink, drive, and return to normal activities immediately unless you received sedation.
The images are sent to a radiologist, a doctor who specializes in interpreting medical imaging. The radiologist reviews the images and sends a written report to the doctor who ordered your scan. Turnaround time varies by facility, but most results are available within one to three business days. Your ordering doctor will then discuss the findings with you and determine next steps based on what the images show.