An MRI scan uses a powerful magnet and radio waves to create detailed images of the inside of your body, with no radiation involved. The process is painless, though it requires you to lie still inside a large tube-shaped machine for anywhere from 25 minutes to over an hour depending on the body part being scanned. Here’s what actually happens before, during, and after.
How the Machine Creates Images
Your body is mostly water, and every water molecule contains hydrogen atoms. Inside each hydrogen atom is a proton that spins on its own axis, generating a tiny magnetic field with its own north and south poles. Normally these protons spin on randomly oriented axes, pointing in all directions. But when you’re placed inside the MRI’s powerful magnet, the protons align with the magnetic field and begin spinning at a specific frequency.
Once the protons are aligned, the machine sends out a pulse of radio waves tuned to that exact frequency. Some of the protons absorb the energy and flip their orientation. When the radio pulse stops, those protons snap back into alignment and release the absorbed energy as a tiny signal. Detectors inside the machine pick up these signals and convert them into electrical data, which a computer assembles into cross-sectional images of your tissues. Different tissues (fat, muscle, fluid, bone marrow) release energy at slightly different rates, which is what gives MRI its remarkable ability to distinguish between soft tissue types.
Preparing for the Scan
For most MRI exams, you can eat and drink normally beforehand. The exception is scans involving contrast dye or sedation, where you may be asked to skip solid food for four to eight hours. Certain abdominal scans have their own fasting windows, sometimes six hours with only water allowed.
The biggest preparation step is removing anything metallic or electronic. The magnet is strong enough to erase credit cards and pull metal objects across the room, so you’ll need to leave behind jewelry, watches, hairpins, eyeglasses, pens, pocketknives, hearing aids, and removable dental work. Clothing with metal zippers, hooks, or buttons also needs to come off. Most facilities provide a gown and a locker for your belongings.
Before you enter the scan room, a technologist will ask about metal inside your body. This includes surgical pins, plates, screws, or staples from previous operations, as well as shrapnel, bullet fragments, or metal shavings in the eyes from welding. Medical devices like pacemakers, implanted defibrillators, cochlear implants, spinal stimulators, insulin pumps, and certain IUDs all need to be disclosed. Even medication patches (nicotine, birth control, nitroglycerin) can contain metal. Some implants are labeled “MR Unsafe,” meaning the scan cannot be performed. Others are “MR Conditional,” meaning the scan can proceed under specific conditions your care team will verify.
What Happens During the Scan
You’ll lie on a padded table that slides into the bore of the machine, a cylindrical tunnel surrounded by the magnet. Depending on which body part is being imaged, the technologist may place a special coil (a device shaped like a cage or frame) over the area to improve image quality. A knee scan, for instance, uses a coil that fits around the knee joint.
Once you’re positioned, the table slides into the tunnel and the scanning begins. You won’t feel the magnetic field or the radio pulses, but you will hear the machine. MRI scanners produce loud knocking, thumping, and buzzing sounds created by electrical currents running through components called gradient coils. The noise is loud enough to potentially affect hearing, so every patient gets earplugs, and many facilities also provide headphones with music. Scans won’t proceed without ear protection in place.
The most important thing you need to do is stay still. Movement blurs the images the same way it blurs a photograph. For certain scans, you may be asked to hold your breath briefly. The technologist operates the machine from an adjacent room and communicates with you through a speaker and microphone built into the scanner. You’ll typically have a squeeze ball or call button to signal if you need a break.
How Long It Takes
Brain and spine exams average about 45 minutes. Joint scans (knee, ankle, hip, elbow, wrist) run 25 to 45 minutes. If your scan requires contrast, add roughly 15 minutes to those times. The technologist will usually give you a time estimate before you go in.
Contrast Dye
Some MRI exams call for a contrast agent, a liquid injected into a vein partway through the scan. The most common type is gadolinium-based. It works by changing how certain tissues interact with the magnetic field, making specific structures (blood vessels, tumors, areas of inflammation) stand out more clearly in the images. A technologist or nurse places a small IV line, typically in your arm, and injects the contrast during a pause in the scanning sequence. You might feel a brief cool sensation at the injection site. The contrast is eliminated from your body through your kidneys over the following hours.
If You’re Claustrophobic
The traditional MRI bore is a relatively tight space, and anxiety about being enclosed is one of the most common concerns patients have. Several options can help. Wide-bore MRI machines have a larger tunnel opening that creates more headroom and space around you. For many exams in a wide-bore system, your head can remain outside the tunnel entirely, which significantly reduces that closed-in feeling. These machines also accommodate patients with broad shoulders or those weighing up to 550 pounds.
If a wide-bore machine isn’t available or isn’t enough, mild sedation is another option. Your ordering physician can prescribe an oral anti-anxiety medication to take before your appointment. In that case, you’ll need someone to drive you home. Talking to the technologist beforehand about what to expect, keeping your eyes closed, and focusing on breathing can also make a real difference.
After the Scan
When the scan is finished, the table slides out and you can get up immediately. If you didn’t receive sedation, there’s no recovery period and you can drive yourself home and resume normal activities right away.
The images go to a radiologist, a physician who specializes in interpreting medical imaging. The radiologist reviews the images, writes a detailed report, signs it, and sends it to the doctor who ordered your scan. That doctor then discusses the findings with you. Turnaround time varies by facility and urgency. Some results are available within a day or two, while others may take longer. Many health systems now post imaging reports to online patient portals, sometimes before your doctor has had a chance to review them, so you may see the report before your follow-up conversation.