This page was reviewed under our medical and editorial policy by
Maurie Markman, MD, President, Medicine & Science
This page was updated on May 2, 2022.
Magnetic resonance imaging (MRI) is a health care imaging tool designed to create detailed, cross-sectional pictures of the insides of the body. Using radiofrequency waves, powerful magnets and a computer, MRI scans are built to distinguish between normal and diseased tissue.
MRI scanners play an important role in diagnosing cancer, as well as staging and treatment planning. The detailed images from an MRI machine may be able to be used to distinguish between normal tissue and abnormalities.. This helps the radiology team precisely pinpoint how cancerous cells have grown within the body. It also may be useful for revealing metastases, when cancer has spread to a new part of the body. MRI provides greater contrast in the soft tissues of the body than a computed tomography (CT) scan. As a result, radiologists often use the machine for medical imaging involving the brain, spinal cord, muscle, ligaments, blood vessels and the inside of bones.
Specialized MRI machines include functional MRI (fMRI) scanners, which evaluate blood flow in the brain, and open MRI scanners, which can be used for patients with claustrophobia.
During an MRI, positively charged particles called protons in the body line up with the magnets of the MRI. The machine then sends pulses of energy called radio waves, which push the protons out of alignment with the magnetic field. When the radio waves are turned off, the protons realign, releasing energy.
MRIs work by detecting this energy and turning it into images. The amount of energy the protons release and how long it takes them to release it depends on their local environment. This difference in energy release is how the machine may distinguish between different types of tissues.
Unlike X-rays and CT scans, an MRI does not use ionizing radiation. Ionizing radiation may damage the genetic material in cells, potentially leading to health issues and increasing cancer risk if the dose is high or frequent. Instead, MRI uses a strong magnetic field and radio waves. These don’t have the same risk of damaging cells.
Because of the large magnets, MRI is not recommended for women in the first trimester of pregnancy or for patients who have electronic devices or metal objects in their bodies, such as bullet fragments, shrapnel, cochlear implants, most pacemakers and defibrillators.
An MRI may be performed with or without the addition of contrast agents. These chemicals are swallowed or given through a vein before or during the MRI. They often contain the element gadolinium.
Contrast speeds up the time it takes protons to realign with the MRI machine’s magnetic field, brightening and sharpening the image.
Sometimes, doctors may be able to tell if a tumor is cancerous or not based on how it looks on an MRI, but images obtained by an MRI may not definitively diagnose cancer. These images aren’t able to tell which genetic changes the cells have undergone or if the tumor is growing and spreading like cancer.
However, MRI images are an important part of the diagnostic process. MRIs are often used to look for signs that cancer has spread to other body parts. They help the care team identify tumors and monitor their growth over time. The images may also help with treatment planning or show how well treatments are working.
MRI is an appropriate way to image non-bony parts of the body. These parts may include:
An MRI may show abnormalities in these types of soft tissues, which makes the scan successful for locating tumors and other abnormalities.
Magnetic resonance imaging is a noninvasive procedure that produces three-dimensional (3D) anatomical images of body parts—for a deeper look than with a standard X-ray. The patient’s doctor may order one to examine soft tissues, muscle, ligaments and tendons, and organs such as the brain or liver.
Before going in for an MRI, a patient should block out enough time on test day. The procedure typically takes 45 minutes to 60 minutes, but it may take up to two hours.
Wearing loose, comfortable clothing may be helpful. Before the MRI, the patient will likely have to change into a medical gown. Since he or she will be asked to remove any metal for the test, it’s best to leave jewelry at home.
A change in diet may be necessary before the procedure. Review the care team’s instructions for food, drink and daily medications. Ask the care team about any restrictions ahead of test day.
If the patient is feeling nervous before the procedure, he or she should ask whether the facility provides ear plugs or headphones during the test. A patient may be able to use music for support or distraction during the procedure.
A patient should have medical records organized. If the MRI technologist doesn’t already have a patient’s personal and medical history, the patient should be prepared to offer details. These may include past scans (for a comparison to new results), surgeries and a list of current medications.
Because of the strong magnetic fields, objects containing iron, some steel and other compounds that may be magnetized aren’t allowed in the room when an MRI is on. That includes piercings, and any medical or other implants that contain metals.
Implants that contain iron or may interfere with the MRI include:
If a patient has any of these implants, talk to the care team. A patient should not enter an MRI machine with these implants unless the doctor and MRI technician say it’s OK.
Tattoos are generally safe for patients having an MRI. According to the U.S. Food and Drug Administration, some people may experience mild swelling or burning in the tattoo area, but this is rare and resolves on its own.
Patients with severe renal failure who are undergoing dialysis should generally avoid using MRI with contrast. It’s possible some types of gadolinium may increase the risk of a serious illness called nephrogenic systemic fibrosis (NSF). Dialysis patients should only get contrast MRI when essential and undergo dialysis as soon as possible after the scan.
Patients with renal disease should know the symptoms of NSF and let their doctor know if they’re experiencing any of them. These symptoms may develop slowly over a few weeks to a few months, and may include:
MRI is generally avoided during pregnancy, especially during the first trimester. No adverse effects have been noted, but the first three months of pregnancy is a sensitive time for the developing fetus. Contrast agents may transfer to the fetus, so they should also be avoided if an MRI is necessary during pregnancy.
Patients who are breastfeeding should inform the technologist before getting an MRI with contrast solution. According to guidelines from the American College of Radiology, breastfeeding is safe after taking the contrast solution, but patients with concerns may opt to wait for 24 hours before breastfeeding again. Some patients express extra milk and store it before the scan, then express and discard the milk for one day after the scan. The solution is cleared from the bloodstream in 24 hours.
Breastfeeding may muddy the MRI image, particularly if there is a breast concern. If possible, patients wait up to six months after stopping breastfeeding, or use an alternate imaging method to get a clearer picture.
During the MRI, to get the most accurate images, a patient may be asked to lie very still while the designated area of the body is being scanned.
Patients should listen to and follow the technologist’s directions carefully. The technologist may ask the patient to hold his or her breath at certain times during the scan to create the clearest images.
Patients are encouraged to speak up if they have any trouble during the scan. It’s not uncommon to feel claustrophobic or anxious during an MRI, so patients should let the technologist know if they’re uncomfortable.
If a patient is feeling anxious, it may help to lean on a loved one for support. Check with the facility to see if there are any restrictions on someone coming with the patient.
Patients with even mild claustrophobia may experience anxiety during an MRI. The process may take up to two hours, and the patient needs to lie still in a small tube in the machine. Techniques like visualization or listening to music may help, as well as medical sedation or anesthesia.
Alternatively, a type of MRI that’s open on all sides (instead of a closed tube) may be an option for patients who experience claustrophobia. Open MRIs don’t create as strong of a magnetic field, so it isn’t the best option for all types of imaging. As a result, a patient may need to repeat the test in a closed-tube MRI to obtain a clearer image.
The MRI process itself is painless. A patient may feel a slight twitching sensation during the test or warmth in the scanned area. The noise may be intense and uncomfortable. If a patient receives contrast with their MRI, an IV or injection may be needed, which may pinch or cause pain.
After the MRI, patients should check in with their doctor about the results. There are typically no side effects or restrictions after an MRI.
Sometimes MRI results are delivered in real-time to the doctor’s office, but patients should follow up after a few days if they haven’t received the results.
It’s possible to have a bad reaction to the MRI contrast material. Potential sensations or side effects may include:
If a patient has any of these side effects, he or she should let the doctor know.