Magnetic Resonance Imaging (MRI)

What Facts Should I Know about MRI?

History of MRI

Working independently, Felix Bloch of Stanford University and Edward Purcell of Harvard University made the first successful nuclear magnetic resonance experiment to study chemical compounds in 1946. Dr Bloch and Dr Purcell were awarded the Nobel Prize for Physics in 1952. In the early 1980s, the first "human" magnetic resonance imaging (MRI) scanners became available, producing images of the inside of the body. Current MRI scanners produce highly detailed two-dimensional and three-dimensional images of the human anatomy.

What is the medical definition of MRI?

  • An MRI is similar to a computerized tomography (CT) scanner in that it produces cross-sectional images of the body. Looking at images of the body in cross section can be compared to looking at the inside of a loaf of bread by slicing it. Unlike a CT scan, MRI does not use X-rays. Instead, it uses a strong magnetic field and radio waves to produce very clear and detailed computerized images of the inside of the body. MRI is commonly used to examine the brain, spine, joints, abdomen, and pelvis. A special kind of MRI exam, called magnetic resonance angiography (MRA), examines the blood vessels.

What is an MRI scan used to diagnose?

  • An MRI of the brain produces very detailed pictures of the brain and is commonly used to study people with such problems as headaches, seizures, weakness, hearing loss, and blurry vision. It can also be used to further evaluate an abnormality seen on a CT scan. During a brain MRI, a special device called a head coil is placed around the person's head to help produce very detailed pictures of the brain. The head coil does not touch the person, and the person can see through large gaps in the coil.
  • Spine MRI is most commonly used to look for a herniated disk or narrowing of the spinal canal (spinal stenosis) in people with neck, arm, back, and/or leg pain. It is also the best test to use to look for a recurrent disk herniation in a person with a history of prior back surgery.
  • Bone and joint MRI can be used to check virtually all of the bones, joints, and soft tissues. MRI can be used to identify injured tendons, ligaments, muscles, cartilage, and bones. It can also be used to look for infections and masses.
  • MRI of the abdomen is most frequently used to look more specifically at an abnormality seen on another test, such as an ultrasound or a CT scan. The exam is usually tailored to look at just the liver, pancreas, or adrenal glands.
  • For women, pelvic MRI provides a detailed look at the ovaries and uterus and is often used to follow up an abnormality seen on ultrasound. It is also used to evaluate the spread of cancer of the uterus. For men, pelvic MRI is sometimes used to check those diagnosed with prostate cancer. Pelvic MRI is also used to look at the bones and muscles of the pelvis.
  • Magnetic resonance angiography (MRA) depicts the blood vessels. The blood vessels in the neck (carotid and vertebral arteries) and brain are frequently studied by MRA to look for areas of constriction (narrowing) or dilatation (widening). In the abdomen, the arteries supplying blood to the kidneys are also frequently examined using this technique.

What Are the Risks of MRI?

MRI is a very safe procedure. The strong magnetic field itself does not hurt people, unless they have certain types of metal implanted in their body. The magnetic field can cause certain types of metal to move, which could potentially cause an injury.

  • People with any metal on or in their bodies should tell the technologist. Most people who have metal in their body after surgery can have an MRI. For example, people with hip or knee replacements can have an MRI as soon as 6 weeks after surgery. Other implanted devices require less time after surgery.
  • Certain devices (heart pacemakers, some implanted pumps, and nerve stimulators) can never go into the MRI machine, as they may malfunction or become damaged. Some brain aneurysm clips also cannot go into the scanner.
  • People who have had prior surgery must inform the technologist prior to the scan. Also, if metal might be in any part of the body from a prior injury or accident, people must inform the technologist prior to the scan. Certain people should not be scanned. For example, in a rare case, one person went blind from being scanned because he had metal in his eye from a welding injury.
  • Some MRI exams require an injection of an MRI contrast or dye. This MRI contrast or dye is very safe and is completely different from the contrast agent or dye used for imaging tests using X-rays, such as an intravenous pyelogram (IVP) or a CT scan. Allergic reactions to the contrast used are possible but extremely uncommon. The doctor and the MRI technologist should be informed beforehand of any allergies.
  • An MRI has no known side effects on pregnancy. Most centers will scan pregnant women in their second and third trimesters.

What Is the Preparation for MRI?

Typically, all metal and electronic devices (watches, jewelry, cellular phones, and credit cards) must be removed from one's clothing and body before the exam. This protects valuables from the effects of the MRI machine.

  • Depending on what part of the body is being imaged, a hospital gown may be necessary. Clothing that has metal snaps or attached metal should be replaced with a gown.
  • No preparation is needed. The only exception is a special study of the bile ducts, called an MRCP (magnetic resonance cholangiopancreatography), in which case eating or drinking is not allowed for 2 to 3 hours before the test. For all other studies, refraining from eating or drinking beforehand is not necessary.
  • A contrast (or dye) may need to be injected into a vein through an IV. This contrast (or dye) helps the doctor to see the inside of the body. The contrast is safe; severe reactions rarely occur.

What Happens During the MRI Procedure?

The study may take place on either an open scanner or a closed scanner. For an open scanner, a person lies on a table face up, and the table slides under the magnet from the side. For a closed scanner, which looks like a tube, a person lies on the table face up and goes in either head-first or feet-first, depending on what part of the body is being scanned.

The MRI scan is performed inside a large magnet, and the person lies on the table in the center. During the procedure, the machine scans the body by turning small magnets on and off. Radio waves are sent into the body. The machine then receives returning radio waves and uses a computer to create pictures of the part of the body being scanned. The radio waves used in the procedure are safe and are similar to the radio waves used in a car radio.

  • The scanner can make a loud knocking sound, so people are given either earplugs or music headphones. The knocking sound is due to the small magnets in the machine being turned on and off.
  • People have to hold the part of their body being scanned motionless for 30 to 60 minutes, which is the length of a typical MRI scan. If a person moves during the scan, some or all of it often has to be repeated. The scans are done in multiple parts. The technologist talks between each part to let the person know how things are going with the scan and to remind the person to hold still.
  • Sedation is sometimes necessary. Infants and young children often require sedation or general anesthesia in order to remain motionless during the scan. Most older children and adults do not require any medication for relaxation or sedation. On occasion, people who are nervous or claustrophobic require oral sedation and, rarely, general anesthesia.
  • Claustrophobia
    • Claustrophobia is a common concern. Many people wonder how far into the scanner they have to go. To get the best pictures possible, the part of the body being studied has to be in the middle of the scanner. For example, if a person is having a brain MRI, the head has to be in the middle of the scanner. If a person is having an ankle MRI, the ankle is in the scanner, but the head is not.
    • Unlike older MRI scanners in which the person was placed in a long tube, many centers now offer new, "short-bore" scanners that are much shorter and more comfortable if a person is claustrophobic. For people with severe claustrophobia, medication can be given to help them relax during the scan. For those people who take medication, someone must drive them home.

What Happens After the MRI Procedure?

If a contrast injection is used, the IV is removed from the arm before the person goes home. No side effects from the scan or the contrast injection should occur.

In the rare circumstance that sedation is needed, that person is sent home once awake and alert. For those people who receive sedation, someone must drive them home. No aftereffects occur from having an MRI.

A radiologist is a medical doctor trained to interpret various imaging studies. The radiologist interprets the results of the scan, and the results are then sent to the doctor. How quickly the doctor receives the report depends on the imaging center where the study is performed.

Medically reviewed by Sandra Mun, MD; Board Certification: Diagnostic Radiology


"Magnetic Resonance Imaging -- A Window into the Human Body." National Institute for Medical Research.

"Magnetic Resonance Imaging (MR/MRI)" Imaginis.

"Magnetic Resonance Imaging (MRI) Scan." Patient Education Institute. National Library of Medicine.