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MRI (Magnetic Resonance Imaging)

J. Taylor Whaley, MD
The Abramson Cancer Center of the University of Pennsylvania

What is an MRI?

  • An MRI scan, or magnetic resonance imaging scan, is a non-invasive radiology test that allows physicians to evaluate internal organs through the use of magnetic fields. Magnetic resonance was discovered in the mid-20th century, however, MRI's became commonly used in the 1990s in the United States. MRI scans heavily rely on computer technology, and with the modernization of computers, MRI scans have become considerably more enhanced, providing better detail. The most critical change has been improved resolution, allowing the radiologist to see very small differences (on the order of millimeters).
  • The use of radiologic imaging, including MRI scans, in medicine has considerably changed how doctors in numerous specialties approach illnesses. By visualizing internal organs noninvasively, from the brain to the neck, chest, liver, muscles, and pelvis, MRI scans can aid doctors by supporting their physical exam and lab work findings.
  • Just as an ultrasound and CT scan provide unique information to the radiologist and your doctor, MRI scans are performed because they also give special details that cannot be seen in other radiology tests.
  • MRI scanners can be found in most cities and are now widely available throughout the U.S. and many countries; however, there are far fewer MRI scanners than CT scanners. Additionally, because MRI scans are very expensive, they are generally performed after other scans.

What is an MRI used for?

  • Although MRI scans are occasionally used emergently, particularly in the setting of a possible stroke, MRI scans are generally used in non-emergent conditions. This is mostly due to the fact that MRI scans take much longer to obtain as well as to interpret compared to CT or ultrasound.
  • MRI scans are particularly useful in the evaluation of brains, spinal cord, muscles, bones, the heart, and the liver.
  • From a cancer perspective, MRI scans are often used in assessing the size of tumors and whether or not they can be surgically removed. Brain and spinal cord tumors are always evaluated with an MRI. Frequently, head and neck tumors located near the base of the skull will be evaluated with an MRI. Some tumors in the chest, particularly near the chest wall or heart, will be assessed with an MRI. Patients with breast cancer will occasionally undergo an MRI to check for breast cancer in the opposite breast. Tumors in the liver and prostate cancers are also frequently evaluated with MRI scans.
  • Unlike CT scans, which are relatively cheap and can be quickly performed, MRI scans are more time consuming, costly, and technically challenging.

How is this test performed?

  • MRI scans are almost always performed as an outpatient procedure, although they can also be done for patients in the hospital.
  • MRI scans involve the use of magnets and radiowaves - and not radiation - to generate images. The computer then processes this to create a slice-by-slice figure of the patient, similar to a loaf of bread that has been sliced. A radiologist, which is a doctor who specializes in looking at different types of images of patients, evaluates the scan and generates a report.
  • For the test, the patient lies on a sliding table that is passed through a large tube (this is the magnet). The tube can be quite loud while it is on and you will frequently wear earplugs to protect your ears. Many people worry about being in a tube during the exam; however, recently, much larger tubes, or bores, have been created. The time to scan a patient can range from 30 minutes to 2 hours, depending on what areas are being scanned.
  • Patients who are claustrophobic can be given a medicine to relax. However, in this case, you will need to arrange for a ride home after the exam as you will not be able to drive.
  • For patients who are still unable to tolerate the MRI, "open" MRI scans which are less claustrophobic may be performed. They are not available as readily, however, and the pictures may not be as useful for your doctor. This should be discussed with your doctor on an individual basis.
  • MRI scans use a very strong magnet, and because of this, no metal is permitted in the MRI scanner. Prior to the exam, you will be asked if you have any metal on or inside you. This can range from cell phones and jewelry to a pacemaker or metal rods. If you have specific questions about metal, you should discuss this with your doctor prior to the exam.
  • It is very important to not move during an MRI. This can cause the results to be blurry and limit the usefulness of the exam.
  • A radiation technologist will be in an adjoining room with a window and microphone to hear and talk to you during the exam.
  • Sometimes, if the doctor wants the blood vessels to be evaluated, IV contrast is given. This contrast is known as gadolinium and is different from the contrast used in a CT scan. If you have an allergy to CT contrast, which is iodine based, there is a good chance you will still be able to have the MRI contrast. Gadolinium cannot be given to anyone with kidney disease because it is can damage the kidneys.

Picture of an MRI machine
Picture of an MRI machine

How do I prepare for an MRI scan?

  • There is very little preparation for an MRI. Other than removing any metal on your body, there is no prep involved. You are not required to fast prior to the exam.

How do I interpret the results of an MRI scan report?

  • The results of the test depend on which part of the body was imaged. Following the scan, the images are processed by a computer and read by a radiologist. The radiologist then generates a report for the medical professional responsible for ordering the MRI scan.
  • The report generally states the patient's name, date of birth, and indication (reason for the MRI scan) at the top of the report. Radiology reports follow a standard outline, regardless of where they are obtained. Radiologists report both normal and abnormal findings in a very systematic approach. For this reason, it is very important to discuss the results with your doctor.
    • The first paragraph typically includes the specific technical information involved in obtaining the scan (i.e. whether contrast was given, what the exam involves, etc).
    • The middle paragraphs generally begin the description of findings, both normal and abnormal. Because reports are generated for other medical professionals, the terminology is often medically oriented and can be difficult to interpret.
    • Following the detailed interpretation above, an impression generally follows. This is a summary of the findings, often generated to answer the question posed by the ordering physician.
    • The radiologist may compare these findings to any previous scans of that body site, if the images are accessible.