MRI (Magnetic Resonance Imaging)

Published On 07/11/2011

Magnetic Resonance Imaging (MRI) is the use of magnetic energy and radio waves to create cross-sectional images or "slices" of the human body.

What is MR Imaging (MRI) of the Head?

MRI (magnetic resonance imaging) uses radio waves and a strong magnetic field rather than x-rays to provide remarkably clear and detailed pictures of internal organs and tissues. This technique has proved very helpful to radiologists in identifying abnormalities of the brain, eyes, inner ear, spine, and joints. It requires specialized equipment and expertise and allows evaluation of some body structures that may not be as visible with other imaging methods.

What are some common uses of the procedure?

MRI is the most sensitive exam for brain tumors, strokes, and certain chronic disorders of the nervous system such as multiple sclerosis. In addition, it is a useful means of documenting brain abnormalities in patients with dementia, and it is commonly used for patients with disease of the pituitary gland. MRI can detect tiny areas of tissue abnormality in patients with disease of the eyes or the inner ear.

How should I prepare for the procedure?

Because the strong magnetic field used for MRI will pull on any metal object implanted in the body, MRI staff will ask whether you have a heart pacemaker (or artificial heart valve), implanted port (brand names Port-o-cath, Infusaport, Lifeport), or any metal plates, pins, screws, or surgical staples in your body. Tattoos and permanent eyeliner may also create a problem. You will be asked if you have ever had a bullet or shrapnel in your body, or ever worked with metal. You will likely be asked to have an x-ray that will detect any metal objects. Tooth fillings usually are not affected by the magnetic field, but they may distort images of the facial area or brain, so the radiologist should be aware of them. The same is true of braces, which may make it hard to "tune" the MRI unit to your body. You will be asked to remove anything that might degrade MRI images of the head, including hairpins, jewelry, eyeglasses, hearing aids, and any removable dental work.

The technologist may ask about drug allergies and whether head surgery has been done in the past. If you might be pregnant, this should be mentioned. Some patients who undergo MRI of the head in an enclosed unit may feel confined or claustrophobic. If you are not easily reassured, a sedative may be administered. Roughly 1 in 20 patients will require medication.

What does the equipment look like?

The conventional MRI unit is a closed cylindrical magnet in which the patient must lie totally still for several seconds at a time, and consequently may feel "closed-in" or truly claustrophobic. However our patient-friendly design is both shorter and wider than a conventional MRI unit.

We also offer an Open MRI at 2 locations.

How does the procedure work?

MRI is a unique imaging method because, unlike the usual radiographs (x-rays), radiostope studies, and even CT scanning it does not rely on radiation. Instead, radio waves are directed at protons, the nuclei of hydrogen atoms, in a strong magnetic field. The protons are first "excited" and then "relaxed," emitting radio signals, which can be computer-processed to form an image. In the body, protons are most abundant in the hydrogen atoms of water -- the "H" of H2O -- so that an MRI image shows differences in the water content and distribution in various body tissues. Even different types of tissue within the same organ, such as the gray and white matter of the brain, can easily be distinguished. Typically an MRI exam consists of two to six imaging sequences, each lasting two to 15 minutes. Each sequence has its own degree of contrast and shows a cross section of the head in one of several planes (right to left, front to back, upper to lower).

How is the procedure performed?

You will be placed on a sliding table and a radio antenna device called a surface coil is positioned around the part of your body being imaged. After positioning you inside the MRI gantry, the technologist leaves the room and the individual sequences are performed. You are able to communicate with the technologist at any time using an intercom. Methodist Diagnostic Center allows a friend or, if a child is being examined, a parent, into the room. Depending on how many images are needed, the exam will generally take from 15 to 45 minutes, although a very detailed study may take longer. You will be asked not to move during the actual imaging process, but between sequences some movement is allowed. Patients are generally required to remain still for only a few seconds at a time. Some patients will require an injection of contrast material to enhance the visibility of certain tissues or blood vessels. A small needle is placed in an arm or hand vein.

When the exam is over you will be asked to wait until the images are examined to determine if more images are needed.

What will I experience during the procedure?

MRI causes no pain, but there may be discomfort from being closed in or from the need to remain still. You may notice a warm feeling in the area under examination; this is normal, but if it bothers you the technologist should be told. If a contrast injection is needed, there may be discomfort at the injection site, and you may have a cool sensation at the site during the injection. Most bothersome to many patients are the loud tapping or knocking noises heard at certain phases of imaging. Ear plugs may help. You may bring your favorite CD to listen to during the exam or we can tune in to your favorite radio station.

Who interprets the results and how do I get them?

Our radiologist will analyze the images and fax a signed report with the interpretation to your physician. The physician's office will inform the patient on how to obtain their results.

What are the benefits of an MRI?

Benefits

  • Images of the brain, spine, joints, and other structures are clearer and more detailed than with other imaging methods.
  • MRI contrast material is less likely to produce an allergic reaction than the iodine-based materials used for conventional x-rays and CT scanning.
  • Exposure to radiation is avoided.
  • MRI enables the detection of abnormalities that might be obscured by bone tissue with other imaging methods.

A variant called MRI angiography provides detailed images of blood vessels in the brain without the need to inject contrast material.

Risks
An undetected metal implant may be affected by the strong magnetic field. MRI is generally avoided in the first 12 weeks of pregnancy. Doctors usually use other methods of imaging, such as ultrasound, on pregnant women, unless there is a strong medical reason.

What are the limitations of MRI of the Head?

MRI may not always distinguish between tumor tissue and edema fluid, and may not detect calcium when this is present within a tumor. In most cases the exam is safe for patients with metal implants, with the exception of a few types of implants, so patients should inform the technologist of an implant prior to the test. The exam must be used cautiously in early pregnancy. MRI often costs more than CT scanning.