MRI of the Head
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 diagnosing tumors of the brain as well as disorders of the eyes and the inner ear. 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 ferromagnetic metal object implanted in the body, MRI staff will ask whether you have a prosthetic hip, heart pacemaker (or artificial heart valve), implanted port (brand names Port-o-cath, Infusaport, Lifeport), intrauterine device (IUD), 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. If there is any question of metal fragments, you may be asked to have an x-ray that will detect any such 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 jewelry, eyeglasses, hearing aids and any removable dental work.
What does the equipment look like?
The conventional MRI unit is a closed cylindrical magnet in which the patient must lie still, and consequently may feel "closed-in" or truly claustrophobic. However more "patient-friendly" designs are rapidly coming into routine use. FMH offers the "short-bore" systems which are wider and shorter and do not fully enclose the patient.
An MRI uses high field-strength magnets and special coils to produce detailed images of body tissues without the use of x-rays. A moving table will position you inside a hollow tube that is open at both ends at all times. You will hear a series of loud, “knocking” sounds. This is the instrument positioning its internal components to get the very best view. You can listen to music or use the earplugs provided, and the technologist will communicate with you throughout the 30-45 minute procedure.
How does the procedure work?
MRI is a unique imaging method because, unlike the usual radiographs (x-rays), radioisotope 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 2 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?
The patient is placed on a sliding table and a radio antenna device called a surface coil is positioned around the upper part of the head. After positioning the patient with the head inside the MRI gantry, the technologist leaves the room and the individual MRI sequences are performed. The patient is able to communicate with the technologist at any time using an intercom.
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. Some patients will require an injection of a contrast material to enhance the visibility of certain tissues or blood vessels. A small needle connected to an intravenous line is placed in an arm or hand vein.
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 radiologist or 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 will be provided.
Who interprets the results and how do I get them?
A radiologist, who is a physician experienced in MRI and other radiology examinations, will analyze the images and send a signed report with his or her interpretation to the patient's personal physician. The personal physician's office will inform the patient on how to obtain their results.