MRI of the Musculoskeletal System
What is MR Imaging (MRI) of the Musculoskeletal System?
MRI (magnetic resonance imaging) uses radio waves and a strong magnetic field rather than x-rays to provide clear and detailed pictures of internal organs and tissues. The parts of the musculoskeletal system that are most frequently imaged with MRI are the knee and shoulder. However, MRI has also been used to study almost every joint in the body, including the spine, hips, wrists and hands. MRI 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?
Because MRI can give such clear pictures of soft tissue structures near and around bones, it is usually the best choice for examination of the body's major joints, the spine for disk disease, and soft tissues of the extremities. MRI is widely used to diagnose sports-related injuries, as well as work-related disorders caused by repeated strain, vibration, or forceful impact.
Using MRI images, physicians can locate and identify the cause of pain, swelling, or bleeding in the tissues in and around the joints and bones. The images allow the physician to clearly see even very small tears and injuries to tendons, ligaments and muscles, and even some fractures that cannot be seen on x-rays.
In addition, MRI images can give physicians a clear picture of degenerative disorders such as arthritis, deterioration of joint surfaces, or a herniated disc. Neurosurgeons often use MRI to evaluate the integrity of the spinal cord after trauma.
Finally, MRI is also useful for the diagnosis and characterization of infections (for example osteomyelitis) and tumors (for example metastases) involving bones and joints.
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, an aneurysm clip in the brain, 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. In most cases, surgical staples, plates, pins and screws pose no risk during MRI if they have been in place for more than 4-6 weeks. Dyes used in tattoos and permanent eyeliner may contain metallic iron oxide and could heat up during MRI; however, this is rare. 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, especially in the orbit, 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.
The technologist may ask if you have any drug allergies and whether you have undergone any surgery in the past. If you are or might be pregnant, please mention it to the MRI staff.
Some patients who undergo MRI in an enclosed unit may feel confined or claustrophobic. You should discuss this with your physician prior to your MRI exam. A mild sedative may be prescribed to help ease your anxiety.
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 more "patient-friendly" designs are rapidly coming into routine use. The "short-bore" systems 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, it does not rely on ionizing 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 body in one of several planes (right to left, front to back, upper to lower).
How is the procedure performed?
The patient is comfortably positioned on a special table which slides into the MRI system opening where the magnetic field is created. Then the technologist leaves the room and the individual MRI sequences are performed. The patient will hear tapping noises during the exam. The tapping is created when magnetic field gradient coils are switched on and off to measure the MRI signal reflecting back out of the patient's body. 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. Patients are generally required to remain still for only a few seconds to a few minutes at a time.
Depending on the part of the body being examined, a contrast material may be used to enhance the visibility of certain tissues or blood vessels.
What will I experience during the procedure?
MRI causes no pain, but some patients can find it uncomfortable to remain still during the examination. Others experience a sense of being "closed in," though the more open construction of newer MRI systems has done much to reduce that reaction. 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 an injection of contrast material 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 are provided to reduce the noise.
Who interprets the results and how do I get them?
A radiologist, 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. New technology also allows for distribution of diagnostic reports and referral images over the internet at some facilities.