• How is the MRI performed?
• How should I prepare for the MRI?
• How does the MRI work?
• What will I experience during the MRI?
• Who interprets the results and how do I get them?
• What are the benefits vs. risks?
How is the MRI performed?
The patient is placed on a special table and positioned inside the opening of the MRI unit. A typical exam consists of two to six imaging sequences, each taking two to 15 minutes. Each sequence provides a specific image orientation and a specified degree of image clarity or contrast. Depending on the type of exam being done, the total time needed can range from 20 to 60 minutes, not counting the time needed to change clothing, and answer questions. When contrast material is needed, a substance called gadolinium is given by IV injection during one of the imaging sequences. It highlights blood vessels and pathology, making them stand out from surrounding tissues. If you are having you shoulder, knee, or other specific joint imaged an intra-articular injection of contrast maybe required. The injection is performed either prior to, or about half-way through the exam (please see section on “MRI Arthrography” for details). The technologist leaves the room during the actual imaging process, but the patient can communicate with them at any time using an intercom. A friend/relative or, if a child is being examined, a parent may stay in the room. When the exam is completed you may be asked to wait to make sure that more images are not needed.
How should I prepare for the MRI?
The magnetic field used for MRI will pull on any iron containing object in the body, such as a heart pacemaker, artificial heart valve, aneurysm clip in the brain, intrauterine device, vascular access port (most are MRI safe), metal plate, pins, screws or staples. If you have an implant you believe is safe in the MRI scanner, please bring the implant identification card or letter from the physician who put the implant in, that confirms the devices safety. You will be given a questionnaire to answer regarding these issues. The technologist should know about any such item and also whether you have ever had a bullet in your body, whether you ever worked with metals, or if you have had a joint replacement. If there is any question of metal fragments, especially in the eye, you may be asked to have an x-ray that will detect any such metal objects. In most cases, joint replacements, surgical staples, artificial heart valves, plates, pins and screws pose no risk during MRI if they have been in place for more than four to six weeks. Braces make it harder to properly adjust the MRI unit, but are safe. You will be asked to remove anything that might degrade the MRI images such as hairpins, jewelry, eyeglasses, hearing aids and any dental work that can be taken out. Some wigs contain metal and must be removed. Dyes used in tattoos and permanent eyeliner may contain metallic iron oxide and could heat up during MRI; however, this is rare. You should report any drug allergies to the technologist and should mention if there's any possibility that you might be pregnant. You can eat normally before the exam (unless told differently) and medications may be taken as usual. A young child should not eat or drink for about four hours if they will receive a sedative. You will wear a lightweight medical gown for the exam.
Some patients will feel uncomfortably confined (claustrophobic) when enclosed in an MRI unit. If necessary, you can schedule your appointment with sedation which will be given fifteen to thirty minutes before your exam to help put you at ease. You will be asked to bring someone to drive you home if you receive a sedative. Roughly one in twenty patients will require medication. Another option is to use an “open” MRI unit. However, the strength of these open magnets is generally less than that of closed magnets, often resulting in lower-quality images.
MRI is a unique imaging method because, unlike the usual radiographs (x-rays), radioisotope studies and even CT scanning, 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 depicts differences in the content and distribution of water in various tissues. With MRI, different types of tissue within the same body structure are clearly displayed in fine anatomic detail. In the spine, for instance, fatty tissue, cerebrospinal fluid and the intervertebral discs contain considerable water, more than is found in bone, cartilage and nerve tissue. MRI is well suited to detecting conditions that increase the amount of fluid, such as tumors, inflammation and infection. Even different types of tissue within the same organ, such as the gray and white matter of the brain, can easily be distinguished. Special imaging techniques are used to image various structures, such as arteries in the case of MRA.
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).
What will I experience during the MRI?
MRI causes no pain, but there may be a feeling of claustrophobia 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 are provided to reduce the noise. Also, if you are not having you head scanned we can provide you with stereo headsets.
Who interprets the results and how do I get them?
A subspecialty trained 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. A preliminary interpretation may be available shortly after the exam and a formal report of the complete results will be sent to the referring physician within a day. Your primary physician or the radiologist may discuss the findings of the MRI examination with you. New technology also allows for distribution of referral images over the Internet from our facility to selected physicians.
What are the benefits vs. risks?
Benefits:
• Images of the brain, spine, joints and soft tissue 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 MR angiography (MRA) provides detailed images of blood vessels in the brain and other body parts-oftentimes without the need for contrast material. The risk of an allergic reaction from MRA contrast is extremely low and kidney damage does not occur.
• New MRI systems can depict brain function, and in this way detect a stroke at a very early stage.
Risks:
• There are no definite side effects from any type of MRI study, including MR angiography.
• An undetected metal implant may be affected by the magnetic field. The procedure should be avoided in any patient with a pacemaker, implanted neurostimulator, certain types of metallic ear implants (cochlear implants) or metallic object within the eye socket. It should also be avoided if the patient has a port for delivering insulin or chemotherapy (however, many ports are detachable so patients can safely undergo MRI examinations). If you have an implant you believe is safe in the MRI scanner, please bring the implant identification card or letter from the physician who put the implant in, that confirms the devices safety.
• 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.
• Women who are breast-feeding should inform the radiologist and ask how to proceed. If necessary they may pump breast milk before the exam for use until the gadolinium contrast material has cleared from the body.
Copyright © 2005 Radiological Society of North America, Inc.
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© 2006 Mink Radiologic Imaging, Inc.