Since its development in the mid 1970s, Magnetic Resonance Imaging, or MRI, has grown into a mainstream diagnostic imaging tool used in a wide variety of conditions. The unique ability of MRI to demonstrate the anatomy, and more recently the function of soft tissues in the body, continues to find new application in patient care. It is not surprising, given the widespread application of MRI, that the occasional myth should find its way into the latest TV medical drama, Web page or even physician/patient discussions. The following are several of the more common myths.
Myth: I can’t have an MRI because I have had a previous joint replacement.
Reality: Because the MRI uses a powerful magnet, it is important to keep metallic objects that are attracted to the magnetic field out of the scanning area. Due to the strength of the MRI magnet, even a small metallic object such as a paper clip, can have substantial force as it is drawn into the magnet, and could cause injury if the object struck someone inside the MRI scanner. For that reason, access to the MRI scanning area is restricted, and all patients are screened to determine if they have metallic devices that may place them at risk prior to their MRI exam. Fortunately, the majority of medical devices used to care for patients are not affected by that magnetic field and can be safely scanned. This includes such common devices such as orthopaedic joint replacements and implanted hardware used to treat fractures.
It is always important to discuss prior operations or implanted devices that you may have had with your doctor before having an MRI. In addition, the technologist performing your MRI examination will ask you a series questions to make sure your MRI can be performed safely. Some devices, such as certain brain aneurysm clips and cardiac pacemakers, may not be safe to place in an MRI scanner, and could result in serious injury or death. With increased use of MRI in the care of patients, more manufacturers are designing and building their medical devices to be safe and compatible with MRI. However, it is never good to simply assume that a device is safe.
To ensure that your scan can be performed safely, it is necessary to know the type of device that was implanted, and in some cases the make, model number, and year that the device was implanted. While some devices may be tested and found to be safe in the weaker magnets used in most clinical MRI scanners, they may not have been tested in the more powerful magnets now being introduced into clinical practice. In practice, most patients with implanted medical devices can have an MRI examination, but it is always important to make the technologists and physicians performing your MRI aware of any implanted devices or metal objects before entering the scanning area.
Myth: The MRI contrast agent will damage my kidneys.
Reality: Some, but certainly not all, MRI examinations may require the injection of a contrast agent to allow the radiologist to make a better diagnosis. Over the 20 years that MRI contrast has been used, it has been a very safe drug with a very low incidence of side effects. Compared to iodinated contrast agents used in some X-ray studies and CT, MRI contrast is less toxic to the kidneys and can be safely given to nearly all patients. While MRI contrast agents alone are considered safe and do not pose a serious risk of kidney damage, they can cause problems when given to patients that already have problems with their kidneys.
In recent years, there has been growing recognition of a condition called nephrogenic sclerosing fibrosis or NSF that is related to exposure to MRI contrast agents. NSF is a rare condition in which patients develop swelling, and thickening of the skin that can progress to fibrosis where the skin feels “woody” and mobility is limited. There is evidence that this condition can occur when patients with pre-existing problems with their kidneys received MRI contrast agents that contain gadolinium. In particular, patients with severe kidney problems requiring dialysis are at risk if they receive MRI contrast agents. Although the cause of NSF remains to be determined, it is thought that it may be related to the limited ability of the damaged kidneys to remove the contrast agent from the blood, allowing gadolinium to accumulate in the skin.
If you are a patient with kidney problems and require an MRI examination, it is important that you discuss this with your physician and the MRI technologist performing your scan. It may be possible to modify the MRI study so that the necessary diagnostic information can be obtained without giving you contrast. If contrast is necessary, the type and amount of contrast may need to be adjusted. It is important to coordinate your MRI study with your dialysis schedule to reduce the risk of NSF. Additional information can be found at the official NSF Web site, http://www.icnfdr.org. (Info from Cowper SE. Nephrogenic Fibrosing Dermopathy [NFD/NSF Website]. 2001-2007. Accessed 03/03/2008.)
Myth: All MRI scanners are the same.
Reality: Of all the techniques that radiologist use to look inside of the human body, MRI provides the weakest signal. In practical terms, making MRI images is like trying to take a photograph at night without a flash. It is possible, but can be very challenging. Just like photography where the film, or in digital photography the detector, must be exposed to a certain amount of light to yield an acceptable photograph, in MRI a certain amount of signal must be accumulated to yield an acceptable image. A photographer has several options to obtain an image under low-light conditions. One option is to purchase an expensive camera that efficiently uses the available light, allowing images to be acquired at reasonable exposure times. Alternatively, it may be possible to use a less expensive camera, place it on a tripod, and hope the subject does not move during the long exposure. The situation is the same with an MRI examination.
In MRI, the strength of the signal is directly proportional to the strength of the magnet. The strength of the MRI magnet is measured in Tesla (T) with most clinical MRI scanners using magnets that range between 0.5 T to 3.0 T. MRI research scanners have been built that are much stronger (7.0 T to 11 T), but are not used routinely used in clinical practice. For comparison, the earth’s magnetic field is approximately 0.00005 T. Over the years the strength of MRI magnets has increased primarily due to the desire to produce better images. At the same time, there is the desire to make the examination as comfortable as possible for the patient. This has led to a variety of different types of MRI scanners that seek to provide acceptable image quality and patient comfort.
A form of MRI scanner that has received much attention is the “open” MRI scanner. In this type of scanner, the magnet consists of two large plates with an open space where the patient is placed during the MRI scan. With early versions of this type of scanner, the magnetic field generated by these plates was relatively weak (< 0.5 T), and thus the time to acquire the image, similar to the exposure time, was quite long. Any movement by the patient during that time blurred the image making them challenging to interpret. While the openness of the MRI made patients more comfortable, this was off set by the longer time patients needed to remain still in the magnet to obtain acceptable images. To address this concern, manufacturers began designing and building open configuration MRI scanners with stronger magnets. Although open scanners comprise the weaker magnets used in clinical practice, they can provide acceptable image quality in reasonable scan times for most routine clinical MRI examinations.
An alternative MRI design is the cylindrical magnet, sometimes erroneously referred to as a “closed” magnet to contrast it with the “open” magnets. These MRI systems generally use strong magnets. Although early magnets were long narrow cylinders sometimes up to 3 meters in length and 55 centimeters in width, more recent scanners use shorter and wider magnets. This design allows the benefits of using a strong magnet, such as shorter imaging time, while providing a less confining environment for the patient.
Although excellent image quality and patient comfort are important, there are many other components necessary to obtaining a high quality MRI examination. The ultimate benefit of the MRI examination is not pretty pictures, but an accurate diagnosis that will improve the care of the patient. For some conditions, an accurate diagnosis can be made with images with limited detail, while for other conditions a high level of detail is essential.
For example, in knee MRI, the strength of the magnet does not appear to have a significant effect on the ability to diagnose tears of the knee meniscus, a common indication for obtaining a knee MRI study; however, diagnosis of injuries to the articular cartilage of the knee requires very high resolution images. In some cases this may require specialized equipment, training, and imaging techniques. Several studies have shown that the experience and training of the interpreting radiologist is one of the most important factors in achieving an accurate diagnosis. In summary, not all MRI examinations are alike, there are many factors that are important to consider in determining where you obtain your study.
Timothy J. Mosher, M.D., is vice chair, radiology research; chief, MRI and musculoskeletal imaging at Penn State Hershey Medical Center.