Since being introduced in the late 1970s, the magnetic resonance imaging device (MRI) has become a valuable diagnostic tool. Unlike X-rays, the MRI does not use potentially dangerous radiation to produce images of the inside of the body. However, because it uses a magnetic field that is about 10,000 times stronger than that of the earth, MRIs can pose a danger to patients who have certain medical devices implanted.
Why Are MRIs Used?
An MRI machine allows doctors to see inside the body. Unlike x-rays, which show only hard structures, the MRI produces images of the soft tissues, such as the heart, lungs and brain. The MRI does this by using magnetic waves, which travel through the tissues at different rates, and as a result can be used to create detailed images, or scans, of the body.
Dangers from Magnetism
A patient inside an MRI machine lies within a huge magnet. Like all magnets, those within the MRI machine attract iron and iron-containing materials. Some objects are classified as "MR Safe," meaning that they are not affected by the MRI. Others are "MR Conditional," which means that they are not known to cause problems within specific conditions; these items must be evaluated on a case-by-case basis to determine if they can safely be around the particular type of MRI device. An item classified as "MR Unsafe" is known to pose a risk in an MRI environment. These items range from cardiac implants to scissors that contain iron. Patients with an implant that contains iron will be at risk of injury if they undergo an MRI because the powerful magnetic fields produced by the machine can make the object move, possibly leading to physical damage for the patient, or cause the device to malfunction Among the objects that are considered "MR Unsafe" are: certain medication pumps (such as insulin pumps) older vascular stents cochlear implants brain-aneurysm clips pacemakers or cardiac defibrillators gastrointestinal clips
In some cases, the metal object within the patient will become hot, possibly hot enough to burn, during an MRI. A catheter with metal parts, for example, could cause a burn injury. The ActiPatch, an electromagnetic device that aids in the healing of soft tissues, should not be worn during an MRI. Not only might the device be damaged, but its potential for heating is cause for concern. Heating is also an issue with body piercings. Depending on the mass of the metal in the piercing, a serious burn can result. Even if a burn does not occur, the movement of the piercing that is caused by the magnetic waves can become uncomfortable or even painful. Similarly, a patient who has a metal fragment near the eye or another organ should not undergo an MRI because the metal piece could become dislodged and damage the organ. This is a common problem facing iron workers and metal workers; before undergoing an MRI, they should have an X-ray to evaluate the location of the fragment.
Contrast Dye Complications
In 2007, Dr. Aneet Deo of St. Vincent's Medical Center in Bridgeport, Connecticut, along with Dr. Mitchell Fogel of the Columbia University College of Surgeons and Physicians and Dr. Shawn Cowper of Yale University School of Medicine, found a connection between the contrast dye used in magnetic resonance imaging and nephrogenic systemic fibrosis (NSF) in patients with impaired kidney function. The dye, which is used in about 5 to 10 percent of MRIs, contains gadolinium, which is normally removed from the bloodstream without harm to a healthy kidney. Damaged kidneys, however, cannot process the gadolinium; approximately 18 months after receiving the dye, patients began to show symptoms of NSF, including the development of thickened skin and subcutaeous tissues. The thickening can affect the tissues of the liver, lungs and the myocardium around the heart and may lead to death. In response to the research, the Food and Drug Administration in 2007 required manufacturers to include a black-box warning on the labels on their products that warns against using gadolinium-containing contrast dye with patients who have impaired kidney function.
Any iron-containing object within the MRI area presents a potential hazard. The magnetic field from the machine can pull on a pen and cause it to fly across the room—turning an innocent item into a potentially dangerous missile. As a result, such objects are not allowed in the scanning area. Before the scan, a patient will remove glasses and any hearing aids and will need to leave them outside. Although it is not a danger to the patient, image distortion can be caused by some metal objects. For example, metal jewelry, dental fillings and spinal rods have been known to disrupt the image. Some tattoos can also affect the quality of the image.