What you should know about safe MR practices.
The guidance document represents the unanimous opinions of a panel of physicists, radiologists, MR technologists, nurses, cardiologists, anesthesiologists and other experts in MR and patient safety. Their recommendations are "reasonably applicable in the real world in which we all live, with all its patient care, financial and throughput pressures and considerations" and in all types of MR settings, the panelists wrote. In particular, the document offers up-to-the-minute guidance on key issues such as contrast administration, emergency preparedness, facility design and signage, and imaging patients who have pacemakers.
Gadolinium-based Contrast Agents
The new guidelines give special attention to gadolinium-based contrast agents used during MR imaging in light of a rare but potentially fatal condition in patients with advanced kidney disease. Considered extremely safe for most patients, gadolinium contrast has been linked to development of nephrogenic systemic fibrosis in about 200 patients with renal disease.
First observed a decade ago and officially described in 2000, NSF increases collagen deposits throughout the body. This thickens and hardens the skin, causes painful and debilitating joint contractures and can lead to fibrosis in the heart, lungs and pulmonary blood vessels. There is no cure, although some treatments have had limited success, including kidney transplantation, steroids and photophoresis. About one in four patients who develops NSF dies of the condition.
According to a study published in August 2006 in the Journal of the American Society of Nephrology, 13 patients with severe renal disease developed NSF between two and 75 days after receiving Omniscan, a gadolinium-based contrast agent manufactured by GE Healthcare. The authors of the study concluded that Omniscan "plays a causative role" in NSF. Other brands of gadolinium contrast also have been associated with NSF, including Magnevist, manufactured by Schering, and OptiMARK, manufactured by Mallinckrodt. However, about 90 percent of reported cases "seem to be clearly associated with Omniscan," most commonly as a result of high-dose administration.
Researchers hypothesized that when patients with impaired renal function receive gadolinium, the kidneys fail to clear it from the body and transmetalation occurs, releasing free gadolinium into the tissues. Gadolinium precipitate has been found in body tissues of patients with end-stage renal disease as long as 11 months after contrast was administered. Researchers also suggested that other, as yet undetermined factors, might contribute to the disease process.
The U.S. Food and Drug Administration updated its guidelines late in 2006 as a result of these findings. The FDA now advises that MR imaging with gadolinium-based contrast agents should be avoided for patients with moderate-to-end-stage kidney disease (i.e., stages 3, 4 or 5) whenever possible. In addition, if these patients do undergo MR scanning with gadolinium contrast, "prompt dialysis following the MRI or MRA (magnetic resonance angiography) should be considered."
Even though the cases of NSF have been linked primarily to Omniscan, the panel believes it is "appropriate to assume for now that a potential association might exist for all five FDA-approved gadolinium-based MR contrast agents until there are more definitive data to suspect otherwise." In addition, the lowest possible dose needed to obtain the necessary diagnostic results is strongly recommended. A half-dose should be considered the default for patients with renal disease, the panel advised.
Peritoneal dialysis does not seem to be effective for clearing gadolinium from the body, and hemodialysis should begin immediately after imaging for patients who currently are receiving hemodialysis, Dr. Kanal said. About 75 percent of contrast is removed during one hemodialysis session, and 95 percent is removed in three sessions, according to Richard C. Semelka, M.D., in a recent MedScape article.
When hurricanes Katrina and Rita ravaged the Gulf Coast in fall 2005, some MR facilities were caught unprepared. In a new appendix, the guidance document discusses preparing for natural disasters and other emergencies.
Electricity is vital to keeping the cryogen in an MR magnet liquid. Without power to cool the cryogen, the substance begins to boil off. In a matter of days, an uncooled MR magnet automatically "quenches" itself, discharging the remaining cryogen gas. This not only runs the risk of damaging the magnet coils, but also can be dangerous to those nearby.
Some facilities attempted to pre-emptively quench their MR magnets during the hurricanes. At one such New Orleans facility, "the quench tube reportedly failed and the pressure from the expanding cryogen blew out the control room radiofrequency window." Manual quenches should "be initiated only after careful consideration and preparation," the new guidelines warn. MR facilities should check the emergency exhaust systems and inspect the cryogen vent pipe before attempting a quench.
Ideally, planned and unplanned quenches can be avoided completely by assuring temporary power during emergency situations through on-site generators. Some larger facilities even generate their own power, the document noted.
Water is another potential threat to MR systems, and "it only takes a small quantity of water in contact with an MRI scanner to incapacitate or destroy the equipment." Facilities should carefully evaluate the potential for flooding in the area and prepare for it. Sturdy sheets of plastic should be taped over gantries and other equipment to prevent water damage from leaking roofs and pipes. Electronic equipment should be elevated off the floor in case of a flood.
Police and firefighters who might respond to an MR facility in an emergency need to be familiar with MR safety. Facilities should make a point of inviting local emergency responders to tour the facility and learn more.
The updated guidelines include a new appendix with architectural and interior design recommendations to improve safety. For example, the panel suggested that setting up a private area for patient screening interviews will increase the likelihood of patients disclosing that they have certain types of implants. They also noted that it's important to designate a temporary storage space for equipment that is unsafe in MR environments, such as ferromagnetic IV poles and stretchers.
Of particular note to radiologic technologists are new recommendations about signage in the MR facility, according to Nancy Sass, R.T.(R)(MR)(CT), an ACR program accreditation specialist who assisted the panel. The new guidelines suggest that every metal or partly metal object that enters the MR magnet room be labeled as "MR safe," "MR conditional" or "not MR safe."
"It really is the technologist's responsibility to check everything that comes into the area," Ms. Sass said.
Pacemakers and Defibrillators
The new guidelines contain stronger, more definitive language than the previous guidelines regarding imaging patients who have pacemakers and implanted cardiac defibrillators. "MRI of patients with pacemakers and ICDs is not routine. As of this writing, no cardiac pacing and/or defibrillating devices are labeled safe or conditionally safe for MRI scanning," the panel warned. It also noted that a few deaths have occurred during scanning of patients with pacemakers and ICDs, and that possible malfunctions include rapid or asynchronous pacing, failure to pace, battery depletion and complete device failure requiring replacement.
If imaging is undertaken, it should be only on a case-by-case basis with cardiologists and radiologists available throughout the procedure and a fully-stocked crash cart on hand.
By Katie Faguy, Radiologic Technology Editor
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|Title Annotation:||magnetic resonance|
|Date:||May 1, 2007|
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