The increase in the use of magnetic resonance imaging (MRI) for diagnostic use in companion animals has increased the demand for anesthesia support in a strong magnetic environment. In many instances, this may necessitate anesthesia being provided by individuals that are unfamiliar with MRI and the hazards associated with it. The objective of this article is to describe the conditions and hazards associated with a strong magnetic field, review considerations for safe and effective anesthetic management of patients during an MRI, and promote close collaboration and communication between personnel in an effort to insure staff awareness and safety. This report describes conditions that exist for the superconducting high field strength magnets, 1.0, 1.5, and 3.0 Tesla, that are commonly used for clinical imaging of companion animals. Many of these same safety and anesthesia considerations can be applied to any MRI facility. Magnetic resonance imaging (MRI) has been used in clinical veterinary diagnostics since the late 1980s. Before the availability of veterinary imaging facilities, MRIs for companion animals were performed at facilities designed for human imaging.
Safety Aspects of MRI
Dangers Associated With MRI
There is no ionizing radiation exposure associated with MRI, but the MRI suite is still a potentially dangerous location because of the presence of a strong static magnetic field, high frequency electromagnetic (RF) waves, and time-varied (pulsed) magnetic gradient fields. Other hazards include high-level acoustic noise, systemic and localized heating, and accidental projectiles. Magnets are considered medical devices but are not regulated by the U.S. Food and Drug Administration (FDA). Anesthetists should be aware of the risks associated with working around this powerful imaging tool to prevent adverse events.
The primary feature of the magnetic resonance imaging system is a powerful magnet. A static magnetic field is created with superconducting coils and is measured in Tesla (T). High field strength, clinically useful magnets have magnetic fields measuring 1.0, 1.5, and 3.0 T. The earth's gravitational pull, or the natural magnetic field, is 0.05 mT or 0.5 Gauss (G). There are 10,000 G in 1 T. Therefore, a 1.5-T magnet is 30,000 times stronger than gravity and a 3.0-T magnet is an amazing 60,000 times the pull of gravity. (Kelmer, 2009)
A large invisible magnetic field, called a fringe field, extends in all directions, well beyond the confines of the magnet and the magnet room. The strength of this fringe field depends on the strength and shielding the magnet. The magnetic field is weakest at its outside edge and its strength increases rapidly in the immediate vicinity of the magnet. It is important to know the location of the 5 Gauss line. This designates a safety point, inside which access should be limited. The increase in magnetic force beyond this point can cause accidents and medical device malfunction. Actively shielded magnets have a more compact fringe field. This means that the rate of field strength increase is greater the closer you get to the magnet. A map of the fringe fields associated with individual magnets can be obtained ...