Safety concerns of 3-Tesla MR scanners

Safety Considerations of 3T MR Scanners

Obviously, the safety issues involve the stronger magnetic field of the 3T system. The most conspicuous concern is the static magnetic field strength and the so-called projectile effect.
True, some guidelines for the MR environment remain the same with the stronger magnets. “Guidelines for general behavior in the MR environment, as far as watching out for strong static fields, will be the same for new systems and systems currently being used. This is especially true for the projectile effect, where ferromagnetic objects close to the static magnetic field can become dangerous projectiles that could injure or kill anyone between the object and the magnet. A 1.5T magnet has 30,000 times the strength of the earth’s magnetic field, while a 3T magnet has the force of 60,000 times the strength. When you reach forces of that level, it’s difficult to draw a line marking the point at which an object becomes a harmful projectile. How much more lethal would an unsecured oxygen tank become in the presence of a 3T system than near a 1.5T system?

Magnetic Field Effects

The most considerable concerns involve the effect of the magnetic field on medical devices and implants. Some medical devices are safe and compatible at 1.5 but not at 3T. A metallic device with weak ferromagnetic qualities in relation to a 1.5 system may experience significant magnetic field interaction at 3T.
Problems presented by 3T for metallic implants include translational attraction and torque. Translational attraction is essentially the projectile effect, when an object moves sideways toward a magnet. By comparison, torque, as it relates to MRI, refers to the shifting or twisting of ferromagnetic medical devices and implants inside the patient’s body. The movement is caused by the static magnetic field and can cause discomfort or injury if an implant is displaced. It can even cause death if the movement involves a life-sustaining device. Most reported cases of MR-related injuries and the few fatalities that have occurred have apparently been the result of failure to follow safety guidelines or of use of inappropriate or outdated information related to the safety aspects of biomedical implants and devices. To prevent accidents in the MR environment, therefore, it is necessary to revise information on biologic effects and safety according to changes that have occurred in MR technology and with regard to current guidelines for biomedical implants and devices.
In the study, which was published in the March 2003 issue of the American Journal of Neuroradiology (“Aneurysm Clips: Evaluation of Magnetic Field Interactions and Translational Attraction by Use of ‘Long-Bore’ and ‘Short-Bore’ 3.0-T MR Imaging Systems,” [Shellock, Jean A. Tkach, Paul M. Ruggieri, Thomas J. Masaryk, and Peter A. Rasmussen]), the researchers evaluated magnetic field interactions for 32 aneurysm clips in association with exposure to “long-bore” and “short-bore” 3T MR systems.Each clip was quantitatively assessed for translational attraction and qualitatively evaluated for torque. The researchers found that 17 of the 32 aneurysm clips showed positive magnetic field interactions. Specifically, 15 aneurysm clips made from commercially pure titanium and titanium alloy displayed no translational attraction, while 17 clips made from stainless steel alloy, Phynox, and Elgiloy displayed positive deflection angles. According to the researchers, the 32 different aneurysm clips passed the deflection angle test by using the long- and short-bore 3T MRI systems, which indicated that they are safe for patients and other persons in MR environments. However, the authors write, only clips made from the titanium and titanium alloy are entirely safe for patients undergoing MR imaging procedures because of the total lack of magnetic field interactions. The remaining clips require characterization of magnetic interactions.

Greater Heat Potential

Other crucial elements of safety are radiofrequency (RF), heat, and the specific absorption rate. RF energy pulses are used in every MR system to generate the signal measured during each scan. The absorption of RF power into the body causes heating of the tissue. Unregulated absorption can lead to injury.