3.0-Tesla MR Safety Information for Implants and Devices
Because previous investigations performed to evaluate MR safety issues for implants and devices used mostly scanners with static magnetic fields of 1.5-Tesla or less, it is crucial to perform ex vivo testing at 3-Tesla to determine possible risks for these objects with respect to magnetic field interactions and heating. Importantly, a metallic object that displayed “weakly” ferromagnetic qualities at 1.5-Tesla may exhibit substantial magnetic field interactions at 3-Tesla.
Furthermore, for elongated devices or those that form a loop of a certain diameter, the effects of MRI-related heating may be substantially different. For example, evidence from a study conducted by Shellock et al. reported that significantly less MRI-related heating occurred at 3-Tesla/128-MHz (MR system reported whole body averaged SAR, 3-W/kg) versus 1.5-Tesla/64-MHz (MR system reported whole body averaged SAR, 1.4-W/kg) for a pacemaker lead not connected to a pulse generator (same lead length, positioning, etc.). This phenomenon, whereby less heating was observed at 3-Tesla/128-MHz vs. 1.5-Tesla/64-MHz, has also been observed for external fixation devices, Foley catheters with temperature sensors, neurostimulation systems, relatively long vascular stents, and other objects (Unpublished Observations, F.G. Shellock, 2012). Therefore, it is crucial to conduct ex vivo testing to assess magnetic field interactions and, for certain devices, MRI-related heating to identify potentially hazardous objects prior to subjecting individuals to the MR environment or patients to an MR examination at 3-Tesla.
Magnetic Field Interactions at 3-Tesla. From a magnetic field interaction consideration, translational attraction and/or torque may cause movement or dislodgment of a ferromagnetic implant resulting in an uncomfortable sensation or injury to a patient or individual. Translational attraction is dependent on the strength of the static magnetic field, the spatial gradient magnetic field, the mass of the object, the shape of the object, and its magnetic susceptibility. The effects of translational attraction on external and implanted ferromagnetic objects are predominantly responsible for possible hazards in the immediate area of the MR system. That is, as one moves closer to the MR system or is moved into the scanner for an examination. An evaluation of torque is also important for a metallic object, especially if it has an elongated configuration. Qualitative and quantitative techniques have been used to determine magnetic field-related torque for implants and devices.
From a practical consideration, in addition to the findings for translational attraction and torque, the “intended in vivo use” of the implant or device must be considered as well as mechanisms that may provide retention of the object in situ (e.g., implants or devices held in place by sutures, granulation or ingrowth of tissue, fixation devices, or by other means) with regard to potential risks for a given metallic object.
Long-Bore vs. Short-Bore 3-Tesla MR Systems. Different magnet designs exist for commercially available 3-Tesla MR systems, including configurations that are conventional “long-bore” scanners and “short-bore” systems. Because of physical differences in the position and magnitude of the highest spatial gradient magnetic field for different magnets, measurements of deflection angles for implants using long-bore vs. short-bore MR systems can produce substantially different results for deflection angle measurements (i.e., translational attraction), as reported by Shellock et al. Studies conducted using 3-Tesla MR systems indicated that, in general, there were significantly (p less than 0.01) higher deflection angles measured for implants in association with exposure to short-bore vs. the long-bore MR systems. The differences in deflection angle measurements for the metallic objects were related to differences in the highest spatial gradient magnetic fields for short-bore vs. long-bore scanners.
The safety implications are primarily for magnetic field-related translational attraction with respect to short-bore versus long-bore 3-Tesla MR systems. For example, the deflection angle measured for an implant on a short-bore can be substantially higher (and thus, potentially unsafe from a magnetic field interaction consideration) compared to the deflection angle measured on a long-bore MR system. Therefore, safety information for measurements of magnetic field interactions for metallic objects must be considered with regard to the specific type of MR system used for the evaluation or, more accurately, with respect to the level of the highest spatial gradient magnetic fields that used for the tests.
Heating of Implants and Devices at 3-Tesla. Ex vivo testing has been used to evaluate MRI-related heating for various metallic implants, materials, devices, and objects of a variety of sizes, shapes, and metallic compositions. In general, reports have indicated that only minor temperature changes occur in association with MR procedures involving metallic objects that are relatively small passive implants (e.g., those that are not electronically-activated). Therefore, heat generated during an MR procedure performed at 3-Tesla involving a patient with relatively small, passive metallic implant does not appear to be a substantial hazard.
However, because excessive heating and burns have occurred in association with implants and devices that have elongated configurations or that form conducting loops of a certain diameter, patients with these objects should not undergo MR procedures at 3-Tesla until ex vivo heating assessments are performed to determine the relative risks. Ex vivo investigations have demonstrated that excessive heating may occur for certain implants related to MRI performed at 3-Tesla under certain operating conditions.
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