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 or higher.
Furthermore, for elongated devices or those that form a loop of a certain diameter, the effects of MRI-related heating may be substantially different at 3-Tesla/128-MHz versus, for example, 1.5-Tesla/64-MHz. Evidence from an ex vivo study conducted by Shellock, et al. (2005) 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 in the phantom, etc.). This phenomenon, whereby less heating was observed at 3-Tesla/128-MHz versus 1.5-Tesla/64-MHz, has also been observed for external fixation devices, Foley catheters with temperature sensors, neurostimulation systems, relatively long peripheral vascular stents, and other objects (Unpublished Observations, F.G. Shellock, 2011). 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 3-Tesla 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. 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 serious 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 at 3-Tesla.
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 the metallic object (for further information on this topic, please refer to the prior section in this textbook, General Information).
Long-Bore vs. Short-Bore 3-Tesla MR Systems. Different magnet designs exist for commercially available 3-Tesla MR systems, including configurations that are older “long-bore” scanners and “short-bore” systems. Because of physical differences in the position and magnitude of the highest spatial gradient magnetic fields 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<0.01) higher deflection angles measured for implants using 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 versus 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 field that was used for the test.
Lenz Effect. In 1835, Heinrich Lenz stated the law that an electric current induced by a changing magnetic field will flow such that it will create its own magnetic field that opposes the magnetic field that created it. These opposing fields, which occupy the same space at the same time, result in a pair of forces. The more current that is generated, the greater the force that opposes it. The so-called “Lenz Effect” occurs with electrically?conductive materials (i.e. not just ferromagnetic materials) that develop magnetic field eddy currents in the presence of high-field-strength static magnetic fields, such as those associated with MR systems.
Forces described by the Lenz Effect may restrict movement for metallic objects (e.g., for an aluminum oxygen tank) or compromise the function of certain implants in the MRI setting, such as those with moving metallic parts. This may be a concern for prosthetic heart valves that have leaflets or discs, especially if the implant is used for mitral valve replacement, where the range of pressures that are present are relatively low (higher pressures are more likely to overcome any significant Lenz Effect).
Condon and Hadley (2000) first reported the theoretical possibility of electromagnetic interaction with heart valve prostheses that contain metallic disks or leaflets. In theory, “resistive pressure” may develop with the potential to inhibit both the opening and closing aspects of a mechanical heart valve prosthesis that has leaflets or disks.
Heating of Implants and Devices at 3-Tesla/128-MHz. 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/128-MHz on a patient with a relatively small, passive metallic implant does not appear to be a substantial hazard.
However, because excessive heating and burns have occurred in implants and devices that have elongated configurations or that form conducting loops of 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 some implants related to MRI performed at 3-Tesla/128-MHz under certain operating conditions.
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