MRI safety: RF-induced heating near straight wires
ABSTRACT One of the most crucial magnetic resonance (MR) safety concerns is related to excessive heating of metallic implants by the radio frequency (RF) magnetic field. In this study, heating by the MR imaging (MRI) RF magnetic field of bare and insulated wires was evaluated to model the heating of medical lead wires. Currents induced in the wire were calculated using the method of moments. The electric field in the tissue surrounding the wire was calculated to determine the power deposition. From the power, the temperature rise was calculated using the bioheat equation. For bare wires, the calculated and maximal temperature rise, which is about 28°C, occurred for a length of 20 cm. For lengths exceeding 20 cm, temperature rises for the insulated wires were greater, and the resonance length exceeded 40 cm.
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ABSTRACT: PurposeThe concept of a “radiofrequency safety prescreen” is investigated, wherein dangerous interactions between radiofrequency fields used in MRI, and conductive implants in patients are detected through impedance changes in the radiofrequency coil. TheoryThe behavior of coupled oscillators is reviewed, and the resulting, observable impedance changes are discussed. MethodsA birdcage coil is loaded with a static head phantom and a wire phantom with a wire close to its resonant length, the shape, position, and orientation of which can be changed. Interactions are probed with a current sensor and network analyzer. ResultsImpedance spectra show dramatic, unmistakable splitting in cases of strong coupling, and strong correlation is observed between induced current and scattering parameters. Conclusions The feasibility of a new, low-power prescreening technique has been demonstrated in a simple phantom experiment, which can unambiguously detect resonant interactions between an implanted wire and an imaging coil. A new technique has also been presented which can detect parallel transmit null modes for the wire. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.Magnetic Resonance in Medicine 03/2014; · 3.27 Impact Factor
Conference Paper: Mitigating RF heating near medical devices in Magnetic Resonance Imaging[Show abstract] [Hide abstract]
ABSTRACT: Because of its excellent soft tissue imaging capability, Magnetic Resonance Imaging (MRI) is a desirable diagnostic modality. This modality, however, is presently denied to many patients with implanted medical devices due to adverse interaction of the device with the electromagnetic fields of the MRI system. These adverse interactions include heating near the tissue-electrode interface due to the radiofrequency (RF) field. This work proposes a technique to mitigate the RF heating by a field reduction scheme. While the work concentrates on simplified structures that can be well understood from an analytical approach, the method shown can be extended to the more complex geometry of an implanted lead.Antennas and Propagation Society International Symposium (APSURSI), 2012 IEEE; 01/2012
Conference Paper: Scattering by implants during MRI: A simplified computational approach[Show abstract] [Hide abstract]
ABSTRACT: During magnetic resonance imaging (MRI), an implanted lead present in a patient's body scatters the radiofrquency (RF) field used for magnetic resonance. The resultant RF field can achieve very high values in the vicinity of the implant structure. The conduction currents flowing in tissue can cause dangerous resistive heating. Therefore scattering by implanted devices is an important safety issue in MR scanning. The scattering problem is large and computationally expensive. However the scattered field produced by the implant has a significant strength only in the vicinity of the implant and does not extend over a large tissue volume. Using this fact, a hybrid finite element-method of moments (FEM-MoM) formulation is used in this paper to compute the scattered fields of different implants.Electromagnetics in Advanced Applications (ICEAA), 2010 International Conference on; 01/2010