Analysis of Conservative and Magnetically Induced Electric Fields in a Low-Frequency Birdcage Coil


Numerical methods are used to evaluate variations of the electromagnetic fields generated by a head-sized birdcage coil as a function of load (“loading effect”). The loading effect was analyzed for the cases of a coil loaded with a conductive cylindrical sample, a dielectric cylindrical sample, and an anatomically precise head model. Maxwell equations were solved by means of finite difference time domain (FDTD) method conducted at 12.8, 64, and 128 MHz. Simulation results indicate that at 12.8 MHz the conservative electric field (Ec) caused by the scalar electric potentials between the coil and the load or within the load was significantly higher than the magnetically-induced electric field (Ei) and was the major component of the total electric field (Etotal). The amplitudes of Ec and Etotal are seen to be lower within a sample than at a corresponding location in an empty coil, but approximately 65% higher in the space between coil and sample than at a corresponding location in an empty coil. This is due to polarization effects generating an additional scalar potential parallel to the original field. The increased electric field between coil and sample may cause increased power deposition at the surface of the sample and may affect the RF-induced currents in external leads used for physiological recording, i.e. ECG, during MRI scanning.

Share and Cite:

B. Park, S. Rajan, C. Collins and L. Angelone, "Analysis of Conservative and Magnetically Induced Electric Fields in a Low-Frequency Birdcage Coil," Journal of Electromagnetic Analysis and Applications, Vol. 5 No. 7, 2013, pp. 271-280. doi: 10.4236/jemaa.2013.57043.

Conflicts of Interest

The authors declare no conflicts of interest.


[1] International Electrotechnical Commission (IEC), “International Standard, Medical Equipment Part 2: Particular Requirements for the Safety of Magnetic Resonance Equipment for Medical Diagnosis,” 3rd Edition, International Electrotechnical Commission, Geneva, Vol. 601, 2010, pp. 2-33.
[2] “Guidance for Industry and FDA Staff: Criteria for Significant Risk Investigations of Magnetic Resonance Di agnostic Devices,” 2003.
[3] B. S. Park, A. G. Webb and C. M. Collins, “A Method to Separate Conservative and Magnetically-Induced Electric Fields in Calculations for MRI and MRS in Electrically-Small Samples,” Journal of Magnetic Resonance, Vol. 199, No. 2, 2009, pp. 233-237. doi:10.1016/j.jmr.2009.05.007
[4] D. I. Hoult and P. C. Lauterbur, “The Sensitivity of the Zeugmatographic Experiment Involving Human Samples,” Journal of Magnetic Resonance, Vol. 34, No. 2, 1979, pp. 425-433.
[5] W. Mao, B. A. Chronik, R. E. Feldman, M. B. Smith and C. M. Collins, “Consideration of Magnetically-Induced and Conservative Electric Fields within a Loaded Gradient Coil,” Magnetic Resonance in Medicine, Vol. 55, No. 6, 2006, pp. 1424-1432. doi:10.1002/mrm.20897
[6] B. S. Park, T. Neuberger, A. G. Webb, D. C. Bigler and C. M. Collins, “Faraday Shields within a Solenoidal Coil to Reduce Sample Heating: Numerical Comparison of De signs and Experimental Verification,” Journal of Magnetic Resonance, Vol. 202, No. 1, 2010, pp. 72-77. doi:10.1016/j.jmr.2009.09.023
[7] D. G. Gadian and F. N. H. Robinson, “Radiofrequency Losses on NMR Experiments on Electrically Conducting Samples,” Journal of Magnetic Resonance, Vol. 34, No. 2, 1979, pp. 449-455.
[8] A. Krahn, U. Priller, L. Emsley and F. Engelke, “Resonator with Reduced Sample Heating and Increased Homogeneity for Solid-State NMR,” Journal of Magnetic Resonance, Vol. 191 No. 1, 2008, pp. 78-92. doi:10.1016/j.jmr.2007.12.004
[9] F. D. Doty, J. Kulkarni, C. Turner, G. Entzminger and A. Bielecki, “Using a Cross-Coil to Reduce RF Heating by an Order of Magnitude in Triple-Resonance Multinuclear MAS at High Fields,” Journal of Magnetic Resonance, Vol. 128 No. 2, 2006, pp. 239-253. doi:10.1016/j.jmr.2006.06.031
[10] P. T. Hardy Jr. and K. M. Weil, “A Review of Thermal MR Injuries,” Radiologic Technology, Vol. 81, No. 6, 2010, pp. 606-609.
[11] Q. X. Yang, “A Method of Utilization of High Dielectric Constant (HDC) Materials for Reducing SAR and Enhancing SNR in MRI,” US Patent No. 20,110,152,670, 2011.
[12] K. R. Minard and R. A. Wind, “Solenoidal Microcoil design-Part II: Optimizing Winding Parameters for Maximum Signal-to-Noise Performance,” Nuclear Magnetic Resonance, Vol. 13 No. 3, 2001, pp. 190-210. doi:10.1002/cmr.1008
[13] C. M. Collins and M. B. Smith, “Calculations of B1 Distribution, SNR and SAR for a Surface Coil Adjacent to an Anatomically-Accurate Human Body Model,” Magnetic Resonance in Medicine, Vol. 45, No. 4, 2001, pp. 692-699. doi:10.1002/mrm.1092
[14] C. M. Collins and M. B. Smith, “Signal-to-Noise Ratio and Absorbed Power as Functions of Main Magnetic Field Strength, and Definition of ‘90?’ RF Pulse for the Head in the Birdcage Coil,” Magnetic Resonance in Medicine, Vol. 45, No. 4, 2001, pp. 684-691. doi:10.1002/mrm.1091
[15] K. Yee, “Numerical Solution of Initial Boundary Value Problems Involving Maxwell’s Equations in Isotropic media,” IEEE Transactions on Antennas and Propagation, Vol. 14, No. 3, 1966, pp. 302-307. doi:10.1109/TAP.1966.1138693
[16] C. Gabriel, T. Y. A. Chan and E. H. Grant, “Admittance Models for Open Ended Coaxial Probes and Their Place in Dielectric Spectroscopy,” Physics in Medicine and Biology, Vol. 39, No. 12, 1994, pp. 2183-2200. doi:10.1088/0031-9155/39/12/004
[17] Federal Communication Commission (FCC), “Body Tissue Dielectric Parameters,” 2010.

Copyright © 2024 by authors and Scientific Research Publishing Inc.

Creative Commons License

This work and the related PDF file are licensed under a Creative Commons Attribution 4.0 International License.