Journal of Electromagnetic Analysis and Applications, 2013, 5, 271-280 http://dx.doi.org/10.4236/jemaa.2013.57043 Published Online July 2013 (http://www.scirp.org/journal/jemaa) 271 Analysis of Conservative and Magnetically Induced Electric Fields in a Low-Frequency Birdcage Coil * Bu S. Park 1# , Sunder S. Rajan 1 , Christopher M. Collins 2 , Leonardo M. Angelone 1 1 Division of Physics, Center for Devices and Radiological Health, Office of Medical Products and Tobacco, Food and Drug Admini- stration, Silver Spring, USA; 2 Department of Radiology, New York University, New York, USA. Email: # bu.park@fda.hhs.gov Received May 16 th , 2013; revised June 17 th , 2013; accepted June 25 th , 2013 Copyright © 2013 Bu S. Park et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT 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   c E 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   i E and was the major component of the total electric field   total E . The amplitudes of c E and 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 addi- tional scalar potential parallel to the original field. The increased electric field between coil and sample may cause in- creased 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. total E Keywords: MRI; FDTD; Loading Effect; Conservative Electric Field; Birdcage Coil 1. Introduction In magnetic resonance imaging (MRI), the signal to noise ratio (SNR) and the specific energy absorption rate (SAR), the dosimetric parameter used to establish safety limits for human subjects by the International Electro- technical Commission (IEC) [1] and the US Food and Drug Administration [2], depend upon the total electric field   total E . The total can be decomposed into a con- servative and magnetically-induced electric fields (E- fields) [3] and a distinction is often needed between the two components. Conservative E-fields E   c E caused by the scalar electrical potential on conductors give rise to a portion of sample loss also referred to as “dielectric loss” [4]. Magnetically-induced E-fields i   E are created by the time-varying magnetic fields [5], and give rise to a portion of sample loss also referred to as “inductive loss” [4]. In some cases it is possible to reduce the losses due to c E without changing the current distribution or magnetic field distribution using a so called “E c -shield” [6], and thus maintaining the desired sensitivity and field of view (FOV) while reducing SAR in the sample and/or the noise received from the sample [6-9]. A previous study [6] showed that this method could be applied to a solenoid coil. This study evaluated whether the method of “E c -shield” could be also extended to a birdcage coil, the most common type of coil used in human MRI. One of the motivations of this study to understand the mecha- nism of thermal injury to skin is currently the most common type of adverse event reported for MRI scans [10]. Another reason for this study is to find the effect of a conductive or a dielectric sample related to the safety assurance in a region of interest (ROI), particularly be- tween the RF coil and the sample. Previous research [9,11] showed that the total electric field inside a coil would be decreased with addition of a loading sample. * Disclaimer: The mention of commercial products, their sources, or their use in connection with material reported herein is not to be con- strued as either an actual or implied endorsement of such products by the Department of Health and Human Services. # Corresponding author. Copyright © 2013 SciRes. JEMAA