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 (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.
Highlights
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 Electrotechnical Commission (IEC) [1] and the US Food and Drug Administration [2], depend upon the total electric field Etotal
This study evaluated whether the method of “Ec-shield” could be extended to a birdcage coil, the most common type of coil used in human MRI
There was approximately 25% reduction in the average Ec and 70% increase (i.e., 231 vs. 134 V/m) in the maximum Ec within the whole sample when the coil was loaded with the conductive, dielectric, or weak saline phantom compared to the empty coil
Summary
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 Electrotechnical Commission (IEC) [1] and the US Food and Drug Administration [2], depend upon the total electric field Etotal. Conservative E-fields Ec caused by the scalar electrical potential on conductors give rise to a portion of sample loss referred to as “dielectric loss”. One of the motivations of this study to understand the mechanism 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), between 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
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