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.
References
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