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.

Slice-Selective Transmit Array Pulses for Improvement in Excitation Uniformity and Reduction of SAR

To overcome challenges of inhomogeneous transmit B1 distribution and high specific energy absorption rate (SAR) in MRI, we compare slice-selective array-optimized composite pulse and RF shimming designed to both improve B1 uniformity and reduce SAR using an 8-channel transmit head array loaded with a head model at various RF pulse excitation times, and compare results with standard quadrature voltage distribution at 3T (128 MHz) and 7T (300 MHz). The excitation uniformity was estimated throughout the 3D brain region and SAR was calculated for the whole head. The optimized composite pulse could produce significantly better homogeneity and significantly better homogeneity when SAR was not constrained, and both significantly better homogeneity and lower SAR when the pulse duration was allowed to be twice that of the quadrature or RF shimmed pulse. When the total pulse durations were constrained to the same length, the relative advantages of the optimized composite pulse for producing better homogeneity and lower SAR simultaneously were diminished. Using the optimization results, the slice-selective composite pulse sequence was implemented on a 3D MRI simulator currently under development, and showed both effective slice selection and improvement in excitation uniformity compared to a conventional quadrature driving method.