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ELECTROMAGNETIC SIMULATION OF PARALLEL TRANSMIT RADIOFREQUENCY COILS AND HIGH PERMITTIVITY MATERIALS USING CIRCUIT-SPATIAL OPTIMIZATION WITH VIRTUAL OBSERVATION POINTS
The recent FDA regulatory clearance for the 7 tesla Magnetic Resonance Imaging (MRI) system has led to increased interest in clinical ultra-high field (UHF) applications. However, to robustly achieve the expected increase in signal-to-noise ratio (SNR) at UHF, the radiofrequency (RF) challenges need to be met, namely, problems with higher RF power, worse B1+ inhomogeneity (signal voids) and increased tissue dielectric properties at higher frequency, all of which usually results in increased specific absorption rate (SAR). The parallel transmission (pTx) techniques are generally accepted as a realistic solution, providing improvement in the B1+ homogeneity with good RF efficiency while reducing peak local SAR. We designed a hybrid circuit-spatial domain optimization to accelerate the design of a double row pTx head coil. The method predicted consistent coil scattering parameters, component values and B1+ field. RF shimming of the calculated field maps matched in vivo performance. To further increase the B1+ homogeneity in tissue, we added high dielectric material (HPM) pads near the coil, as the displacement currents in the HPM induced secondary B1+ in tissue. This raises a RF safety question of how to monitor millions of local SAR (complex valued Q-matrix) in the tissue voxels, for any weightings (forward voltages) applied to the pTx system. We implemented VOPs based on singular value decomposition to compress the Q-matrices with a compression ratio >100, effectively monitoring the maximum peak local SAR values at given weighting amplitudes.