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Abstract:
We investigated the influence of tissue anisotropy on SAR and B1 value at NMR frequencies of 100, 200 and 300 MHz, using both finite element and finite difference time domain electromagnetic field simulation software packages. An insert of anisotropic tissue, of varied dimensions, was positioned inside a uniform isotropic tissue phantom of cubic shape, occupying most of the volume within the rf transmit coil assembly. Different types and levels of tissue anisotropy were investigated – anisotropy only in electrical conductivity, only in relative permittivity, and in both of these parameters. For comparison, simulations were performed using an isotropic tissue insert, whose electrical conductivity and relative permittivity were set to the maximum or minimum value for the corresponding anisotropic insert. The results, for small insert dimensions, show a significant influence of tissue anisotropy on SAR, but rather weak effects on the B1 maps. As expected, the effect is stronger if the tissue anisotropy axis is parallel to the electrical field.
We also present comparisons between the finite element method and the finite difference time domain results. There are noticeable differences in the maps of SAR value, which will receive further investigation.
