COMPUTATION OF ELECTROMAGNETIC-FIELDS FOR HIGH-FREQUENCY MAGNETIC-RESONANCE-IMAGING APPLICATIONS

A numerical method is presented to compute electromagnetic fields insi de a 2 mm high resolution, anatomically detailed model of a human head for high-frequency magnetic resonance imaging (MRI) applications. The method uses the biconjugate gradient algorithm in combination with th e fast Fourier transform to solve a matrix equation resulting from the discretization of an integrodifferential equation representing the or iginal physical problem. Given the current distribution in an MRI coil , the method can compute both the electric field (thus the specific en ergy absorption rate (SAR)) and the magnetic field, also known as the B-1 field. Results for the SAR and B-1 field distribution, excited by a linear and a quadrature birdcage coil, are calculated and presented at 64 MHz, 128 MHz and 256 MHz, corresponding to the operating frequen cies of the 1.5 T, 3 T and 6 T MRI systems. It is shown that compared with that at 64 MHz, the SAR at 128 MHz is increased by a factor over 5 and the SAR at 256 MHz is increased by a factor over 10, assuming th e same current strength in the coil. Furthermore, compared with the li near excitation, the average SAR for the quadrature excitation is redu ced by a factor over 2 and the maximum SAR is reduced by a factor over 3. It is also shown that the B-1 field at high frequencies exhibits a strong inhomogeneity, which is attributed to dielectric resonance.