Jin J M, Chen J, Chew W C, Gan H, Magin R L, Dimbylow P J
Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign 61801-2991, USA.
Phys Med Biol. 1996 Dec;41(12):2719-38. doi: 10.1088/0031-9155/41/12/011.
A numerical method is presented to compute electromagnetic fields inside 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 the fast Fourier transform to solve a matrix equation resulting from the discretization of an integrodifferential equation representing the original physical problem. Given the current distribution in an MRI coil, the method can compute both the electric field (thus the specific energy absorption rate (SAR)) and the magnetic field, also known as the B1 field. Results for the SAR and B1 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 frequencies 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 the same current strength in the coil. Furthermore, compared with the linear excitation, the average SAR for the quadrature excitation is reduced by a factor over 2 and the maximum SAR is reduced by a factor over 3. It is also shown that the B1 field at high frequencies exhibits a strong inhomogeneity, which is attributed to dielectric resonance.
本文提出了一种数值方法，用于计算高频磁共振成像（MRI）应用中分辨率为2毫米、具有详细解剖结构的人体头部模型内部的电磁场。该方法将双共轭梯度算法与快速傅里叶变换相结合，以求解由表示原始物理问题的积分微分方程离散化产生的矩阵方程。给定MRI线圈中的电流分布，该方法可以计算电场（从而计算比吸收率（SAR））和磁场，也称为B1场。计算并给出了由线性和正交鸟笼线圈激发的SAR和B1场分布在64 MHz、128 MHz和256 MHz时的结果，这些频率分别对应于1.5 T、3 T和6 T MRI系统的工作频率。结果表明，假设线圈中的电流强度相同，与64 MHz时相比，128 MHz时的SAR增加了5倍以上，256 MHz时的SAR增加了10倍以上。此外，与线性激励相比，正交激励的平均SAR降低了2倍以上，最大SAR降低了3倍以上。研究还表明，高频下的B1场表现出很强的不均匀性，这归因于介电共振。
