Jin J, Chen J
Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801-2991, USA.
Magn Reson Med. 1997 Dec;38(6):953-63. doi: 10.1002/mrm.1910380615.
Birdcage coils are widely used as a radiofrequency (RF) resonator in magnetic resonance imaging (MRI) because of their capability to produce a highly homogeneous B1 field over a large volume within the coil. When they are employed for high-frequency MRI, the interaction between the electromagnetic field and the object to be imaged deteriorates the B1-field homogeneity and increases the specific absorption rate (SAR) in the object. To investigate this problem, a finite-element method (FEM) is developed to analyze the SAR and the B1 field in a two-dimensional (2D) model of a birdcage coil loaded with a 2D model of a human head. The electric field, magnetic field, and SAR distributions are shown, and a comprehensive study is carried out for both linear and quadrature birdcage coils at 64, 128, 171, and 256 MHz. It is shown that to generate the same value of the B1 field, the SAR is increased significantly with the frequency, and for the same imaging method the SAR produced by a quadrature coil is significantly lower than that of a linear coil. It is also shown that the B1-field inhomogeneity is increased significantly with the frequency.
鸟笼式线圈因其能够在线圈内的大体积区域产生高度均匀的B1场，而被广泛用作磁共振成像（MRI）中的射频（RF）谐振器。当将它们用于高频MRI时，电磁场与待成像物体之间的相互作用会降低B1场的均匀性，并增加物体中的比吸收率（SAR）。为了研究这个问题，开发了一种有限元方法（FEM），以分析加载有二维人头模型的二维鸟笼式线圈模型中的SAR和B1场。给出了电场、磁场和SAR分布，并对64、128、171和256 MHz的线性和正交鸟笼式线圈进行了全面研究。结果表明，为了产生相同值的B1场，SAR会随着频率显著增加，并且对于相同的成像方法，正交线圈产生的SAR明显低于线性线圈。还表明，B1场的不均匀性会随着频率显著增加。
