On the SAR and field inhomogeneity of birdcage coils loaded with the human head

Abstract

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 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.