Adjustable RF Transmitter Head Coil: Improving Transmit Efficiency With SAR Management for 7-T Magnetic Resonance Imaging

Abstract

In ultrahigh field magnetic resonance imaging (MRI) using a static magnetic field over 7 T, the inhomogeneity in the time-varying magnetic field ( $B_{1}$ ), which is caused by its short wavelength, is mitigated by using a multichannel radio frequency (RF) coil as an antenna. However, the conventional multichannel RF coil has a fixed configuration; consequently, its transmit efficiency (TE) changes according to the head shape of the patient. In this study, a geometrically adjustable eight-channel RF transmitter head coil for 7-T MRI was proposed. All resonators and supporting structures (called bridges) in the coil have interlocking teeth at their sides, allowing each channel to adjust its geometric location according to the head shape of the patient. Scenarios for conventional and proposed coils in three different configurations were simulated and compared in terms of the transmitted component of the $B_{1}$ field ( $B_{1}^{+}$ field) and specific absorption rate (SAR) maps. The simulation results indicate that, for a given configuration and head shape, the TE of the proposed coil was higher than that of the conventional coil, while the SAR efficiency remained unchanged. Furthermore, the S-parameter matrix of the proposed coil and the measured $B_{1}$ field produced by the two channels facing each other show that the proposed coil resonates at 298 MHz with acceptable mutual coupling and effectively improves the $B_{1}$ field intensity. Moreover, the obtained signal-to-noise ratio (SNR) maps and magnetic resonance (MR) images prove that the proposed coil is an efficient candidate for an RF transmitter or transceiver head coil used in 7-T MRI.