Metamaterial-based transmit and receive system for whole-body magnetic resonance imaging at ultra-high magnetic fields

Abstract

Magnetic resonance imaging (MRI) at ultra-high fields (UHF), such as 7 T, provides an enhanced signal-to-noise ratio and has led to unprecedented high-resolution anatomic images and brain activation maps. Although a variety of radio frequency (RF) coil architectures have been developed for imaging at UHF conditions, they usually are specialized for small volumes of interests (VoI). So far, whole-body coil resonators are not available for commercial UHF human whole-body MRI systems. The goal of the present study was the development and validation of a transmit and receive system for large VoIs that operates at a 7 T human whole-body MRI system. A Metamaterial Ring Antenna System (MRAS) consisting of several ring antennas was developed, since it allows for the imaging of extended VoIs. Furthermore, the MRAS not only requires lower intensities of the irradiated RF energy, but also provides a more confined and focused injection of excitation energy on selected body parts. The MRAS consisted of several antennas with 50 cm inner diameter, 10 cm width and 0.5 cm depth. The position of the rings was freely adjustable. Conformal resonant right-/left-handed metamaterial was used for each ring antenna with two quadrature feeding ports for RF power. The system was successfully implemented and demonstrated with both a silicone oil and a water-NaCl-isopropanol phantom as well as in vivo by acquiring whole-body images of a crab-eating macaque. The potential for future neuroimaging applications was demonstrated by the acquired high-resolution anatomic images of the macaque’s head. Phantom and in vivo measurements of crab-eating macaques provided high-resolution images with large VoIs up to 40 cm in xy-direction and 45 cm in z-direction. The results of this work demonstrate the feasibility of the MRAS system for UHF MRI as proof of principle. The MRAS shows a substantial potential for MR imaging of larger volumes at 7 T UHF. This new technique may provide new diagnostic potential in spatially extended pathologies such as searching for spread-out tumor metastases or monitoring systemic inflammatory processes.

A Preliminary Study for Reference RF Coil at 11.7 T MRI: Based on Electromagnetic Field Simulation of Hybrid-BC RF Coil According to Diameter and Length at 3.0, 7.0 and 11.7 T

“…In the obtained normalized |B 1 + |-field results, the central pixel of the BC coil in the |B 1 + |-field maps was calculated by assuming that the flip angle was π/2, and thus, the RF pulse was normalized at 1.957 µT. This normalization factor corresponds to a π/2 flip angle with a 3 ms rectangular RF pulse [61,62]. It also indicates the signal sensitivity of the unnormalized |B 1 + |-field.…”

A Comparative Study of Birdcage RF Coil Configurations for Ultra-High Field Magnetic Resonance Imaging

“…A 90 • flip-angle with a 3-ms rectangle RF pulse was assumed to be applied to the center of the BP-BC RF coil and the target subject, and the normalized coefficient (Norm-COEF) was calculated based on the assumption that the signal sensitivity of the central voxel in the acquired |B + 1 |-field was 1.95 µT [57,58]. The normalized coefficient was calculated by applying the B + 1 transmission efficiency (1.95 µT) obtained using the Fast Low-Angle Shot (FLASH) sequence in the actual 7.0 T MRI experiment to the calculated |B + 1 |-field.…”

Interleaved and simultaneous multi‐nuclear magnetic resonance in vivo. Review of principles, applications and potential

“…To improve the overall dual-tuned coil performance and to allow for acquisition of high-quality 1 H data, innovative and organ-specific coil designs have been developed. 37,60,[112][113][114][115][116][117][118][119][120][121][122] An in-depth discussion on the trade-offs for single-and multi-structure dual-tuned RF coils designs (focused on the brain but applicable to other anatomical targets) can be found elsewhere. 123…”