Design of a RF-resonant set improving locally the B1+ efficiency. Applications for clinical MRI in andrology and urology

Volumetric wireless coils for breast MRI: A comparative analysis of metamaterial-inspired coil, Helmholtz coil, ceramic coil, and solenoid

This study comprehensively assesses radiofrequency (RF) volumetric wireless coils utilizing artificial materials for clinical breast MRI. In particular, we evaluated the transmit efficiency, RF safety, and homogeneity of magnetic field amplitude distribution for four structures electromagnetically coupled with a whole-body birdcage coil: extremely high permittivity ceramic coil, solenoid coil, Helmholtz coil, and metamaterial-inspired coil based on periodically coupled split-loop resonators. These coils exhibit favorable attributes, including lightweight construction, compactness, cost-effectiveness, and ease of manufacturing. The results of this study demonstrated that the metamaterial-inspired coil outperforms other wireless coils considered for addressing a specific problem in terms of the set of characteristics. In particular, the metamaterial-inspired coil achieved 85% and 88% homogeneity in magnetic field amplitude distribution at 3 T and 1.5 T MRI, respectively. Also, the 1.5 T metamaterial-inspired coil demonstrated the best performance, increasing the efficiency gain of the birdcage coil by 4.93 times and improving RF safety by 2.96 times. This research explains the limitations and peculiarity of utilizing the volumetric wireless coils in 1.5 and 3 T MRI systems.

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

Magnetic resonance imaging (MRI) systems must undergo quantitative evaluation through daily and periodic performance assessments. In general, the reference or standard radiofrequency (RF) coils for these performance assessments of 1.5 to 7.0 T MRI systems have been low-pass-type birdcage (LP-BC) RF coils. However, LP-BC RF coils are inappropriate for use as reference RF coils because of their relatively lower magnetic field (B₁-field) sensitivity than other types of BC RF coils, especially in ultrahigh-field (UHF) MRI systems above 3.0 T. Herein, we propose a hybrid-type BC (Hybrid-BC) RF coil as a reference RF coil with improved B₁-field sensitivity in UHF MRI system and applied it to an 11.7 T MRI system. An electromagnetic field (EM-field) analysis on the Hybrid-BC RF coil was performed to provide the proper dimensions for its use as a reference RF coil. Commercial finite difference time-domain program was used in EM-field simulation, and home-made analysis programs were used in analysis. The optimal specifications of the proposed Hybrid-BC RF coils for them to qualify as reference RF coils are proposed based on their B₁⁺-field sensitivity under unnormalized conditions, as well as by considering their B₁⁺-field uniformity and RF safety under normalized conditions.