Coupling strategies for ultra-high field dielectric resonators

Introduction: Recently cylindrical dielectric resonators (CDR) have been used instead of traditional RF coils in MRI probes at high-field strength (B0 ≥ 7T) [1] [2] [3] [4] [5]. However, the optimal coupling scheme for the CDRs in not yet explored. In this study, different coupling methods for TE_{01 δ} mode were investigated and compared using S parameters.
Methods: A cylindrical hollow bore dielectric resonator made of ceramic CaTiO3 (rel. permittivity of 156) with a Q-factor of 2225 was used in TE_01δ mode to create a MRI probe head for 14 Tesla. Five coupling methods (totally 6 setups) were evaluated (see Fig. 1). The CDR was placed in a copper shield during all the evaluations of these coupling methods and a sensor probe was placed in the bore center of the CDR to measure the S21 and its Q-factor using a network analyzer. [6]

Results: The Q-factors and their S21 values of five coupling methods are presented in Table 1. By replacing a small coupling loop on aside of the resonator with a large coupling loop surrounding the resonator, we were able to increase the S21 value with a sacrifice in Q-factor, which means a gain in B1+ field strength with a trade-off in SNR. Overall, the triple loop yielded the best coupling scheme with a Q-factor around 716.2 and S21 value of -9.20 dB.
Discussions: The measured Q-factors shown in Tab 1 are related to other contributions in the resonant system as 1/Q =1/Q_c+1/Q_d+ 1/Q_rad+1/Q_ex, where Q_c, Q_d, Q_rad, and Q_ex, are the Q-factors that are due to losses in conductor, dielectric, radiation, and external circuitry. The Q_ex value decreases when the external coupling increases, so there is a trade-off between S21 and Q_ex [6] [7] . By using large and multi-turn loops, we managed to balance the effects of coupling on Qex with the additional contributions Q_c, and Q_rad. Such that, the S21 value was improved by 91.7% with the Q-factor decrease of 34.6%.