Determination of the fundamental resonance mode of a birdcage resonator

Determining the resonance modes of a birdcage resonator may become a challenging task, especially when the design is more sophisticated. One possible approach is a numerical analysis of the resonator's fields. However, the results of such analysis may differ from the parameters of the fabricated resonator (due to the neglected non-ideal behaviour). Another possible way is to confirm the well-known rules for the determination of the resonance modes by an experimental characterisation measurement at the desired frequency, which requires a previous knowledge of the resonant modes. The approach, presented here, consists of analysing transmission parameters obtained by measuring current induced in the magnetic field probe close to the individual longitudinal resonator's elements (legs). The results are values of the module and phase of transmission of the birdcage resonator across the entire frequency range. From such data, the frequency of the fundamental resonance mode can be easily and conclusively extracted. The functionality of the proposed method is proven by the results of the analysis of four different birdcage resonators working in the frequency range of around 400 MHz.