A mutual inductive tuning scheme of birdcage coil for MR imaging

The birdcage RF-coil is one of most widely used in MR imaging, but it is the most difficult to be constructed due to the multiple resonance modes. Once the right mode for the specific frequency is selected, it must be tuned and matched for the optimal energy transfer. Typically, capacitive tuning is widely used for birdcage coils using a non-magnetic, high break-down voltage (few tens kV), variable capacitor. The inductive tuning method of the birdcage coil for MR imaging was used in this study. A set of inductive rings was overlaid onto one end of the birdcage, and varying of the resonance frequency was accomplished by sliding the eddy-rings relative to the main coil. The tuning scheme, including it´s construction, was very simple, and more than 1 MHz of tuning range was easily obtained. The impedance Z changed less than 5% over the wide range of tuning with sliding the eddy-coil without adjusting the matching of the coil assembly