the mri rf coil known as the balanced high - pass - weighted hybrid birdcage is modified to obtain two homogeneous degenerate resonances , capable of generating circular polarization at widely separated frequencies , by including inductors in parallel with the ring tuning capacitors such that their isolated parallel resonant frequency is greater than the desired low - frequency homogeneous resonance and not greater than 10 % more than the desired high - frequency homogeneous resonance . the coil is advantageous for the frequency - diameter product range of 10 to 60 mhz - m . the coil is preferably balanced at both frequencies such that the electric potentials vanish on the central axial plane at both the lf and hf homogeneous modes . additional parasitic - mode - shifting reactive elements , either capacitors or inductors , may be added between adjacent rungs near their ends , and the rungs may each consist of two parallel bands shorted together at their ends , as in the crozier birdcage . note that a high - pass - weighted hybrid birdcage is defined as a hybrid birdcage in which the reactance of the rung series capacitors is significantly less than the reactance of the ring series capacitors .