A Degenerate Frequency Spectrum for Parallel Magnetic Resonance Imaging

A recent advance designed to speed up magnetic resonance imaging (MRI) is the simultaneous processing of data from multiple radiofrequency (RF) coils. To implement this, the multiple current loops require the combination of inductive decoupling and a degenerate frequency spectrum. In a ladder coil, this is impossible for more than two loops due to non-neighboring mutual inductance coupling of the rungs, independent of the addition of capacitances on every leg. However, in a planar array consisting of two concentric squares with four spokes, exactly degenerate modes are possible if a small center loop is included. The center loop of the array fills the vital role of decoupling the non-adjacent outer loops. This result of partial degeneracy can be applied to a four-legged birdcage because the two arrays are topologically congruent. A coil has been constructed utilizing the capacitance network predicted by the theoretical work to give the necessary degeneracy, and a successful experimental test has been carried out.