Stray Capacitance Between Magnetic Resonance Imaging Coil Elements: Models and Application to Array Decoupling

The role of capacitive coupling between RF coil elements of MRI arrays is studied extensively in this paper and comprehensive models are obtained. At higher MRI frequencies, capacitive coupling greatly impacts coil coupling and therefore cannot be neglected. We compare lumped and distributed capacitance models and show that they correctly predict impedance variations due to stray and intentional mutual capacitance. The distributed model takes into account the dispersed nature of the mutual impedance, and it is more accurate than the lumped model. The value of stray capacitance obtained by fitting simulated results is shown to depend strongly on substrate and phantom permittivity, while being mostly invariant to changes in frequency. A novel result of this investigation is a method of completely eliminating mutual impedance between adjacent coils, including the mutual resistance. Until recently it was believed that only mutual reactance could be eliminated with lossless passive circuits. The SNR performance of two and four coil arrays with varying degrees of coupling is compared, showing that there is no SNR degradation when the mutual resistance is eliminated in addition to mutual reactance. This result has applications in arrays used in transmission, where mutual resistance leads to losses that degrade transmit efficiency.