Control of Mutual Coupling in High-Field MRI Transmit Arrays in the Presence of High-Permittivity Liners

In high-field magnetic resonance imaging, transmit arrays and high-permittivity inserts are often used together to mitigate the effects of RF field inhomogeneities due to short wavelength. However, array performance is limited by mutual impedance between elements which must be closely spaced around the volume of interest. Mutual impedance plays a substantial role at high frequencies and is increased by the presence of dielectric pads which are used to increase the homogeneity of the RF magnetic field. This paper describes a decoupling strategy for an eight-channel transmit/receive array in the presence of a high permittivity dielectric liner. The elements are decoupled using capacitive bridges between adjacent elements. In spite of the higher mutual impedance due to the liner, both mutual resistance and reactance can be removed between adjacent elements (isolation better than 30 dB), and coupling between nonadjacent elements is maintained below 15 dB. The effects of decoupling on the transmit performance of the array in presence of high permittivity liners are investigated in terms of coupling, magnetic field intensity, SAR and transmit efficiencies.