B1 propagation properties of combined circularly polarised mode in ultra-high-field MRI

Abstract

In ultra-high-field magnetic resonance imaging (MRI), inhomogeneity of the radiofrequency transmit field (|B-1(+)|) occurs due to wavelength shortening that results from interactions between the coil and the high dielectric subject. Even if a multiple transmit/receive (Tx/Rx) coil array is employed using the first-order clockwise circularly polarised (|CP1+|) mode, |B-1(+)| inhomogeneity is an insurmountable obstacle. A scheme utilising the CP1+ mode for the odd-numbered coil elements and the third-order clockwise CP mode for the even-numbered coil elements in the multiple-element Tx/Rx coil array was investigated with the objective of increasing the |B-1(+)| homogeneity at 300 MHz. The results of an electromagnetic simulation and a 7 T MRI experiment are presented and to illustrate the improvements achieved using the optimum CP mode combination is discussed.