Conventional volume coil and travelling-wave antenna for homogeneous excitation of the human head in MRI at 300 MHz

Two competing factors make for RF engineering challenges in MRI: (1) MRI at higher RF field frequencies provides higher signal-to-noise ratio and (2) MRI typically works best with a homogeneous (e.g., long wavelength) circularly-polarized RF magnetic field to excite nuclei in the region of interest. Conventional RF coils can be thought of as a series of antennas placed about the circumference of a cylinder and generating RF fields that travel into the body in the radial direction. Recently an alternative approach, with a circularly-polarized antenna sending fields along the axis of the cylinder has gained much interest. Here we use numerical methods to compare and combine the two approaches for homogeneous excitation of the human head at 300 MHz.