Design of RF Coils for Ultra-high Field MRI
A handful of groups worldwide are equipped with magnets stronger than 7T. The excellence of MRI results from these unique scanners depends ultimately on the RF instrumentation, techniques to control the RF field distribution and acquisition methods. Two state-of the art RF coil setups were developed for 1H and 23Na imaging at 9.4T. At high Larmor frequencies, the wavelength in tissue is comparable or even smaller than the sample dimensions and the RF field distribution exhibits traveling wave behavior, resulting in an inhomogeneous image. Arranging transmit array elements in multiple rows provides additional degree of freedom to correct the inhomogeneities and to achieve whole-brain excitation. Receive arrays shaped to the contours of the anatomy increase the image signal-to-noise. A 16-element dual-row transmit array and a 31-element receive array is combined to achieve whole brain excitation and high-SNR at 9.4T. Multi-nuclei imaging also benefits from higher field strength. However, such setups should have 1H imaging capability for B0 shimming purposes. A novel coil arrangement, consisting of a combination of three coil arrays was designed to achieve high SNR, efficient transmit excitation and B0 shimming capability for 23Na MRI at 9.4T. Design requirements, coil design, phantom studies and invivo results from these two set-ups will be presented.