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Coaxial waveguide for travelling wave MRI at ultrahigh fields



At high magnetic fields the performance of a volume-type body coil inside a human sized MR-scanner is influenced by the waveguide action of the scanner's bore. This can result in undesirable strong radio frequency fields inline image) outside the coil's target volume. A radio frequency (RF) transmit system, exploiting this waveguide action of the bore, is proposed in this work. A coaxial waveguide section is introduced between the antenna and the imaging region. It is shown that the coaxial waveguide has several advantages over the initially proposed travelling wave setup based on the cylindrical waveguide. First, a novel radio frequency matching principle (based on the transmission line impedance matching) is feasible with the coaxial waveguide achieving better radio frequency transmission characteristics, such as homogeneity and power efficiency of inline image field. In case of body torso imaging, the coaxial waveguide prevents unwanted specific absorptive rate (SAR) deposition outside the target region and thus, effectively decreases local peak SAR values by factor of 5. A 3-fold inline image gain in the prostate can be achieved with the coaxial waveguide in comparison with the initially proposed travelling wave setup. Magn Reson Med 70:875–884, 2013. © 2012 Wiley Periodicals, Inc.