A radio-frequency (RF) signal wavelength of proton MRI at 7T becomes comparable to the circumference of the human MR scanner bore (1 m vs. 1.9 m) and the bore starts to act as a cylindrical waveguide for the RF signal. Moreover, in human tissue the RF wavelength shrinks to about 15 cm only. The operating wavelength shrinkage at 7T challenged the traditional design of large volume RF probes which was based on near field coupling and was successfully used at lower magnetic field strengths. On the other hand, because of the relatively small wavelength, microwave engineering principles could be borrowed to design novel RF probes. This opened a new RF research area for investigation and was called travelling wave NMR or waveguide MRI. In our work we explored waveguide principles and applied them for human body MRI at 7T and 9.4T. Three key attributes for any high field RF probe were addressed from the waveguide perspective: efficiency, signal uniformity by means of shimming and RF tissue heating.
At the end I will give a short overview of my clinical MR experience: MR spectroscopy at 7T, a thermal noise based motion sensor (noise navigator) and bone marrow segmentation by means of MRI only.