Coaxial waveguide setup for multinuclear highfield-MRI with traveling waves

Due to the increased Larmor frequencies in high-field MR imaging (B0≤7 Tesla), resonant volume coils can no longer be used to generate the homogenous spin excitation field within the human torso. As an alternative approach a coaxial waveguide setup to transmit traveling electromagnetic waves to a defined imaging area is proposed in this work. The object of interest is placed in the hollow inner conductor of the setup and the imaging area is formed by a gap in that conductor. The proposed method is characterized with respect to its transmission efficiency, field homogeneity and the resulting radiofrequency exposure of the patient. In this work numerical simulations (FDTD method) were used for evaluation. Furthermore, a prototype setup with reduced scale was constructed and MRI experiments with multiple test objects and anatomical samples were performed. Higher modes of propagation (TE) were also tested besides the basic coaxial mode (TEM). Due to the frequency independence of the TEM mode, the coaxial waveguide setup could be operated at five different Larmor frequencies from 29 to 297 MHz and MRI experiments with various nuclei (1H, 23Na and 35Cl) were performed. Using a frequency diplexer, combined MRI of 1H and 23Na was possible. The results of this work showed that the proposed method can be used for MRI at multiple frequencies and is therefore suitable for multinuclear MR studies. The achieved transmission efficiency of 0.5-6 μT/√(kW) and the variation in field homogeneity (one order of magnitude) were found to be similar to those of other traveling wave approaches, but are still not generally appropriate for clinical MRI at high fields.