Travelling Wave Approach for High Field Magnetic Resonance Imaging Ibrahim A. Elabyad, A. Omar Chair of Microwave and Communication Engineering University of Magdeburg Magdeburg, Germany T. Herrmann, J. Mallow, J. Bernarding Institute for Biometry and Medical Informatics University of Magdeburg Magdeburg, Germany Abstract—A theoretical investigation of the travelling-wave approach for high field magnetic resonance imaging (MRI) is presented. The travelling wave probes excite the fields in the longitudinal direction of the bore, so that the effects of the attenuation constant due to high permittivity and lossy tissue need to be calculated. In addition, the travelling wave modes should affect the 1 + Β field homogeneity inside high permittivity and lossy tissue. The full-field solution of infinite waveguide loaded with a lossy dielectric cylinder for 7T travelling wave phenomenon is presented. The transcendental equation was solved numerically to find the attenuation and propagation constants. Results indicate that, the 1 + Β field is independent of the antenna type, other than the magnitude as the purpose is just exciting modes. In this paper, the traveling wave probes were designed, fabricated, and tested for 7-tesla (7T) MRI. Keywords: Travelling-wave approach, Travelling RF-probes, RF-shield, 7-tesla (7T) MRI, Transcendental equation. I. INTRODUCTION At high and ultra high field (UHF), the RF-wavelength in the tissue becomes significantly smaller than the size of the human head. In addition, with higher magnetic field strengths, RF-coil design faces some problems in designing and tuning. Although, the challenges associated with high and ultra high field MRI, the signal-to-noise-ratio (SNR) will be increased for such systems [1-4]. The promising approach for addressing these challenges is to use large coil arrays for parallel RF transmission and reception. Transceiver arrays become a popular method to overcome some difficulties at ultra high magnetic fields. MRI at UHF requires different transmit coils for excitation of different body parts because the required large transmit volume coil is usually unavailable; since the construction of one large transmit body coil similar to those at lower fields is difficult. Moreover, at 7T the transmit 1 + Β field is inhomogeneous as the RF-wave length within the object is smaller than the object dimensions. In addition, the transmit 1 + Β profile generated by the large volume coil at ultrahigh fields is highly non-uniform in the human body owing to high permittivity and lossy tissue. Alternatively, a promising approach when working with ultrahigh fields is to build transceiver coil arrays that allow for independent phase and amplitude control for each coil element [5-13]. Recently, the travelling wave approach has been proposed in [14] by using a large circularly polarized patch antenna that propagates the RF-fields in the axial direction of the RF-shield of the MRI-system bore. Based on the travelling wave approach, the NMR signal can be transmitted and received by an antenna located at one end of the RF-shield of the MRI imaging bore system. The antenna can be positioned inside the bore, and propagates RF-fields in the longitudinal direction. The high value of the attenuation constant due to high permittivity and lossy tissue will decrease the intensity of the transmit 1 + Β field in the longitudinal direction. The travelling wave concept offers the potential to overcome some of the following restrictions. First, the usable excited 1 Β field in traditional RF-coils is restricted to dimensions and geometry of the RF-coil itself, contrary to this in the travelling wave concept the usable 1 Β field is restricted to the dimensions of the waveguide (RF-shield) only. Second, while standard transmit coils at 7T excite rather small volumes, the travelling wave MRI allows exciting large volumes depending on the length of the RF-shield. For an antenna with a resonance frequency of 300 MHz the approximate wavelength is about 1m. Thus the RF-shield of the gradient coil with a diameter of 64 cm can be used as a circular waveguide. When enclosing only air, the RF-shield has a cut-off frequency of 275 MHz for the propagating 11 TE mode, which is below the proton Larmor frequency at 7T. The larger diameter of the RF-shield at 7T Siemens whole body scanner, in compare to the 7T Phillips Achieva whole body scanner [14], is an advantage. Thus, the wave propagates through the whole RF-shield with much less damping. The travelling wave MRI can be used to solve the standing wave problem of the conventional volume resonators at high magnetic field strengths [14]. However, the travelling wave MRI suffers from high electromagnetic field attenuation. In addition, the modes of the travelling wave also have patterns which results inhomogeneity. The effect of attenuation constant on the electromagnetic fields of the first propagating modes at 7T should be explored. Although this problem is already known in microwave engineering but due to high permittivity and lossy tissue, it is expected that, the travelling wave will be highly attenuated for 7-tesla (7T) MRI. For these reasons, the attenuation and propagation constants for the propagating modes need to be explored and investigated.