Magnetic resonance imaging (MRI) is one of the most important non-invasive examination methods in the modern medicine. To increase the range of medical applications, MRI systems with more powerful magnetic fields are required. Standard high-field whole body (1.5T-3T) MRI Systems (Fig. 1) use a body coil for the excitation. MRI at ultra-high-field (UHF) requires different Tx-coils for excitation of different body parts since the construction of one large body coil, similar to those at lower fields, is too difficult. Moreover, at 7T B1 is inhomogeneous as the RF-wave length within the object is smaller than the object extensions. While the usable B1-field in RF-coils is restricted to the dimensions and geometry of the RF-coil itself, with the new traveling wave concept (described by Brunner ) the usable B1-field is restricted to the dimensions of the waveguide (RF-shield) only. Thus the MR traveling wave concept allows excitation of large volumes depending on the length of the RF-shield. For an antenna with a frequency of 297MHz, the approximate wavelength is about 1m. Thus the RF-shield of the gradient coil with a diameter of 64cm can be used as a waveguide because of the cut-off frequency. The cut-off frequency is the minimum frequency where a wave fits into the waveguide without damping. This study examines the use of the traveling wave concept as an efficient body coil replacement in UHF MRI-System with the support simulations in CST MICROWAVE STUDIO® and measurements. Therefore two different types of antennas have been simulated and produced. The B1-field distribution of a dipole and a patch antenna were simulated and compared with B1-field measurements in a 7T MRI System. The efficiency compared to a 1.5T body coil was investigated. Further research goals are to create biological models based on anatomical MRI-Dataset for use in field-simulation software with dynamic thermal solver for more realistic SAR calculation. However the remaining problems of exposing sensitive body parts, such as the human head, to increased SAR needs to be solved for next generation UHF MRI-Systems.
Tim Herrmann, Johannes Mallow, OvG University Magdeburg