## Global optimization of default phases for parallel transmit coils for ultra-high-field cardiac MRI

Please always quote using this URN: urn:nbn:de:bvb:20-opus-265737
• The development of novel multiple-element transmit-receive arrays is an essential factor for improving B$$_1$$$$^+$$ field homogeneity in cardiac MRI at ultra-high magnetic field strength (B$$_0$$ > = 7.0T). One of the key steps in the design and fine-tuning of such arrays during the development process is finding the default driving phases for individual coil elements providing the best possible homogeneity of the combined B$$_1$$$$^+$$-field that is achievable without (or before) subject-specific B$$_1$$$$^+$$-adjustment in the scanner. ThisThe development of novel multiple-element transmit-receive arrays is an essential factor for improving B$$_1$$$$^+$$ field homogeneity in cardiac MRI at ultra-high magnetic field strength (B$$_0$$ > = 7.0T). One of the key steps in the design and fine-tuning of such arrays during the development process is finding the default driving phases for individual coil elements providing the best possible homogeneity of the combined B$$_1$$$$^+$$-field that is achievable without (or before) subject-specific B$$_1$$$$^+$$-adjustment in the scanner. This task is often solved by time-consuming (brute-force) or by limited efficiency optimization methods. In this work, we propose a robust technique to find phase vectors providing optimization of the B-1-homogeneity in the default setup of multiple-element transceiver arrays. The key point of the described method is the pre-selection of starting vectors for the iterative solver-based search to maximize the probability of finding a global extremum for a cost function optimizing the homogeneity of a shaped B$$_1$$$$^+$$-field. This strategy allows for (i) drastic reduction of the computation time in comparison to a brute-force method and (ii) finding phase vectors providing a combined B$$_1$$$$^+$$-field with homogeneity characteristics superior to the one provided by the random-multi-start optimization approach. The method was efficiently used for optimizing the default phase settings in the in-house-built 8Tx/16Rx arrays designed for cMRI in pigs at 7T.