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Zusammenfassung:
Purpose/Introduction: Remotely placed antennas may find application as a 1H “body coil” substitute in setups where free space in the scanner is crucial or where transmit/receive coils for other nuclei are already present around the subject's head [1]. However, one weakness of the conventional traveling-wave approach [2] is the small number of RF channels (usually the two TE11 modes) which limits B1 + management and parallel receive capabilities.
In this work, we utilize the natural propagation of the TM01 mode in the RF screen of our scanner. Human brain images acquired with the TE11 and TM01 modes are combined using the TIAMO approach [3] to investigate possible
improvements in image uniformity for traveling-wave MRI at 9.4 T.
Subjects and Methods: Experiments were conducted on a Siemens Magnetom 9.4 T scanner equipped with SC72 gradients. A 3-port antenna [4] (Fig.1B) was constructed by integrating a monopole into a tunable patch antenna [5]
for additional excitation of the TM01 mode. Two modes were excited separately (Fig.1A): The CP mode, i.e. both TE11
modes with a 90° phase difference, as well as the monopole. In both cases, signal reception was performed with both patch antenna channels; the monopole was not used for reception in these initial experiments. First in vivo FLASH
images and B1 + field maps were acquired on a single subject with an antennasubject distance of 50 cm. Numerical simulations were performed with CST Studio Suite (CST, Germany).
Results: The B1 + patterns of the well-decoupled CP and monopole modes show complementary features (Fig.1C) and their root-sum-squares (RSS) combination, which is a measure of transmit uniformity using TIAMO reconstruction
[3], yields improved homogeneity compared to the single modes. Improved image quality can also be observed using a simple RSS reconstruction of FLASH images (Fig.1D) that were acquired by transmitting with each mode separately.
Discussion/Conclusion: TIAMO reconstruction of monopole and CP mode excitations can be used to yield images with an improved homogeneity. With full control over the three transmit/receive channels, optimized modes may be found that complement each other even better. Hence, a simple 3-port antenna may be a reasonable tradeoff between imaging performance and setup-complexity for traveling-wave neuroimaging at 9.4 T.
