Investigation of the Optimal Transmit Only/Receive Only (ToRo) RF Coil for Brain Imaging at 9.4T

A, transmit efficiency; B, B1+ field homogeneity and C, SAR efficiency depending on the phase shift between the rows for six simulated coils.

In this work we investigate the optimal design of the Transmit only/Receive only (ToRo) head coil for 9.4T. The developing coil is inspired by another unshielded ToRo design consisting of 16 transmit only loops in the two rows and 31 receive only tight-fit helmet [1]. In addition, it was observed previously that enlarging coil length may improve the transmit efficiency in the superior part of the brain. Thus, this hypothesis has to be tested in this work. Moreover, it was observed, that in the state-of-the-art design we can improve the loading factor. By loading factor, we mean the percentage of power that is absorbed by the tissues. The optimal solutions to achieve our goal would be to make the overlapping between the rows, that not only enlarge the coil length but also simplifies the design due to avoiding the transformer decoupling between the rows.
To confirm our hypothesis, we conducted the full-wave simulation analysis of the transmit coil using commercial CST software. For that purpose, we studied 6 setups split into the two groups. First group contains the coils having transformer decoupling between the rows (gap). Second group has overlapping between the rows. As a reference coil we took the coil with the gap of 14 mm between the rows. This coil has an elliptical cross section with semi axis of 243 mm and 281.5 mm. The width of each loop was 88.3 mm. The length of the reference loop is 88 mm that makes the total length of the coil 190 mm. Then, the coil was enlarged leftward, while retaining rightward position (close to the shoulder) to 20 and 40 mm. The same procedure was done for the coil with overlapping. It is also important to not that overlapping between the rows does not spoil the isolations between the elements. Worst case isolations for the coil with gap was -13.1 dB, and -13.2 dB for the coil with the overlapping.

1.
G. Shajan, M. Kozlov, J. Hoffmann, R. Turner, K. Scheffler, and R. Pohmann:
A 16-channel dual-row transmit array in combination with a 31-element receive array for human brain imaging at 9.4 T.
Magn. Reson. Med., vol. 71, no. 2, pp. 870–879, 2014.
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