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Abstract:
Purpose/Introduction: Tight-fit human head ultra-high field (UHF, C7T) transceiver (TxRx) surface loop phased arrays (1, 2) improve transmit (Tx)-efficiency in comparison to Tx-only arrays, which are larger to fit receive (Rx)-only arrays inside (3). However, the number of TxRx-array elements is restricted by the number of Tx-channels (commonly B16), which limits Rx-performance. In this work we developed a method of increasing the number of Rx-elements without
necessity of increasing the array size. We constructed a prototype of a human head array, which consists of 8 TxRx-surface ‘‘horizontal’’ loops, and 8 Rx-only ‘‘vertical’’ loops positioned perpendicularly in
the center of each TxRx-loop.
Subjects and Methods: The entire array measures 20 cm 9 23 cm, while horizontal and vertical loops measure 10.5 cm 9 10 cm and 3.3 cm 9 10 cm, respectively (Fig. 1). Overlapping of adjacent horizontal loops provides decoupling below -30 dB (Fig. 1) (4). Vertical loops are decoupled by preamplifier decoupling and actively detuned during transmission. The array is shielded. Electromagnetic simulations of B1 + and SAR were performed using CST Studio Suite 2015 (CST, Darmstadt, Germany) with all 16 loops included into
simulations. Three voxel models were used, i.e. a head/shoulder(HS) phantom (3), which was constructed to match tissue properties at 400 MHz (3), and two voxel models, ‘‘Duke’’ and ‘‘Ella’’. Experimental B1 + maps were obtained using the AFI sequence (5). All data were acquired on a Siemens Magnetom 9.4T human imaging system.
Results: EM simulations showed that introduction of vertical loops have negligible effect on the local SAR and B1 +. Experimentally measured (Fig. 2) B1 + was also similar (̵1
+[*12.5uT/HkW) for both 16- and 8-channel arrays. Introduction of the vertical loops improved SNR up to 40, and at the center by *28 (Fig. 2). Parallel Rx-performance was also improved (Fig. 3).
Discussion/Conclusion: Combining horizontal and vertical loops is similar to combining surface loops with dipole antennas (6,7). This combination is required to optimize SNR at UHF (6,7). In our case instead of dipoles we used vertical loops, which are much smaller, and easier to construct. As a proof of concept we developed a method of increasing the number of Rx-elements in a human head tight-fit TxRx-array. We constructed a prototype of a 16-channel 9.4T array, which consists of 8 horizontal TxRx-surface loops and 8 Rx-only vertical loops. We
demonstrated both experimentally and numerically that addition of the vertical loops substantially improves the Rx-performance without compromising the Tx-performance and the local SAR. To further improve SNR and longitudinal coverage, increasing the overall number of array elements, (double-row 16-TxRx-element/32-Rxelement array) is required.
