Purpose

To improve both the transmit (Tx) and receive (Rx) performance of a human head array and provide for the whole-brain longitudinal coverage at 9.4T, a new 32-element tight-fit array was developed, constructed and tested.

Introduction

Tight-fit human head ultra-high field (UHF,>7T) transceiver (TxRx) surface loop phased arrays (1-3) improve transmit (Tx)-efficiency (B₁⁺/√P) in comparison to Tx-only arrays, which are larger to fit multi-channel receive (Rx)-only arrays inside (4). A drawback of the TxRx-design is that the number of array elements is restricted by the number of available RF Tx-channels (commonly<16). This element count is not sufficient for an optimal SNR and parallel Rx-performance. Previously we developed a method of increasing the number of Rx-elements in a TxRx-array without moving Tx-elements further away from the subject, which compromises the Tx-performance. We constructed a 16-element human head array, which consisted of 8 TxRx-surface loops circumscribing a head, and 8 Rx-only “vertical” loops positioned along the central axis of each TxRx-loop perpendicularly to its surface (5). In this work, we extended the new approach by designing and constructing a 9.4T (400MHz) 32-element tight-fit human head array consisting of 18 TxRx surface loops and 14 Rx-only vertical loops with the total number of Rx-elements equal to 32, i.e. the number of available Rx-channels.

Methods

The array consists of 18 TxRx surface loops and 14 Rx-only vertical loops with the total number of Rx-elements equal to 32. Fig.1A shows the TxRx-part of the array, i.e. 16 surface loops and two perpendicular vertical loops on the top of the array, both measure 50mm in height. All the loops were constructed of 1.5mm copper wire. In addition, 14 Rx-only vertical loops were placed perpendicularly in the center of each surface loop (Figs.1B and 1C) accept the two surface loops located across the eyes. In the 2nd row the array measures 20cm in width (left-right) and 23cm in height (anterior-posterior). The array length is 17.5cm. To fit a human head, the array was tapered at row 1. Optimized overlapping the surface loops provides for very good decoupling (Fig.2A) without additional decoupling strategies(6). The array is shielded with the cylindrical shield located at 4-cm distance from the coil elements. We compared the performance of the tight-fit array with a larger 16-element Tx-only / 31-element Rx-only (ToRo) surface loop array (28cm - diameter, 19cm - length) described previously for brain studies at 9.4T (4). Electromagnetic (EM) simulations (Figs.2C,D) of the transmit B₁⁺ and the local specific absorption rate (SAR) were performed using CST Studio Suite 2015 (CST, Darmstadt, Germany) and the time-domain solver based on the finite-integration technique (FIT). Three voxel models were used, i.e. a head/shoulder (HS) phantom (4), which was constructed to match tissue properties at 400MHz (ε=58.6, σ=0.64S/m), and two virtual family multi-tissue models, “Duke” and “Ella”. Experimental B₁⁺ maps were obtained using the AFI sequence (7). All data were acquired on a Siemens Magnetom 9.4T human imaging system.

Results and Discussion

The tight-fit array provides for a full-brain coverage. It also delivers ~50% greater B₁⁺ averaged over the entire brain than the ToRo-array (Fig.3). Most importantly, it provides for ~30% SNR improvement near the brain center(Fig.4). It is well known that increasing the number of smaller surface loops in a helmet human-head Rx-array only improves peripheral SNR, while SNR near the center practically doesn’t change (8,9). However, when surface and vertical loops are combined, SNR near the center is substantially improved. Both arrays provide for comparable Rx parallel performance (Fig.5). The general idea of our design approach is that the total number of array elements should not exceed the number of available Rx-channels, e.g. 32. During designing, first, the required number of surface TxRx-loops is placed around the object tightly to provide for high Tx-performance. The rest of the loops are used as Rx-only elements, which are positioned to minimize interaction with the TxRx-loops, e.g. vertically between the surface loops and the RF shield (5). In comparison to the common ToRo-design, this method preserves tight fit of the TxRx-loops and, thus, does not compromise the Tx-performance. It also minimizes the total number of array elements and the number of active detuning circuits, which aren’t required for the TxRx-elements.

Conclusion

The new approach of constructing the phased array consisting of both TxRx and Rx-only elements simplifies the array construction by minimizing the total number of elements and makes the entire design more robust and, therefore safe. Overall our work provides a recipe for a very Tx- and Rx-efficient design suitable for parallel transmission and reception as well as whole brain imaging at UHF.

Acknowledgements

No acknowledgement found.

References

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