Purpose:

To optimize the transmit (Tx) performance of a human head transceiver (TxRx) phased array and provide for the longitudinal coverage suitable for the whole brain imaging at 9.4T.

Introduction:

It has been previously demonstrated for ultra-high field (UHF, >7T) human brain imaging that a simple increase of the length of a single-row Tx-array cannot provide an adequate longitudinal coverage (along the magnet axis) of a whole brain (1-4). Multi-row (2 and above) arrays together with RF shimming have to be used instead (2,3,5). Also tight-fit surface loop TxRx-phased arrays (2,4) improve Tx-efficiency in comparison to Tx-only arrays (5), which are larger to fit smaller multi-channel Rx-only arrays inside. Previously, based on the analytical modeling (6) we demonstrated that at 9.4T both the magnetic and electric coupling between two heavily loaded loops can be compensated at the same time simply by overlapping, and excellent decoupling can be obtained between adjacent loops of a single-row 8-element human head tight-fit TxRx-array without additional decoupling strategies (7). In this work, we extended this idea by constructing a 9.4T tight-fit human head 16-loop double-row (2x8) TxRx-array decoupled entirely by overlapping. The developed transceiver array provides efficient transmission and the whole brain coverage at the same time.

Methods:

Loop size (10.5 cm x 10 cm) was first evaluated analytically (Fig.1) and then adjusted on a bench. The array (Fig.2) measures 20 cm in width (left-right), 23 cm in height (anterior-posterior), and 17.5 cm in length. Overlapping provides very good decoupling (Fig.2D). To decrease radiation losses the array is shielded with the cylindrical shield located at 4-cm distance from the coil elements. Loaded Q-factor, Q_L, measured between 17 (bottom) and 30 (top), which corresponds to a Q_U/Q_L ratio between 8 and 14. We compared the performance of the tight-fit 16-channel TxRx-array with a larger 16-element Tx-only/ 30-element Rx-only (ToRo) surface loop phased array (28 cm – diameter, 19 cm - length) described previously for human head application at 9.4T (5). Electromagnetic (EM) simulations 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 (5), which was constructed to match tissue properties at 400 MHz (ε = 58.6, σ =0.64 S/m), and two virtual family multi-tissue models, “Duke” and “Ella”. Experimental B₁⁺ maps were obtained using the AFI sequence (8). All data were acquired on a Siemens Magnetom 9.4T human imaging system.

Results and Discussion:

After performing safety evaluation of the array, which included EM modeling (B₁^+, SAR) and phantom imaging, we conducted in-vivo experiments (Fig.3). The 16-channel tight-fit TxRx-array provided ~40% improvement in B₁⁺ efficiency (B₁^+/√P) compared to the larger 16-channel To- / 30-channel Ro-array, which agrees well with results obtained from EM simulations that indicate a ~2 fold increase in the RF power deposited into the tissue. Circular polarized (CP) mode (45º phase shift between adjacent loops in each row; 22.5º - between the rows) doesn’t provide a very homogeneous B₁⁺ field at UHFs (Figs3A, 3B). However, introduction of a 70º-phase shift between the rows (Fig.3C) provides an improvement of the homogeneity (9). In-vivo images (Fig.4A) demonstrate coverage of the whole brain. Figs 4B and 4C show a comparison of the SNR maps obtained in-vivo using both arrays. While the 30-element Ro-array still has a better peripheral SNR, SNRs in the center of the head are similar. We also evaluated the dependence of the maximum local SAR_10g (10g-averaged) on the phased shift, φ, introduced between the rows for the homogeneity improvement (Fig.5) (9). At φ~70º the maximum SAR_10g value decreased by 13% as compared to the CP Mode. Additional drop (~30% in total) of the maximum SAR_10g can be obtained by unbalanced driving of the array (Fig.5C) as suggested in (9).

Conclusion:

We constructed a 9.4T (400 MHz) 16-loop (2x8) overlapped transceiver head array based on the results of the analytical modeling. We demonstrated that simply by overlapping a very good decoupling can be obtained without additional decoupling strategies. This provides a recipe for a simple, robust as well as very Tx-efficient design suitable for parallel transmission (pTx) and whole brain imaging at UHFs.

Acknowledgements

No acknowledgement found.

References

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