Double‐row 18‐loop transceive–32‐loop receive tight‐fit array provides for whole‐brain coverage, high transmit performance, and SNR improvement near the brain center at 9.4T

Solomakha

et al. 2019

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Purpose To improve the receive (Rx) performance of a human head transceiver (TxRx) array at 9.4T without compromising its transmit (Tx) performance, a novel 16‐element array was developed, constructed, and tested. Methods We designed and constructed a phased array, which consists of 8 TxRx surface loops placed in a single row and circumscribing a head, and 8 Rx‐only short folded dipole antennas. Dipoles were positioned along the central axis of each transceiver loop perpendicular to its surface. We evaluated the effect of Rx dipoles on the Tx efficiency of the array and maximum local specific absorption rate (SAR) as compared to the array of 8 surface loops only. We also compared the new array to a 16‐channel array of the same size consisting of 8 TxRx surface loops and 8 Rx‐only vertical loops in terms of Tx efficiency, SAR, and signal‐to‐noise ratio (SNR). Results The new array improves both peripheral (up to 2 times) and central (1.17 times) SNR as compared to the 16‐element array of the same geometry consisting of 8 TxRx surface loops and 8 Rx‐only vertical loops. We demonstrated that an addition of actively detuned Rx‐only dipole elements produces only a small decrease (~7%) of the B1+ transmit field and a small increase (<7%) of the maximum local SAR. Conclusion As a proof of concept, we developed and constructed a prototype of a 9.4T (400 MHz) head array consisting of 8 TxRx surface loops and 8 Rx‐only short optimized folded dipoles. We demonstrated that at ultra‐high field, dipoles outperformed Rx‐only vertical loops in vivo.

Section: Discussionmentioning

confidence: 73%

Evaluation of short folded dipole antennas as receive elements of ultra‐high‐field human head array

Solomakha

et al. 2019

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“…While the coverage (both Tx and Rx) is still worse compared with the 1 H ST 2 × 8 surface loop array, it is much better than that of the 1 H ST 1 × 8 array. As previously shown, combining a pair of cross‐loops with a 2 × 8 TxRx surface loop array further improves the longitudinal coverage of the 1 H array . However, such a design, i.e., the 18 TxRx loop array, would be even harder to combine with a multi‐channel 31 P array within a single layer.…”

Section: Discussionmentioning

confidence: 98%

Double‐tuned ³¹P/¹H human head array with high performance at both frequencies for spectroscopic imaging at 9.4T

Dorst

et al. 2020

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Purpose:To develop a robust design of a human head double-tuned 31 P/ 1 H array, which provides good performance at both 31 P and 1 H frequencies for MR spectroscopic imaging at 9.4T. Methods: Increasing the number of surface loops in a human head array improves the peripheral signal-to-noise ratio (SNR), while the central SNR doesn't substantially change. High peripheral SNR can contaminate MR spectroscopic imaging data at both 1 H and 31 P frequency. To minimize this effect, we limited the number of elements in the 31 P array to 10, i.e., 8 transceiver surface loops circumscribing the head and 2 receive "vertical" loops placed at the superior location. The 1 H-portion of the array also consists of 10 elements, i.e., 8 transceiver surface loops circumscribing the head and 2 transceiver "vertical" loops at the superior location of the head. Both the 31 P array and 1 H array are placed in a single layer at the same distance to the head, which provides high loading and, thus, a good performance for both arrays. Results: Transmit efficiency of the 1 H-portion of the double-tuned array was very similar to that of the single-tuned arrays of similar size. Also, addition of the crossloops substantially improved the brain coverage. Conclusion: We developed a novel 31 P/ 1 H double-tuned array for MR spectroscopic imaging of a human brain at 9.4T. Placing both 31 P and 1 H loops in a single layer provides for high transmit efficiency at both frequencies without compromising SNR near the brain center at the 31 P-frequency. Addition of the cross-loops at the superior location improves the brain coverage.

“…However, this does not diminish the benefits of the new dipole array, which was developed primarily to improve and simplify the design of a multichannel human‐head Tx array at UHF. The SNR and Rx parallel imaging performance can be further improved by combining the TxRx dipole array with Rx‐only elements (eg, surface loops), as previously described 48,49 …”

Section: Discussionmentioning

confidence: 99%

Bent folded‐end dipole head array for ultrahigh‐field MRI turns “dielectric resonance” from an enemy to a friend

Solomakha

et al. 2020

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Purpose To provide transmit whole‐brain coverage at 9.4 T using an array with only eight elements and improve the specific absorption rate (SAR) performance, a novel dipole array was developed, constructed, and tested. Methods The array consists of eight optimized bent folded‐end dipole antennas circumscribing a head. Due to the asymmetrical shape of the dipoles (bending and folding) and the presence of an RF shield near the folded portion, the array simultaneously excites two modes: a circular polarized mode of the array itself, and the TE mode (“dielectric resonance”) of the human head. Mode mixing can be controlled by changing the length of the folded portion. Due to this mixing, the new dipole array improves longitudinal coverage as compared with unfolded dipoles. By optimizing the length of the folded portion, we can also minimize the peak local SAR (pSAR) value and decouple adjacent dipole elements. Results The new array improves the SEE (< B1+>/√pSAR) value by about 50%, as compared with the unfolded bent dipole array. It also provides better whole‐brain coverage compared with common single‐row eight‐element dipole arrays, or even to a more complex double‐row 16‐element surface loop array. Conclusion In general, we demonstrate that rather than compensating for the constructive interference effect using additional hardware, we can use the “dielectric resonance” to improve coverage, transmit field homogeneity, and SAR efficiency. Overall, this design approach not only improves the transmit performance in terms of the coverage and SAR, but substantially simplifies the common surface loop array design, making it more robust, and therefore safer.