A 32‐element loop/dipole hybrid array for human head imaging at 7 T

Abstract

Purpose To improve whole‐brain SNR at 7 Tesla, a novel 32‐element hybrid human head array coil was developed, constructed, and tested. Methods Our general design strategy is based on 2 major ideas: Firstly, following suggestions of previous works based on the ultimate intrinsic SNR theory, we combined loops and dipoles for improvement of SNR near the head center. Secondly, we minimized the total number of array elements by using a hybrid combination of transceive (TxRx) and receive (Rx) elements. The new hybrid array consisted of 8 folded‐end TxRx‐dipole antennas and 3 rows of 24 Rx‐loops all placed in a single layer on the surface of a tight‐fit helmet. Results The developed array significantly improved SNR in vivo both near the center (∼20%) and at the periphery (∼20% to 80%) in comparison to a common commercial array coil with 8 transmit (Tx) and 32 Rx‐elements. Whereas 24 loops alone delivered central SNR very similar to that of the commercial coil, the addition of complementary dipole structures provided further improvement. The new array also provided ∼15% higher Tx efficiency and better longitudinal coverage than that of the commercial array. Conclusion The developed array coil demonstrated advantages in combining complementary TxRx and Rx resonant structures, that is, TxRx‐dipoles and Rx‐loops all placed in a single layer at the same distance to the head. This strategy improved both SNR and Tx‐performance, as well as simplified the total head coil design, making it more robust.

Dipolectric antenna for high‐field MRI

“…A simple extrapolation of our results (assuming that central SNR for the 38‐channel array is 2.3‐fold higher vs. the 8‐channel array) would result in a 1.6‐fold SNR gain versus the Nova coil. Recently, Avdievich 16 reported a 20% SNR gain in the center of human brain using their 32‐channel loop–dipole array versus the Nova coil. This would mean that the 38‐channel dipolectric antenna array could provide a 1.4‐fold gain versus the Avdievich design.…”

“…The theory prescribes that a combination of dipole antennas with standard loop coils is essential to approach the ultimate intrinsic SNR. 13 This was first experimentally verified by Wiggins et al in a cylindrical phantom 14 ; then by Ertürk et al for body applications 15 ; and recently by Avdievich et al, who developed a 32-channel loop-dipole array for brain MRI at 7 T. 16 A very effective approach, which was proposed to enhance receive performance of standard RF coils at UHF, was to combine them with dielectric materials with high 𝜀 r (high-permittivity materials [HPM] 17 ). HPMs can be either used as passive (also known as dielectric pads) or active elements, that is, dielectric resonator antennas (DRA).…”

“…This comparison is even more relevant because a very similar design was used in a 32-channel loop-dipole combined array for brain MRI at 7 T published very recently. 16 Peak B + 1 efficiency (0.67 μT/ √ W) in the center of Duke's head was found to be 8% higher for the 8-channel dipole antenna as well as B + 1 homogeneity (0.42 ± 0.09 μT/ √ W), but an averaged B + 1 efficiency across the central axial slice was higher for the 16-channel dipolectric antenna array driven in dipole-only (DR-open) mode: 0.48 ± 0.17 μT/ √ W. Due to higher pSAR 10g for the 8-channel folded dipole antenna, SAR efficiency in the center of Duke's head was slightly lower when compared with the 16-channel dipolectric antenna array (0.96 vs. 0.97 μT/ √ [W/kg]). Also, note that TX performance provided by the folded and "straight" dipole elements was comparable (Figure 5) (Table 1).…”

“…Interestingly, in the case of loop-dipole combination, receive-only loops can have a detrimental effect on TX performance of a dipole antenna array. 10,16 In the case of a dipolectric antenna, on the contrary, the DRAs have a beneficial effect on TX performance (what can be tentatively called a "dielectric pad effect"), and this can be considered an advantageous feature of dipolectric antenna.…”

“…The theory prescribes that a combination of dipole antennas with standard loop coils is essential to approach the ultimate intrinsic SNR 13 . This was first experimentally verified by Wiggins et al in a cylindrical phantom 14 ; then by Ertürk et al for body applications 15 ; and recently by Avdievich et al, who developed a 32‐channel loop–dipole array for brain MRI at 7 T 16 …”