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

Abstract

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 $B_1^{+}$ 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.

Keywords: RF head array; SNR enhancement; folded dipole; receive arrays; ultra-high-field MRI.