An 8-Channel Transceiver Coil for Carotid Artery Imaging at 7T Using an Optimized Shield Design

Objective: To design and fabricate a transmit/receive (T/R) radiofrequency (RF) coil array for MRI of the carotid arteries at 7T with optimal shielding to improve transmit performance in parallel transmit (pTx) mode. Methods: The carotid coil included 8 total RF elements, with left and right subarrays, each consisting of 4 overlapping loops with RF shields. Electromagnetic (EM) simulations were performed to optimize and improve the transmit performance of the array by determining the optimal distance between the RF shield and each subarray. EM simulations were further used to calculate local specific absorption rate (SAR) matrices. Based on the SAR matrices, virtual observation points (VOPs) were applied to ensure safety during parallel transmission. The efficacy of the coil design was evaluated by measuring coil performance metrics when imaging a phantom and by acquiring in-vivo images. Results: The optimal distance between the RF shield and each subarray was determined to be 45 mm. This resulted in a maximum B₁⁺ efficiency of 1.23 μT/$\surd W$ in the carotid arteries and a peak, 10-g-average SAR per Watt of 0.86 kg⁻¹ when transmitting in the nominal CP⁺ mode. Optimizing the RF shield resulted in up to 37% improvement in B₁⁺ efficiency and 14% improvement in SAR efficiency compared to an unshielded design. Conclusion and significance: Optimizing the distance between the RF shield and coil array provided significant improvement in the transmit characteristics of the bilateral carotid coil. The bilateral coil topology provides a compelling platform for imaging the carotid arteries with high field MRI.