The design and evaluation of single-channel loopole coils at 7T MRI

Abstract

Objective: Improving the local uniformity of B_1^+ field for awake monkey brain MRI at ultra-high fields while facilitating convenient placement and fixation of MRI-compatible multimodal devices for neuroscience study, can eventually advance our understanding of the primate’s brain organization. Approach: A group of single-channel RF coils including conventional loop coils and loopole coils sharing the same size and shape were designed for comparison; their performance as the transmit coil was quantitatively evaluated through a series of numerical electromagnetic (EM) simulations, and further verified by using 7T MRI over a saline phantom and a monkey in vivo. Main results. Compared to conventional loop coils, the optimized loopole coil brought up to 23.5% B_1^+ uniformity improvement for monkey brain imaging in EM simulations, and this performance was further verified over monkey brain imaging at 7T in vivo. Importantly, we have systematically explored the underlying mechanism regarding the relationship between loopole coils' current density distribution and B_1^+ uniformity, observing that it can be approximated as a sinusoidal curve. Significance: The proposed loopole coil design can improve the imaging quality in awake and behaving monkeys, thus benefiting advanced brain research at UHF.

Improving Specific Absorption Rate Efficiency and Coil Robustness of Self-Decoupled Transmit/Receive Coils by Elevating Feed and Mode Conductors

“…For simplicity and clarity, we optimized D mode and D feed separately, which is reasonable considering that the SAR efficiency and coil robustness are separately determined by D feed and D mode . In addition to the loop-type self-decoupled coil studied here, this elevated conductor design could be extended to loopole-mode [ 39 , 40 ] self-decoupled coils where the feed conductor orientates along the z-direction instead of perpendicular to the z-direction. In this case, the feed conductor plays two roles in B 1 + SAR efficiency: its lift-off will change B 1 + as well as the maximum SAR.…”