Enhanced Transmit Efficiency of a Loop Array Combined With Out-of-Phase Passive Dipoles for Thoracic Spinal Cord MRI...

The design of radiofrequency (RF) coils tailored for the target anatomy is necessary to overcome the transmit field homogeneity and penetration issues at Ultra-High-Field (UHF). For the spinal cord, the optimal design has not yet been determined. This work intends to demonstrate the added value of a pair of passive dipoles placed under a conventional thoracic spine 4 × 2-loop array.

The conventional loop array produces a broad transmit field with unnecessary lateral spread. To address this, two passive dipoles were incorporated and tuned out-of-phase to actively suppress these lateral side-lobes and increase field penetration the spinal cord region. The design was numerically optimised using a homogeneous phantom, followed by human voxel model simulations to evaluate performance and specific absorption rate (SAR). Finally, a prototype was tested in a 7T scanner using a homogeneous phantom to acquire experimental $B_1^{+}$ maps.

Human voxel model simulations demonstrated that the passive dipoles improve the $B_1^{+}$ transmit field distribution and predicted a 35% gain in SAR efficiency in the spinal cord region. Correspondingly, phantom experiments showed a 25% gain in transmit efficiency and 15% gain in homogeneity in a selected region of interest compared to the loop-array-only reference.

Incorporating passive dipoles effectively suppresses unwanted lateral field spread, increasing penetration depth and $B_1^{+}$ efficiency in the thoracic spinal cord region. While homogeneous phantom results offer proof of concept, numerical simulations suggest reliable performance in realistic spine anatomy as well as substantial SAR reduction.