# Towards Large Diameter Transmit Coils for 7‐T Head Imaging: A Detailed Comparison of a Set of Transmit Element Design Concepts

## TL;DR

This paper compares nine transmit coil designs for 7-T MRI head imaging to determine which performs best in large-diameter coils.

## Contribution

The study provides a systematic evaluation of nine transmit element designs for large-diameter 7-T MRI coils using multiple performance metrics.

## Key findings

- Loop-based transmit elements showed highest power and SAR efficiency in most head regions.
- Passively fed dipoles had lowest mutual coupling at -31.23 dB.
- Meander elements provided best homogeneity in the hippocampus with a 5.1% coefficient of variation.

## Abstract

Many different transmit (Tx) coil concepts and designs for 7‐T magnetic resonance imaging of the head have been proposed. Most of them are placed close to the head and in combination with the receive coils creating a helmet‐like structure. This limits the space for additional equipment for external stimuli. A large diameter transmit coil can increase the ease using supplementary measurement devices. Therefore, this study systematically evaluated nine different Tx elements regarding their performance within a large diameter transmit coil with a diameter > 350 mm. Each Tx element was examined regarding its power and specific absorption rate (SAR) efficiencies, its loading dependence, intrinsic decoupling, and its radio frequency (RF) shimming capability. Additionally, an experimental validation of |B1+|‐maps was performed. The loop‐based Tx elements (circular and rectangular loop) provided the highest power and SAR efficiency with at least 15.5% and 21.2% higher efficiencies for a single channel and 22.1% and 18.0% for the eight‐channel array, respectively. In terms of voxel‐wise power efficiency, the circular loop was the superior Tx element type within most of the head. Looking at the voxel‐wise SAR efficiency, the loop‐based elements manifest themselves as the most efficient type within most of the central brain. The mutual coupling was lowest for the passively fed dipole (− 31.23 dB). The highest RF shimming capability in terms of sum of normalized singular values was calculated for the rectangular (4.21) and the circular loop (4.36), whereby the L‐curve results showed that the arrays have only minor |B1+| shimming performance differences for the transversal slice. For the hippocampus, the meander element provided the highest overall homogeneity with a minimal coefficient of variation (CoV) of 5.1%. This work provides extensive and unique data for single and eight‐channel Tx elements applying common performance benchmarks and enables further discourse on multi‐channel evaluations towards large diameter Tx coils at 7‐T head imaging. On the bases of the provided results, the preferable Tx element type for this specific application is loop‐based.

This work compares nine different transmit element types towards their potential as large diameter (> 350mm) transmit head coils in single channel and eight channel array configurations. For most of the ROIs within the human head, the loop‐based elements were superior, and the RF shimming capability differed only marginally; however, in the deeper brain regions, dipole‐based elements gained in efficiency. For this particular application, the loop‐based transmit elements were superior to the remaining elements.