On the Contribution of Curl‐Free Current Patterns to the Ultimate Intrinsic Signal‐to‐Noise Ratio at Ultra‐High Field Strength

The ultimate intrinsic signal‐to‐noise ratio (SNR) is a coil independent performance measure to compare different receive coil designs. To evaluate this benchmark in a sample, a complete electromagnetic basis set is required. The basis set can be obtained by curl‐free and divergence‐free surface current distributions, which excite linearly independent solutions to Maxwell's equations. In this work, we quantitatively investigate the contribution of curl‐free current patterns to the ultimate intrinsic SNR in a spherical head‐sized model at 9.4 T. Therefore, we compare the ultimate intrinsic SNR obtained with having only curl‐free or divergence‐free current patterns, with the ultimate intrinsic SNR obtained from a combination of curl‐free and divergence‐free current patterns. The influence of parallel imaging is studied for various acceleration factors. Moreover results for different field strengths (1.5 T up to 11.7 T) are presented at specific voxel positions and acceleration factors. The full‐wave electromagnetic problem is analytically solved using dyadic Green's functions. We show, that at ultra‐high field strength (B0⩾7T) a combination of curl‐free and divergence‐free current patterns is required to achieve the best possible SNR at any position in a spherical head‐sized model. On 1.5‐ and 3T platforms, divergence‐free current patterns are sufficient to cover more than 90% of the ultimate intrinsic SNR. We quantitatively compare the ultimate intrinsic SNR obtained with having only curl‐free or divergence‐free current patterns, with the ultimate intrinsic SNR obtained from a combination of curl‐free and divergence‐free current patterns. At ultra‐high field strength (B0 ≥ 7 T) a combination of curl‐free and divergence‐free current patterns is required to achieve the best possible SNR at any position in a head‐sized sphere. On 1.5‐ and 3T platforms divergence‐free current patterns are sufficient to cover more than 90% of the ultimate intrinsic SNR.