In Vivo Brain B₁⁺ Inhomogeneity Correction and NOE Image Enhancement at 7 T via Flexible Metasurfaces

Nuclear Overhauser effect (NOE) MRI has been used for in vivo brain imaging to assess lipid and protein composition and benefits from 7 T field strengths due to the larger chemical shift dispersion. However, a continuing challenge is signal drop off observed in regions such as the medial temporal lobes due to "standing wave" effects from shorter radiofrequency (RF) wavelengths at ultra-high fields. 2D periodic unit cell metasurfaces have been a promising approach for providing improvements in anatomical imaging but have not yet been evaluated in chemical exchange saturation transfer (CEST)-based sequences. Here, we report the use of metasurfaces for enhancement of NOE imaging as well as for improvement of Lorentzian line fitting of full Z-spectrum data. 3D NOE image data, B₁ ⁺ maps, and B₀ maps were acquired on five healthy volunteers using a 7 T MRI system with and without metasurfaces positioned near the temporal lobes. A frequency offset range of -5 to +5 ppm with additional separate acquisitions of ±20 and ±100 ppm offset images. A five-pool Lorentzian line fitting model was employed to fit and quantitatively compared magnetization transfer (MT), amide proton transfer (APT), amine, and relayed NOE (rNOE) metabolite pools. NOE_MTR-weighted contrast maps were also calculated via Z-spectrum asymmetry analysis. The metasurfaces globally enhanced the transmit efficiency within the imaging slab by approximately 9.6% and reduced B₁ ⁺ inhomogeneity by approximately 16.6% and increased transmit efficiency by 55.8% in the temporal lobes. Amplitude fit maps showed decreases in contrast magnitude ranging from 1 to 16% and changes in image uniformity ranging from a 4.3 decrease to a 34.7% increase, while NOE_MTR-weighted contrast maps demonstrated similar changes. The results presented here demonstrate that metasurfaces can enhance CEST-based techniques complementing previously reported benefits in anatomical imaging.