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, B 1 + maps, and B 0 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 B 1 + 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.