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

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biophysics
• Metamaterials

Background:

• Nuclear Overhauser effect (NOE) MRI at 7 Tesla offers enhanced chemical shift dispersion for assessing in vivo brain lipid and protein composition.
• Signal drop-off in medial temporal lobes due to standing wave effects at ultra-high fields remains a challenge for NOE MRI.
• Periodic unit cell metasurfaces have shown promise for anatomical imaging but haven't been applied to chemical exchange saturation transfer (CEST) sequences.

Purpose of the Study:

• To evaluate the efficacy of 2D periodic unit cell metasurfaces in enhancing NOE MRI.
• To assess the improvement of Lorentzian line fitting for full Z-spectrum data using metasurfaces.
• To investigate the application of metasurfaces in CEST-based MRI sequences.

Main Methods:

• Acquisition of 3D NOE image data, B1+ maps, and B0 maps on five healthy volunteers using a 7T MRI system with and without metasurfaces.
• Utilized a frequency offset range from -5 to +5 ppm, with additional acquisitions at ±20 and ±100 ppm.
• Employed a five-pool Lorentzian line fitting model to analyze magnetization transfer (MT), amide proton transfer (APT), amine, and relayed NOE (rNOE) metabolite pools.

Main Results:

• Metasurfaces globally enhanced transmit efficiency by ~9.6% and reduced B1+ inhomogeneity by ~16.6%.
• Temporal lobe transmit efficiency increased by 55.8% with metasurface application.
• Metasurfaces demonstrated potential for enhancing CEST-based techniques, complementing prior anatomical imaging benefits.

Conclusions:

• Metasurfaces effectively enhance NOE MRI and improve Z-spectrum analysis at 7T.
• The technology addresses signal drop-off issues in challenging brain regions.
• Metasurfaces show promise for advancing CEST-based MRI techniques.