SNR-Efficient Inhomogeneous Magnetization Transfer (ihMT) for Clinical Applications at 7 T

Inhomogeneous MT (ihMT) is an MRI modality sensitive to Myelin, the lipid-rich membrane surrounding the axons. A novel ihMT saturation strategy and its associated $B_1^{+}$ correction at UHF are proposed. The aim is to generate a strong ihMT effect within a clinically compatible scan time while complying with SAR constraints at 7 T.

The MT preparation module of the proposed approach was shortened, reducing power deposition and allowing for shorter scan time while maximizing SNR efficiency. Optimization of the sequence was validated on $N=4$ (2 mm iso.) on a 7 T clinical scanner. Example high resolution images were acquired (1-1.4 mm iso.). Model-based $B_1^{+}$ corrections were applied and their impact on ihMT quantification was studied. Numerical simulations and error quantifications were performed to complement experimental data.

Fewer bursts ( $N_B=5$ ) of cosine-modulated MT preparation pulses ( $N_P=1$ ) performed at maximal $B_1^{\mathrm{rms}}$ yielded strong ihMT signal with better SNR efficiency as compared to previously proposed long saturation trains ( $N_B=10$ ). $B_1^{+}$ correction showed a reliability range ( $B_{1,\mathrm{rel}}^{+}\in \left[80,120\right]\%$ ) which challenged applicability in strong hypo-intense $B_1^{+}$ areas. High resolution acquisitions showed acceptable SNR ( $\ge 25$ ) for resolutions down to 1 mm iso., enabling studies of fine structures (e.g., deep gray nuclei) with added specificity provided by ihMT.

ihMT applications at 7 T are enabled. While whole-brain imaging requires further work to compensate for $B_1^{+}$ inhomogeneities, localized applications (e.g., Thalamus) are immediately feasible within clinically compatible scan time (<  $8$ min). Similar sequence improvements are expected at lower field strengths (e.g., $B_0=3T$ ).