Abstract
PURPOSE: 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.
METHODS: 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.
RESULTS: Fewer bursts ( N B = 5 ) of cosine-modulated MT preparation pulses ( N P = 1 ) performed at maximal B 1 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 , rel + ∈ [ 80 , 120 ] % ) which challenged applicability in strong hypo-intense B 1 + areas. High resolution acquisitions showed acceptable SNR ( ≥ 25 ) for resolutions down to 1 mm iso., enabling studies of fine structures (e.g., deep gray nuclei) with added specificity provided by ihMT.
CONCLUSION: 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 = 3 T ).
