Quantitative muscle water T₂ mapping using RF phase-modulated 3D gradient echo imaging

Area of Science:

• Magnetic Resonance Imaging
• Biomedical Engineering
• Musculoskeletal Imaging

Background:

• Water T2 relaxation time is a crucial biomarker for skeletal muscle tissue characterization.
• Accurate T2 estimation is challenging in skeletal muscles due to motion and B1 field inhomogeneities.
• Existing methods often lack the speed and robustness required for in vivo applications.

Purpose of the Study:

• To develop and validate a motion-robust 3D magnetic resonance imaging (MRI) sequence for accurate water T2 estimation in skeletal muscles.
• To assess the sequence's performance in phantoms and in vivo, including its ability to account for confounding factors like B1 variations and fat content.

Main Methods:

• A partially spoiled gradient echo (pSPGR) sequence was employed, acquiring 10 image volumes with varied RF phase-cycling and flip angles.
• A bi-component water/fat model was used to fit the complex signal evolution, extracting T2 values while correcting for B1 and fat fraction.
• The sequence was implemented in both Cartesian and radial trajectories, with validation through numerical simulations, phantom studies, and in vivo experiments on thigh and tongue muscles.

Main Results:

• Phantom studies showed high correlation (R² > 0.8) between the proposed method and reference spectroscopy/multi-spin echo techniques.
• In vivo, T2 values correlated well with reference methods in healthy (R² = 0.69) and pathological muscles (R² = 0.87), demonstrating robustness to B1 inhomogeneities (R² = 0.06).
• The radial implementation significantly reduced the standard deviation of T2 values in the tongue muscle by 28% compared to the Cartesian approach.

Conclusions:

• The proposed 3D sequence enables efficient and accurate water T2 estimation in skeletal muscles, including small, moving structures like the tongue.
• This technique expands the potential for characterizing heterogeneous muscle impairments.
• The method's compatibility with clinical research durations makes it a valuable tool for muscle tissue assessment.