Optimized MR pulse sequence for high-resolution brain 3D-T1ρ mapping with weighted spin-lock acquisitions.

Purpose: To implement and evaluate the feasibility of brain spin-lattice relaxation in the rotating frame (T1ρ) mapping using a novel optimized pulse sequence that incorporates weighted spin-lock acquisitions, enabling high-resolution three-dimensional (3D) mapping.

Methods: The optimized variable flip-angle framework, previously proposed for knee T1ρ mapping, was enhanced by integrating weighted spin-lock acquisitions. This strategic combination significantly boosts signal-to-noise ratio (SNR) while reducing data acquisition time, facilitating high-resolution 3D-T1ρ mapping of the brain. The proposed sequence was compared with magnetization-prepared angle-modulated partitioned k-space spoiled gradient-echo sequence snapshots (MAPSS).

Results: The newly developed pulse sequence, tested for brain 3D-T1ρ mapping for the first time, obtained maps in 4 min with quality comparable to a 20-min MAPSS sequence. Specifically, the voxel-wise median absolute percentage difference between these MR sequences at a resolution of 0.9 × 0.9 × 3 mm is 13.1%. If high resolution is desired, with a voxel size of 0.5 × 0.5 × 3 mm, the new sequence can acquire T1ρ maps in 8 min, surpassing a 20-min (and resolution of 0.9 × 0.9 × 3 mm) MAPSS in SNR. The weighted spin-lock acquisition combined with optimized variable flip angle improved the SNR over optimized variable flip angle alone by about 28%.

Conclusion: Compared with the 20-min MAPSS sequence for brain T1ρ mapping, the proposed learned high-resolution 3D pulse sequence simultaneously achieved a 2.3-fold improvement in effective (3.2-fold nominal) spatial resolution, a 1.1-fold improvement in SNR, and a 2.5-fold reduction in scan time.