Performance of MR learned pulse sequences for 3D bi-exponential, stretched-exponential, and mono-exponential T₂ and T_1ρ mapping of knee cartilage

Purpose: To compare the performance of a learned magnetization-prepared gradient echo (L-MPGRE) sequence against a commonly used sequence for 3D T₂ and T_1ρ mapping of the knee joint, the magnetization-prepared angle-modulated partitioned k-space spoiled gradient echo snapshots (MAPSS), on bi-exponential (BE), stretched-exponential (SE), and mono-exponential (ME) relaxation models.

Methods: We used a combined differentiable and non-differentiable optimization to learn pulse sequence structure and its parameters for 3D T₂ and T_1ρ mapping of the knee joint using ME, SE, and BE models. The learned pulse sequence framework was used to improve quantitative accuracy and SNR and to reduce filtering effects. We compare the measured multi-compartment values between the two sequences (n = 8), and their repeatability (n = 4) in healthy volunteers (n = 12 total).

Results: The voxel-wise median absolute percentage difference (MAPD) between the T₂ and T_1ρ maps obtained with each sequence was 18.6% and 19.9%, respectively. The T₂ and T_1ρ repeatability tests showed a MAPD of 18.5% and 19.1% for MAPSS, and 16.8% and 15.5% for L-MPGRE. Bland-Altman region of interest (ROI)-wise analysis shows that bias is small, close to -1.5%, and the coefficient of variation is around 5.5% when comparing ROIs from both sequences.

Conclusion: The L-MPGRE sequences can be used as a replacement for MAPSS for T₂ and T_1ρ mapping in the knee cartilage with advantages, achieving similar accuracy and 15% better repeatability in only half of its scan time.