Improving Accuracy and Reproducibility of Cartilage T Mapping in the OAI Dataset Through Extended Phase Graph Modeling.

Background: The Osteoarthritis Initiative (OAI) collected extensive imaging data, including Multi-Echo Spin-Echo (MESE) sequences for measuring knee cartilage T relaxation times. Mono-exponential models are used in the OAI for T fitting, which neglects stimulated echoes and B inhomogeneities. Extended Phase Graph (EPG) modeling addresses these limitations but has not been applied to the OAI dataset.

Purpose: To assess how different fitting methods, including EPG-based and exponential-based approaches, affect the accuracy and reproducibility of cartilage T in the OAI dataset.

Study Type: Retrospective.

Population: From OAI dataset, 50 subjects, stratified by osteoarthritis (OA) severity using Kellgren-Lawrence grades (KLG), and 50 subjects without OA diagnosis during OAI duration were selected (each group: 25 females, mean ages ~61 years).

Field Strength/sequence: 3-T, two-dimensional (2D) MESE sequence.

Assessment: Femoral and tibial cartilages were segmented from DESS images, subdivided into seven sub-regions, and co-registered to MESE. T maps were obtained using three EPG-based methods (nonlinear least squares, dictionary matching, and deep learning) and three mono-exponential approaches (linear least squares, nonlinear least squares, and noise-corrected exponential). Average T values within sub-regions were obtained. Pair-wise agreement among fitting methods was evaluated using the stratified subjects, while reproducibility using healthy subjects. Each method's T accuracy and repeatability varying signal-to-noise ratio (SNR) were assessed with simulations.

Statistical Tests: Bland-Altman analysis, Lin's concordance coefficient, and coefficient of variation assessed agreement, repeatability, and reproducibility. Statistical significance was set at P-value <0.05.

Results: EPG-based methods demonstrated superior T accuracy (mean absolute error below 0.5 msec at SNR > 100) compared to mono-exponential methods (error > 7 msec). EPG-based approaches had better reproducibility, with limits of agreement 1.5-5 msec narrower than exponential-based methods. T values from EPG methods were systematically 10-17 msec lower than those from mono-exponential fitting.

Data Conclusion: EPG modeling improved agreement and reproducibility of cartilage T mapping in subjects from the OAI dataset.

Evidence Level: 3 TECHNICAL EFFICACY: Stage 1.