T2 relaxation-time mapping in healthy and diseased skeletal muscle using extended phase graph algorithms

PURPOSE: Multi-echo spin-echo (MSE) transverse relaxometry mapping using multi-component models is used to study disease activity in neuromuscular disease by assessing the T2 of the myocytic component (T2water ). Current extended phase graph algorithms are not optimized for fat fractions above 50% and the effects of inaccuracies in the T2fat calibration remain unexplored. Hence, we aimed to improve the performance of extended phase graph fitting methods over a large range of fat fractions, by including the slice-selection flip angle profile, a through-plane chemical-shift displacement correction, and optimized calibration of T2fat .

METHODS: Simulation experiments were used to study the influence of the slice flip-angle profile with chemical-shift and T2fat estimations. Next, in vivo data from four neuromuscular disease cohorts were studied for different T2fat calibration methods and T2water estimations.

RESULTS: Excluding slice flip-angle profiles or chemical-shift displacement resulted in a bias in T2water up to 10 ms. Furthermore, a wrongly calibrated T2fat caused a bias of up to 4 ms in T2water . For the in vivo data, one-component calibration led to a lower T2fat compared with a two-component method, and T2water decreased with increasing fat fractions.

CONCLUSION: In vivo data showed a decline in T2water for increasing fat fractions, which has important implications for clinical studies, especially in multicenter settings. We recommend using an extended phase graph-based model for fitting T2water from MSE sequences with two-component T2fat calibration. Moreover, we recommend including the slice flip-angle profile in the model with correction for through-plane chemical-shift displacements.