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

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 (T_2water ). Current extended phase graph algorithms are not optimized for fat fractions above 50% and the effects of inaccuracies in the T_2fat 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 T_2fat .

Simulation experiments were used to study the influence of the slice flip-angle profile with chemical-shift and T_2fat estimations. Next, in vivo data from four neuromuscular disease cohorts were studied for different T_2fat calibration methods and T_2water estimations.

Excluding slice flip-angle profiles or chemical-shift displacement resulted in a bias in T_2water up to 10 ms. Furthermore, a wrongly calibrated T_2fat caused a bias of up to 4 ms in T_2water . For the in vivo data, one-component calibration led to a lower T_2fat compared with a two-component method, and T_2water decreased with increasing fat fractions.

In vivo data showed a decline in T_2water 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 T_2water from MSE sequences with two-component T_2fat calibration. Moreover, we recommend including the slice flip-angle profile in the model with correction for through-plane chemical-shift displacements.