T(2) 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 T(2) 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). METHODS: 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. RESULTS: 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. CONCLUSION: 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.