T<sub>2</sub> relaxation‐time mapping in healthy and diseased skeletal muscle using extended phase graph algorithms
Kevin R. Keene, Jan‐Willem M. Beenakker, Melissa T. Hooijmans, Karin J. Naarding, E. Niks, Louise Otto, W. Ludo van der Pol, Martijn R. Tannemaat, Hermien E. Kan, Martijn Froeling
Abstract
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.