• Julian Emmerich, Sebastian Flassbeck, Simon Schmidt, Peter Bachert, Mark E Ladd, and Sina Straub.
• Division of Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany.
• J Magn Reson Imaging. 2019 May 1; 49 (5): 1253-1262.

BackgroundUsing lower refocusing flip angles in multi-echo turbo spin echo (ME-TSE) sequences at ultra-high magnetic field leads to non-monoexponential signal decay and overestimation of T2 values due to stimulated and secondary echoes.PurposeTo investigate the feasibility of a fast and accurate reconstruction of quantitative T2 values using an ME-TSE sequence with reduced refocusing flip angles at 7 Tesla, a dictionary-based reconstruction method was developed and is presented in this work.Study TypeProspective.SubjectsPhantom measurements with relaxation phantom, four healthy volunteers.Field Strength/Sequence7 Tesla MRI, multi-echo turbo spin echo (ME-TSE), spin echo (SE), and B1 mapping.AssessmentBased on Bloch simulations and the extended phase graph model, signal decay curves were calculated to account for nonrectangular slice profile, B1 inhomogeneity, and reduced refocusing flip angles and stored in a dictionary. Data obtained with an ME-TSE sequence at 7 Tesla were matched to this dictionary to obtain T2 values. To compare the proposed method to reference T2 values, a spin echo sequence with different echo times was used.Statistical TestsWelch's t-test was used to compare T2 values in phantom measurements.ResultsT2 values obtained with the proposed ME-TSE method coincided with the T2 values from the spin echo experiment in phantom measurements (P = 0.89 for 120° flip angle, P = 0.75 for 180° flip angle). Only for very low B1 transmit fields, a slight overestimation of T2 values was observed. In vivo measurements showed lower T2 values in gray matter (55 ± 2 millisecond) and white matter (39 ± 5 millisecond) compared with literature values of 3 Tesla data.Data ConclusionsThe proposed dictionary-based ME-TSE approach provided accurate T2 values in short measurement time at 7 Tesla with low specific absorption rate burden due to the reduction of refocusing flip angles. Therefore, it can provide new opportunities in clinical high-field MRI to further improve radiographic diagnosis by using quantitative imaging.Level Of Evidence1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;49:1253-1262.© 2018 International Society for Magnetic Resonance in Medicine.

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