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Rapid and accurate dictionary‐based T 2 mapping from multi‐echo turbo spin echo data at 7 Tesla
Author(s) -
Emmerich Julian,
Flassbeck Sebastian,
Schmidt Simon,
Bachert Peter,
Ladd Mark E.,
Straub Sina
Publication year - 2019
Publication title -
journal of magnetic resonance imaging
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.563
H-Index - 160
eISSN - 1522-2586
pISSN - 1053-1807
DOI - 10.1002/jmri.26516
Subject(s) - flip angle , imaging phantom , fast spin echo , spin echo , echo (communications protocol) , nuclear magnetic resonance , physics , echo time , signal (programming language) , pulse sequence , magnetic resonance imaging , optics , computer science , medicine , computer network , radiology , programming language
Background Using 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 T 2 values due to stimulated and secondary echoes. Purpose To investigate the feasibility of a fast and accurate reconstruction of quantitative T 2 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 Type Prospective. Subjects Phantom measurements with relaxation phantom, four healthy volunteers. Field Strength/Sequence 7 Tesla MRI, multi‐echo turbo spin echo (ME‐TSE), spin echo (SE), and B 1 mapping. Assessment Based on Bloch simulations and the extended phase graph model, signal decay curves were calculated to account for nonrectangular slice profile, B 1 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 T 2 values. To compare the proposed method to reference T 2 values, a spin echo sequence with different echo times was used. Statistical Tests Welch's t‐ test was used to compare T 2 values in phantom measurements. Results T 2 values obtained with the proposed ME‐TSE method coincided with the T 2 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 B 1 transmit fields, a slight overestimation of T 2 values was observed. In vivo measurements showed lower T 2 values in gray matter (55 ± 2 millisecond) and white matter (39 ± 5 millisecond) compared with literature values of 3 Tesla data. Data Conclusions The proposed dictionary‐based ME‐TSE approach provided accurate T 2 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 Evidence: 1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;49:1253–1262.