Rapid and accurate dictionary-based T ₂ mapping from multi-echo turbo spin echo data at 7 Tesla

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.