Andreia C. Freitas^{1}, Inês Sousa^{1}, Andreia S. Gaspar^{1}, Rui P.A.G. Teixeira^{2}, Joseph V. Hajnal^{2}, and Rita G. Nunes^{1,2}

Multi spin-echo (MSE) sequences have been prescribed for efficient T2 mapping. This can be further improved by matching to pre-computed echo-modulation curves (EMC). Previous use of this method to estimate T_{2} and B_{1} resulted in bias in the latter. We investigated the possibility to improve B_{1} by taking advantage of its spatial smoothness, using a fusion bootstrap moves solver (FBMS). The two methodologies were compared using a numerical phantom and in-vivo brain data. While T_{2} estimation was accurate and equivalent, B_{1} accuracy was improved using the FBMS. Future work is required to tune the regularization parameters of the FBMS algorithm.

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Figure 1 - Numerical phantom simulations. (a) Reference T_{2} map, ranging from 0 to 400 ms. (b,g) T_{2} and B_{1} maps estimated with EMC dictionary match. (c,h) T_{2} and B_{1} maps estimated with FBMS, run at 10 iterations and smoothing factors of α_{B}_{1} = 5 and α_{T2} = 2. (d,i) Relative difference image between T_{2} and B_{1} maps obtained with FBMS and EMC. (e,j) Relative difference image between T_{2} and B_{1} maps obtained with FBMS and reference map. (f) Reference B_{1} field map, ranging from 60 to 140%.

Figure 2 - Example of in-vivo brain axial slice located above the ventricles. (a) T_{2} map obtained with a monoexponential fit to the even echoes, here used as reference. (b,g) T_{2} and B_{1} parametric maps obtained from EMC matching. (c,h) T_{2} and B_{1} parametric maps obtained from FBMS α_{B1} = 5 and α_{T2} = 2 and 10 iterations. (d,i) Relative difference image between T_{2} and B_{1} maps obtained with FBMS and EMC. (e,j) Relative difference image between T_{2} and B_{1} parametric maps obtained with FBMS and reference map (monoexponential fit or AFI). (f) B_{1} map obtained with AFI, here used as reference.

Figure 3 - Quantitative results for mean T_{2} and B_{1} parametric estimations in three selected regions corresponding to the CSF, white matter and grey matter brain tissues. Comparison of FBMS and EMC matching estimation results to reference values. (a,b) T_{2} and B_{1} correlation plots compared to reference T_{2} (monoexponential fit) and B_{1} (AFI) using an axial brain slice encompassing the ventricles (increased presence of CSF with long T_{2} values). (c,d) T_{2} and B_{1} correlation plots compared to reference T2 (monoexponential fit) and B_{1} (AFI) using axial slice which did not include the ventricles.