Myelin water fraction mapping from multiple echo spin echoes and an independent B map.

PURPOSE

Myelin water fraction (MWF) is often obtained from a multiple echo spin echo (MESE) sequence using multi-component T fitting with non-negative least squares. This process fits many unknowns including B to produce a T spectrum for each voxel. Presented is an alternative using a rapid B mapping sequence to supply B for the MWF fitting procedure.

METHODS

Effects of B errors on MWF calculations were modeled for 2D and 3D MESE using Bloch and extended phase graph simulations, respectively. Variations in SNR and relative refocusing widths were tested. Human brain experiments at 3 T used 2D MESE and an independent B map. MWF maps were produced with the standard approach and with the use of the independent B map. Differences in B and mean MWF in specific brain regions were compared.

RESULTS

For 2D MESE, MWF with the standard method was strongly affected by B misestimations arising from limited SNR and response asymmetry around 180°, which decreased with increasing relative refocusing width. Using an independent B map increased mean MWF and decreased coefficient of variation. Notable differences in vivo in 2D MESE were in areas of high B such as thalamus and splenium where mean MWF increased by 88% and 31%, respectively (P < 0.001). Simulations also demonstrated the advantages of this approach for 3D MESE when SNR is <500.

CONCLUSION

For 2D MESE, because of increased complexity of decay curves and limited SNR, supplying B improves MWF results in peripheral and central brain regions where flip angles differ substantially from 180°.