MEF refers to the multi-echo FLASH sequence.

MEF provides several advantages over simple FLASH sequence or even the MPRAGE sequence, in that it has less distortion, higher contrast-to-noise ratio for subcortical structures, and it allows the direct estimation of underlying tissue parameters. One disadvantage is that the large dimensionality of the data (multiple-volumes for the same subject) makes it harder for the data analysis.

FreeSurfer is not optimized for the processing of MEF data yet, and this page describes some possible processing schemes.

Scheme #1 (synthesize a single image and then apply the standard FreeSurfer pipeline):

Due to the difference in image contrast of images acquired at different flip angle or echo time settings, a simple average would fail. Weighted average or nonlinear combination is thus necessary.