Bias Correction in FSL

I want to have non-parametric and non-uniform bias correction (N3/N4 Bias Field Correction) on RAW structural MRI using fsl. Is there any way to achieve this without FAST segmentation ? If no, Is the fsl FAST tool the better for this task when compared to ANTS.
And is this bias correction of RAW sMRI images prior to Registration and Stripping a common approach ?