Function field_mapping_with_offset 

fn field_mapping_with_offset(
    phases: &[&[f64]],
    mag_slices: &[&[f64]],
    mask: &[u8],
    metadata: &ScanMetadata,
    config: &FieldMappingConfig,
    _n_voxels: usize,
    nx: usize,
    ny: usize,
    nz: usize,
    vsx: f64,
    vsy: f64,
    vsz: f64,
    progress: &mut dyn FnMut(usize, usize),
) -> Result<FieldMappingResult, PipelineError>

Path A: Multi-echo with phase offset removal.

MCPC-3D-S (crate::utils::phase_offset_removal) leaves the phase NaN where its offset is undefined, as MriResearchTools does. ROMEO is given that NaN phase and the brain mask, exactly as ROMEO.jl is in the authors’ pipeline, so it never grows through those voxels. The other unwrappers (QSM.rs’s options here) take no NaN: they get the phase zeroed there and the mask brain ∩ valid. The echoes are combined inside brain ∩ valid, the field is 0 outside it, and that mask is returned as FieldMappingResult::valid_mask.