Function nan_box_smooth_3d 

pub fn nan_box_smooth_3d(
    image: &mut [f64],
    sigma: [f64; 3],
    mask: &[u8],
    grid: &Grid,
)

Masked 3D smoothing by repeated NaN-aware box filtering — the mask branch of gaussiansmooth3d! in MriResearchTools.jl (4 passes per axis, alternating direction on even passes, box widths from gaussian_box_sizes). Voxels outside mask become NaN before filtering and are only given values where the filter extrapolates across the boundary; the result is written in place.