mni152reg: Get fsaverage (MNI305) to MNI152 transformation matrix.
In freesurferformats: Read and Write 'FreeSurfer' Neuroimaging File Formats

The uses the 4x4 matrix from the FreeSurfer CoordinateSystems documentation.

1	mni152reg()

There are better ways to achieve this transformation than using this matrix, see Wu et al., 'Accurate nonlinear mapping between MNI volumetric and FreeSurfer surface coordinate system', Hum Brain Mapp. 2018 Sep; 39(9): 3793–3808. doi: 10.1002/hbm.24213. The mentioned method is available in R from the 'regfusionr' package (GitHub only atom, not on CRAN).

1
2
3

    coords_tf = doapply.transform.mtx(c(10.0, -20.0, 35.0), mni152reg());
    coords_tf; #  10.695, -18.409, 36.137
    doapply.transform.mtx(coords_tf, solve(mni152reg()));