[Freesurfer] change ants N4 bias field correction parameter in recon-all pipeline

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Hello FreeSurfer Developers,

Some T1s we collected have higher intensity in the occipital cortex. The default recon-all failed skull stripping and wm/gm segmentation in that part of the brain. After several tests, we found that doing ants N4 bias field correction with a shrink factor of 1 could fix the problem in most cases. We are aware that the same correction is included in recon-all, while with default parameters. We wonder if there is one way to specify the parameter for the bias correction in recon-all? If not, what would you recommend in this situation? A bias correction before recon-all, and still do another correction inside recon-all? The freesurfer we use is 7.1.1. Thanks!

best, Mengxing