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tickbox var: $chkgradampflag -- consider gradient amp
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default: 0 (FALSE)

The "ckgr" tickbox (F3 panel, lower middle)
toggles whether a gradient amplitude volume is
checked during shrink() operations.

The gradient amplitude is calculated/loaded using
the currently loaded main image volume ("im:") by
the calc_gradamp() function.  This is typically a
T1-weighted image, so the gradient amplitude will
be increase at the gray/white border and the
pial/CSF border.

The calc_gradamp() function also writes out
gradient amplitude to:

  $SUBJECTS_DIR/$subject/mri/gradamp.mgz
