Convert number of NN smoothing iterations to a FreeSurfer Gaussian smoothing FWHM.

Inverse of fwhm2niters: convert a number of nearest-neighbor (NN) smoothing iterations into the equivalent Gaussian smoothing kernel width, given as the full width at half maximum (FWHM) in millimeters.

Usage

niters2fwhm(niters, surface = NULL, avg_vertex_area = NULL, k = 1L)

Arguments

niters: positive integer, the number of NN smoothing iterations (the num_iter parameter of pervertexdata.smoothnn).
surface: a mesh, represented as an fs.surface instance from the freesurferformats package, a tmesh3d instance from rgl, or a character string representing the path of a surface file to load. Used to compute the mean vertex area. Ignored if avg_vertex_area is given.
avg_vertex_area: numeric scalar, the mean vertex area of the mesh in square millimeters (squared mesh units). If NULL (the default), it is computed from surface. See details for how FreeSurfer's own numbers differ.
k: positive integer, the k-ring neighborhood size to use for NN smoothing. The FreeSurfer-based calibration below is only valid for the 1-ring neighborhood (k=1); a warning is issued otherwise.

Value

numeric scalar, the FWHM in millimeters of the Gaussian smoothing kernel that the given number of NN smoothing iterations approximates.

Details

This is the inverse of fwhm2niters, using the same empirically calibrated FreeSurfer formula (utils/mrisutils.cpp in the FreeSurfer source):


  gstd = sqrt((7 * avg_vertex_area * niters) / (1.14 * 4 * pi))
  fwhm = gstd * sqrt(log(256))

It is useful to check what amount of Gaussian smoothing (FWHM) a given number of NN smoothing iterations corresponds to, e.g., to report the equivalent FWHM in a paper or to compare settings across methods. The calibration is only valid for the 1-ring neighborhood (k=1).

Examples

if (FALSE) { # \dontrun{
mesh = freesurferformats::read.fs.surface(system.file("extdata", "fsaverage_mesh_lh_white", package = "haze", mustWork = TRUE));
fwhm = niters2fwhm(92, surface = mesh);       # ~10 mm FWHM
} # }