Converts an image to raw pgm format.
Results go to stdout and should be redirected.
Usage:   mritopgm [-pp] input_image
Example: mritopgm fred.001 | ppmtogif > fred.001.gif

  The '-pp' option expresses a clipping percentage.
  That is, if this option is given, the pp%-brightest
  pixel is mapped to white; all above it are also white,
  and all below are mapped linearly down to black.
  The default is that pp=100; that is, the brightest
  pixel is white.  A useful operation for many MR images is
    mritopgm -99 fred.001 | ppmtogif > fred.001.gif
  This will clip off the top 1% of voxels, which are often