The scaling results from the minimal and maximal value
of the image. Actually the real minimal and maximal values
of the image could be just some single casual pixels
which are not relevant values for the rescaling. For this
reason, a statistical method is used to count the minimal
and maximal values used in the rescaling process. Hence,
the values of percentile 35 and percentile 99.9 are calculated.
This is because the typical large black background
of MR images area covers at least 35% of the image and the relevant high intensity area (cranium) covers at least
0.1% of the image.