[Insight-users] Units of MaximumRMSChange in SegmentationLevelSetImageFilter

[Koen Vermeer]

Hi,

In the documentation of SegmentationLevelSetImageFilter, the accepted
range for MaximumRMSChange is 0.0-1.0. What does this actually mean?
Given this fixed range, I would assume that the units for this metric is
something else than the units of the image itself. Is it some kind of
normalized value? If so, how is this done?

My assumption was that it works a bit like this:
Let LS_i(x) denote the value of the levelset function (not a specific
level set) at iteration i at location x. Now, after each iteration, the
RMS value for LS_i(x)-LS_i-1(x) is calculated over all values of x
within the image (or within the narrow band?). If this value is smaller
than MaximumRMSChange, stop, otherwise do another iteration.

Obviously, this cannot be correct, because in that case it would be very
reasonable to have values larger than 1.0 for MaximumRMSChange, as the
values of the level set function also depend on the spacing of the
image.

Can anyone shed some light on this?

Best,
Koen