[Insight-users] SparseFieldLevelSetImageFilter Boundary Conditions

[Brady McCary]

insight-users,

I have found evidence that makes me strongly believe that this is in
fact a bug in ITK.

In order to understand this discussion, note that the Sparse Field
algorithm has to keep track of the STATUS of every pixel (whether it
is moving between layers, etc.) so that it can properly track and
construct the layers used in the algorithm. In particular, there is a
status value which signifies that a particular pixel is on the
boundary of the image. This status should (logically) be constant
throughout the entire iteration. However, if the initialization of
level set is such that the zero level set intersects the boundary of
the image, then the ``on-the-boundary'' status is lost.

I.e., the status of a pixel being on a boundary is assumed to be
constant, but in fact this status depends on the initialization of the
level set, because if the zero level set intersects the boundary of
the image, the status values associated with the layers close to the
zero level set overwrite the ``on-the-boundary'' status value.

I have been viewing the the source code and I am not clear why the
``on-the-image-boundary'' status is significant, but I am looking. If
the author of this class could comment, it would be greatly
appreciated.

I have posted two sequences of images to illustrate my findings. In
particular, the images named sta_NNNN.nii are the status of each pixel
at iteration NNNN. In the sequence which has an initialization where
image boundary pixels are INSIDE, the

http://www.utdallas.edu/~bcm052000/example.tar.bz2

Brady

On Fri, Feb 6, 2009 at 1:00 PM, Koen Vermeer <koen at vermeer.tv> wrote:
> On Fri, 2009-02-06 at 10:53 -0600, Brady McCary wrote:
>> As an update to this thread, I have observed the following. To make
> [...]
>> dynamics in my problem, it is clear that this is not appropriate.
>
> Not very useful, but: I experienced the same issues. In my case, I'm
> looking at objects that are larger than the image, meaning that the
> level set at the boundary should move - but often it doesn't. I simply
> ignored this problem for now, as I'm just exploring possible solutions.
>
> So, basically, this is just a 'me too'.
>
> Best,
> Koen
>
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