[Insight-users] SparseFieldLevelSetImageFilter Boundary Conditions

[Brady McCary]

insight-users,

I have implemented some particular level set methods for segmentation.
Right now, the underlying class I am using is
SparseFieldLevelSetImageFilter. There appear to be some artifacts on
the boundary of the image. In particular, it appears that some (but
not all) boundary pixels are receiving strange updates. In particular,
the output of the filter (i.e., the level set) has values that are the
background value (-3 in this case) adjacent to foreground vales (3 in
this case) and no narrow band between. My understanding is that this
is contrary to the sparse field algorithm, so I am inclined to think
one of the following:

1. I am making some abuse of the boundary condition.
2. There is a bug in the sparse field implementation concerning the
boundary condition.

I have posted the output as a sequence of images in an archive so you
can see what I am talking about. Most files in this archive have
evident names and all images are in nifti format. Files named
phi_NNNN.nii are the level set at iteration NNNN.

http://www.utdallas.edu/~bcm052000/mpr021_Slice_1.png.test.tar.bz2

Please let me know if you need more information, because I want this
anomaly resolved and I am willing to hunt this down if it happens to
be bug in ITK. For the time being, I am going to modify the
NarrowBandLevelSetImageFilter and DenseFiniteDifferenceImageFilter
implementations to include the CalculateUpdateValue which is present
in the sparse field implementation (because in this method I am
performing a necessary computation). I will then test my
LevelSetFunction in these FDSs to see if similar boundary artifacts
appear.

As a small aside, I have been digging in the internals of ITK much as
of late, especially the FDSH. It might be beneficial in the long-term
for FiniteDifferenceImageFilter to implement CalculateUpdateValue (as
it currently appears in SparseFieldLevelSetImageFilter) so all
subclasses can have easy access to it.

Brady