[Insight-users] Is it acceptive to set seeds outside of region of interest in Geodesic Active Contour Level Set?

[Chris McIntosh]

Hi Goo,

Firstly, if you want to experiment with GAC without programming things 
or worrying that you got something wrong
then you  may start out with the insight applications:

http://www.itksnap.org/
http://www.itk.org/HTML/GeodesicActiveContour.htm
http://www.itk.org/HTML/ShapeDetectionLevelSet.htm

Personally I recommend starting with itk-SNAP, because its very easy to use.

1) You can use seeds outside the object you want to segment but it 
certainly makes the segmentation
take longer, and more difficult. To my knowledge when you set seeds and 
then run the fast-marching you create
small circles (or spheres) in 3D as the initial signed distance 
transform. You can verify this by taking the output
of the fast-marching and writing it to file. Note, however, depending on 
the setting of the parameters the GAC
method may prefer to shrink or expand the shape. If your going to shrink 
it's best to start with the target completely
encompassed, if your going to expand its best to start completely inside.

2) Every Level Set method requires an initial signed distance function 
as input. You can specify that input as seeds via
fast marching, or if you have a rough approximation of your segmentation 
you can initialize using the signed distance
transform of the binary segmentation estimate.

In general though I can tell you for many structures its not trivial to 
get the standard level set formulations like GAC,
etc to segment the image, unless you are able to initialize very close 
to the segmentation (and sometimes even then).

Cheers,

Chris

Goo wrote:
> Hi all:
> I intend using Geodesic Active Contour Level Set to segment tissue.
> It's common using fast marching as initialization mechanism in ITK 
> level set methods.
> However, I doubted of the feasibility if the seeds are outside of the 
> region to segment.
> For this reason, there are two questions in here.
>
> (1) Can I set seeds that are outside of region I want to segment?
> (2) Whether such methods that using level set without fast marching or 
> without requirement to give seeds.
>
> Regards.
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