[Insight-users] question about distance map

[Zachary Pincus]

Hello -

First off, make sure to cc the ITK list so that others might also be  
able to help you.

Second, note that technically you don't need to provide a full "level  
set" image to the algorithm to start. While the signed distance map  
of the thresholded input is good, you could technically just pass the  
thresholded input (provided the "inside value" is less than the  
"outside value") as the initial level set, and set the "level set  
value" member variable to the number halfway between the inside and  
outside values. (If this doesn't make sense, it is worth meditating  
on, and if still not, asking for more details.)

Nevertheless, the fact remains that you think there's a problem with  
the initial level set, which is to say a problem with the initial  
conditions you are passing to the geodesic active contour level sets  
algorithm. Without getting two far into the details, it's important  
to provide good initial conditions for the geodesic active contours  
level sets method: while the initial level set needn't be completely  
inside the object to be segmented, it needs to be close enough to get  
"sucked in" to the image edges via the advection term. If you can't  
generate such an initial segmentation, consider (a) generating an  
initial level set completely outside of the bone, and setting the  
"reverse propagation direction" (or whatever; consult the doxygen)  
flag, or (b) more advanced methods (such as providing your own speed  
image that is negative outside of the bone, causing level sets in  
outside regions to "shrink" back to the bone area).

I'm not sure what you mean when you say that "it works better"  
without the signed distance map. What, precisely, are you providing  
as the initial level set in that case?

Of course, the input is but one of many parameters that can be tuned  
to make the geodesic level sets algorithm work better: there's the  
parameters of the class itself, plus all of the parameters of the  
preprocessing steps. I suggest that you look at each of the output  
images of these steps to ensure that the features that are meaningful  
to your needs are the ones getting hilighted by the preprocessing.  
Otherwise it's all a waste of time.

Zach

On Oct 19, 2005, at 2:07 PM, OnezerO wrote:

> Hi,
> Thanks for the interest.
> I'm gonna try to explain (my english is kind of bad).
> I want to create a 3D model of a bone structure that's inside of a  
> serial of
> CT images. For this i'm using the geodesic active contour filter,  
> in the
> guide of itk said that it's need 2 inputs images, one corresponding  
> to the
> distance map of the image and another preprocessed image, in this  
> context i
> dont understand the use of the fastmarching filter, but like the  
> manual said
> thas use of this filter is to obtain a distance map i think to use
> danielsson filter.
>
> The process is the next
>
> InputImage -> CurvatureAnisotropicDiffusionImageFilter ->
> GradientMagnitudeRecursiveGaussianImageFilter -> SigmoidImageFilter ->
> PotentialEdgeImage
>
> InputImage ->
> ThresholdImageFilter->SignedDanielssonDistanceMapImageFilter- 
> >DistanceMapIma
> ge
>
> PotentialEdgeImage-
>              |--> GeodesicActiveContourLevelSetImageFilter ->
> BinaryThresholdImageFilter -> VTKImageExport ->3D View
>   DistanceMapImage-
>
> When i write the result are bad i mean that the segmentation  
> obtained at the
> end does't work, i don't obtain the 3d view, if i remove the  
> danielsson
> filter i get a sementation and hence a 3d image, but this  
> segmentation is
> not optimal.
>
> I hope now i explain better what my problem is
>
> Thanks
>
> -----Mensaje original-----
> De: Zachary Pincus [mailto:zpincus at stanford.edu]
> Enviado el: Miercoles, 19 de Octubre de 2005 15:44
> Para: OnezerO
> CC: insight-users at itk.org
> Asunto: Re: [Insight-users] question about distance map
>
> Could you clarify a bit about what you need the distance map for?
> Even better would be to back up and explain what your desired  
> outputs are,
> what your input images are, what your approach has been, and where  
> you are
> seeing problems.
>
> When you say "the results are not good" what results do you refer to?
> The Danielsson filter produces good distance maps, so presumably  
> the problem
> is what you're doing afterward with that distance map.
>
> If you explain your problem and goals a little more, hopefully I or  
> someone
> else can help.
>
> Zach Pincus
>
> Program in Biomedical Informatics and Department of Biochemistry  
> Stanford
> University School of Medicine
>
>
> On Oct 19, 2005, at 12:59 PM, OnezerO wrote:
>
>
>>
>>
>>
>> Hello
>>
>> How can i get a distance map without using FastMarching, because i
>> dont get the method itself, i'm trying to use Danielsson Filter as
>> well but the results are not good, all the above in the context that
>> i'm trying to do a example of geodesic active contour to make a 3D
>> view of any bone. Or is there another method to extract in a better
>> way the bone inside de images?.
>>
>>  sorry for the english.
>>
>> Thanks
>>
>> _______________________________________________
>> Insight-users mailing list
>> Insight-users at itk.org
>> http://www.itk.org/mailman/listinfo/insight-users
>>
>>
>
>