<DIV>Hello All,</DIV>
<DIV> </DIV>
<DIV>I am looking for structuring element in ITK that has rectnagular structuring element, since I need to have separate radius for X and Y. The default one in ITK is BallStructuringElement which applies same value in X and Y. how can i get structuring element with different values for x and y? do i have to define such element or there is one already avilable in itk?<BR><BR>thanks<BR>neha<BR><B><I>Luis Ibanez <luis.ibanez@kitware.com></I></B> wrote:</DIV>
<BLOCKQUOTE class=replbq style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #1010ff 2px solid"><BR>Hi Neha,<BR><BR>The purpose of the speed image is to make the level set<BR>slow down close to the edges of anatomical structures,<BR>and to make it go faster on the internal regions.<BR><BR>Therefore you want the speed image to have low<BR>(dark) values on the pixels close to edges of<BR>anatomical structures and to have high (bright)<BR>values in the middle.<BR><BR>This is explained in detail on the LevelSets chapter<BR>of the ITK Software Guide.<BR><BR>http://www.itk.org/ItkSoftwareGuide.pdf<BR><BR><BR><BR>If you use the Gradient magnitude directly as<BR>a speed image, you will get the opposite effect.<BR>You will make the level set to move fast across<BR>edges, and to slow down inside the structures.<BR><BR>That's the reason why you need to revert the<BR>intensities of the Gradient Magnitude image.<BR><BR><BR><BR>Note that you don't have to use the Sigmoid.<BR>Any other filter
that reverts the image intensities<BR>will work here.<BR><BR>A filter that is commonly used in the LevelSet<BR>literature is the negative exponential<BR><BR>http://www.itk.org/Insight/Doxygen/html/classitk_1_1ExpNegativeImageFilter.html<BR><BR>You can also use the EdgePotential image filter.<BR><BR>http://www.itk.org/Insight/Doxygen/html/classitk_1_1EdgePotentialImageFilter.html<BR><BR><BR><BR>Note that the choice of the filter is critical, since<BR>the speed image (or feature image) is the most important<BR>information used for controlling the evolution of the<BR>LevelSet. The segmentations that you obtain from LevelSets<BR>are as good or as bad as your feature image.<BR><BR><BR><BR>Regards,<BR><BR><BR>Luis<BR><BR><BR><BR><BR>-------------------<BR>neha k wrote:<BR><BR>> Hello,<BR>> <BR>> For Shape Detection Level Set Filter and other Level Set filters, speed <BR>> image is obtained from Gradient Magnitude. Then the speed is calculated <BR>> from Speed Image which is
obtained from Sigmoid filter. How is speed <BR>> calculated from it? It looks like output of sigmoid is obtained by <BR>> inverting gradient magnitude ouput. Is it possible to use gradient <BR>> magnitude output as a speed image? I am not clear on real advantages and <BR>> purpose of using sigmoid filter. Any input of this is appreciated.<BR>> <BR>> Thanks,<BR>> <BR>> Neha<BR>> <BR>> <BR>> <BR>> <BR><BR><BR><BR></BLOCKQUOTE><p>
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