I need to separate the crack from the background. I have uploaded one slice but it is actually a CT scan with several slices. What I want is to have only the crack, removing the large area that is only background (air). If I was not very clear then in the picture it looks like a like with rivers, I just want the rivers. I have done filters, dilation and analyse particle to just have what you can see, but I do not know how to separate crack from air as they are both the same greyscale.
This scheme might help. I took the already-processed image
you posted and eroded it down until the “rivers” were gone, and
them dilated it back up to restore the edge of the “lake.”
Then I took the difference of this image with the original to isolate
the rivers. Here is the result:
Specifically, I ran Process > Binary > Erode
eight times on your image, and then ran Process > Binary > Dilate
twelve times. (I chose eight and twelve just by playing around.)
I then ran Process > Image Calculator...
and used it to subtract the eroded-dilated image from your
original image. (I calculated original - eroded-dilated instead
of the other way around because your original image has an
inverted LUT, that is black is 255 and white is 0.)