Hello all. I'm an absolute beginner with ImageJ/Fiji. I've been able to apply a few useful built-in tools to my CT scans, but many of the filters fail because my images have a background. The kind of images I look at can be found here: https://blog.highmarkhealth.org/low-dose-ct-scans-show-promise-in-detecting-early-stage-lung-cancer/lungcancer/
As you can see, anything outside the inscribed circle is useless background. My question to the community is: How do you suggest I get rid of this background? Can I do this using the existing functions, or do I have to write my own script or plug-in? (additional detail that may be of help: for my purposes, it would be OK to merge the background with the dark, textured parts at the top and bottom of the disk.) Thanks in advance!!