Edge detector and Hough transform

Dear all,

As suggested I am creating a new post for my doubt!

Well, I am newby in Image processing with matlab and I am facing a trouble, for which I need some help/advice.

I have a set of greyscale images where is slighly visible a lighter curve that I want to stand out for further analysis. I tried to do that with the 'Canny' edge detector, but in those images I have also white styrofoam beads (needed for another analysis) that are detected and surrounded. I attach to this post an image to make more clear my case.

My doubt is: how can I just keep the "curve" (that it is not so bright as the styrofoam beads) for further analysis as the determination of its length and radius?

I thought in using the Hough transform for the last step, but that "curve" is not constant as well it has not a known shape.

Thank you very much in advance,