they can tell the 'degenerate and dehydrated' part because of the loss of the whiteness where the disc is supposed to be. if you look higher in the spine, you'll see that the area between the vertebra, where the discs are, is mostly black, with a white foggy center (mmmm, oreos). however, the discs right above and right below L4/L5 are almost all black. that means they're losing part of the watery part of the disc, leaving only the collagenous part of the disc. this makes them less 'springy'.
