Other commenters have given what's probably the right answer, but in the spirit of exploration, I wonder about electrical impedance tomography. I'd think a salty cheese would be a pretty good medium to pass electricity through, and since the wheels are very uniform in shape, you could make a chuck that held electrodes in contact with the wheel. We don't really need precise information about the shape and location of the voids, we're not going to do surgery on the cheese, so the coarseness of EIT might not be an issue.