Why in the world would they need an x-ray to know haw many bones there are in the back. Do you really think they couldn’t examine a skeleton if they wanted.
In addition, the Gemara Berachos (28b) is obviously only referring to vertebrae that bend while bowing properly. I am sure that if one were to examine which vertebrae bend while bowing the number will be 18. It never says that there are only 18 vertebrae.