I'm not sure s9(3) applies. An x-ray isn't "computer generated": it is now produced digitally because nobody wants to faff around with film-based x-rays if they can avoid, but it isn't "computer generated" any more than a visible light image taken with a digital camera is "computer generated". I just checked Hansard and the House of Lords inserted that clause specifically to deal with "works to which it would not be possible to ascribe human authorship" with a nod towards "artificial intelligence" (which is surprisingly forward-looking for the Lords in the late 80s).

Authorship exists under s9 of CDPA regardless, and it'll be either the dentist (or the NHS trust, if it were done in a dental hospital, or someone with a contractual relationship to the dentist) and certainly not the patient.