We had a similar situation a couple of years ago with my wife. As part of her breast cancer screening her med onc ordered an MRI instead of the usual CT scan (they have been following a spot on one lung) and the clinic didn't think it needed preapproval. Insurance denied it as "investigative and not pre-approved". We ended up successfully appealing the claim, but made it clear to the clinic that they would eat the charge if the appeal was unsuccessful. They agreed.
One of the problems is that we are forever signing papers presented by the doctors, clinics, and hospitals. I think they come down to three basic areas: first, that we are financially responsible for everything; second, that we understand all the risks; and third, that we go along with what they decide to do to us. Simply saying that we didn't order the test isn't enough of a defense when insurance denies a claim. However, it is well established that the practitioners owe us a level of care and diligence that includes getting the pre-approval before performing the test or procedure. That your doctor didn't get the pre-approval means that you should eventually win, but you must move quickly to protect your credit while you fight. They will very quickly turn it over to a collection firm and you have to stay ahead of it.