Who is responsible for medical bill

My sick child was sent by his pediatrician to a local hospital out patient radiology for an abdominal CT.My pediatricians office contacted Hmo prior to procedure for preauthorization it was granted.Several weeks later I receive a denial of claims notice.When I call the insurance they state they paid for abdominal CT but the hospital had done a Cat scan of the pelvis as well.The insurance wont pay it cause no preauthorization and my doctor did not want this test.They never contacted my md priorMy son was exposed to additional radiation he didnt need with this other test.The hospital is demanding I pay out of pocket for a test we didnt ask for or they will submit to collections.I have tried to reason with the hospital but they are still demanding we pay.Do I need a lawyer and if so what kind