You can sue them IF under the terms of your insurance policy (which is a contract) they should be paying for your CAT scan in these circumstances; in that case, you could sue them for the cost of the CAT scan, for "breach of contract" (for violating their contractual obligation of when and what to pay. You have to be able to show that you are entitled to this coverage under the terms of the policy, for this purpose; and if you can, are entitled to the amount of money (i.e. the cost of the scan) they should have paid. That is all you would be entitled to--what they should have paid in the first place.