Your recourse would be to sue the at-fault driver (you sue the driver, not his/her insurer) for the cost of the MRI and any other medical costs or property damage not paid by insurance (and possibly for pain and suffering if you have suffered long-lasting significant life impairment, though doing this makes the lawsuit *much* more complex and expensive and may not be worthwhile; you could sue for costs in small claims court as you own attorney, but for pain and suffering, should really hire a lawyer and *must* hire a medical expert witness to examine you, right a report, and testify). If you can prove in court that that the cost(s) were directly caused by the accident, you can get a court order requiring the at-fault driver to pay, at which point either he or his insurer should do so.