They must have given you some reason for not approving for an MRI. Did they site any reason ??
If you are sure that you have got done all your pre-tests and MRI is the only requirement / option left, then you will need to appeal against Medicaid. I would advice you to contact / get in touch with your case manager at medicaid and he will help you and guide you through the appeals process. Typically, you will need to give an appeal letter to the Medicaid appeals representative in your area. It should be addressed to the Bureau of Appeals. Some states may also give you an option to call in the appeal to their office.
Fill out the appeals form. Most departments provide you with an appeals form for Medicaid decisions. You'll give your Medicaid information along with the reason that you are appealing the claim. Get a statement from your physician. A physician's statement is often required during the appeals process. The statement will give the reasons that you needed to get the service that was performed.
The following link details the Medicaid Appeals process http://docs.google.com/viewer?a=v&q=cache:VqOYVQD0Ab IJ:https://www.cms.gov/MLNProducts/downloads/MedicareAppealsprocess.pdf+medicaid+appeals+process&hl=en&gl=in&pid=bl&srcid=ADGEESgkR3ZhLXvORGNI30_IE-XlH3IJqnJvX6CV0xlQpNY0k1bg6UMFPSHCZsG2nDYh71BFNoH7TFq9r8YvJ3es8umuVBSir28sCiSxNx8IPiYt1b6XsGYnXmvOLI--bMSCraNtJPk_&sig=AHIEtbSRJck_RWN6IbafBEoiEewd9MfmnA
I am sure this would help.. Wish you good health...