MRI needed for cancer patient

My mother is a stage 4 breast cancer survivor who has had 3 recurrences over the last 10 years. Recently she had a mammogram done which showed no abnormalities but was sent for an MRI. The MRI showed that she had three tumors located in her breast. Her oncologist recommended another MRI to make sure that they haven't grown but her insurance company is denying the claim stating that her cancer is NOT life threatening and they would only cover an ultrasound. Her oncologist states that an ultrasound will only show the silicone implant she has and not the masses. I don't know where to go with this because she is my best friend and has always been here for me! I served 7 years active duty military and still serve today. She has been my biggest support and with everything she is still a true inspiration. She helps raise money for troops and would drop everything to help a stranger out. It isn't fair to see her go through this and what if it is life threatening? I am not ready to lose my mom! Please help me raise this money for her! I would pay for it myself if I had the money because I know she would do it for me!