Hello dear and welcome to the medhelp forum. The nerves supplying sensation to the knee arise from the lower back and provide hip, leg, and ankle sensation. This knee pain can arise from the knee itself or referred from conditions affecting the hip, ankle or the lower back. MRI spine will reveal the cause whether it is a vertebral fracture, slipped intervertebral disc causing nerve impingement etc since these symptoms started with the back injury. You could take oral anti anxiety medicines like benzodiazepines under medical supervision. If it is difficult, an anesthesiologist could be requested for intravenous sedation or even general anesthesia. You need to tell the doctor if you are suffering from any other co morbidity like hypertension or heart disease. Wishing you all the best.
I would see if they could give you a muscle relaxant or something like valium or xanax for you procedure. They help relax the muscles and can lessen the spasms. Also see if they will set a couple of pillows under your legs when they do the mri. I am not positive they will be able to do the pillows because it may interfere with the image but I have been able to have them put a small pillow or wedge under my knees for me. It really helps with the twitching spasms. Best wishes, Sissie.