Seconding what Karlin said about MRIs, I was talking to a neurologist recently, as Oliver is slowly getting more symptoms of his CM/SM, about the need to have a full MRI (so far he's only had two mini scans) to find out what is going on. She said that though it would be interesting to see how his syrinx and dilated ventricles had developed, it wasn't essential, and I would do better to keep my money to spend on medication for him. Unless surgery is an option, whatever a scan says, you can still really only treat the symptoms you and your vet (and above all, a neurologist) can see. Finding the right medication is almost always a matter of trial and error for a few months, and then adjustment from time to time, but when the right combination is found for your dog, more or less normal life should be able to be resumed!
Kate, Oliver and Aled