Hi, I'm so sorry you're going through this with Sherman. Has your vet totally checked out his tail pocket area to rule out any infections? Or could it be an anal gland problem. I would have those areas thoroughly checked out first before making a decision to do a MRI, since they are so expensive, especially since his foot is not affected, and he's not showing any other neurological symptoms like seizures or tremors, or balance problems. I hope it isn't something serious and you find the problem. Could it be itchy due to food related allergies? What type of dogfood is Sherman eating? Has he been tested for worms or other parasites that could cause itching in the back area? I would try ruling out these things first before going with an expensive MRI procedure. Good luck. Please keep us posted.