It is true that only a MRI
scan will reveal MS in most patients.
Coming to the Medtronic spinal stimulator implant, I would suggest you to collect details of the stimulator and make sure the implant is MRI compatible and also take surgeon's advise on the same.
On the other hand you could please contact your nearest MRI center and have a talk with the attending Radiologist
Please note that MRI machines vary only in their magnetic field strength and the patient bore (closed or open).
MS can be diagnosed in all MRI machines.
Hope this helps.