I would ring and query. Does it say ultrasound of what/where? Ultrasound isn’t a usual diagnostic tool in MS investigations, but I don’t know the full background, so don’t know what else they might be looking for.
I’ve had both a pelvic ultrasound and one of my feet, before. The first was because of abnormal bleeding between periods (turned out to be cervical erosion, which is benign), and the feet was because of pain in them, which I’m now 99% certain was the beginnings of MS, but at the time, they were looking for evidence of arthritic changes.
Don’t suppose either of these helps? But each time, the appointment did specify which bit of me was going to be scanned. It didn’t just say “an ultrasound”.
Personally, although I’m not sure the results of even the MRI will be back and ready for discussion by the 18th (the actual pictures are available almost immediately, but they may not have them reviewed and written up just four working days later), I wouldn’t ring and postpone the appointment, because you don’t know how long you might have to wait for another. I’d take the appointment while you can, regardless that the consultant might not have much (anything?) to go on.
At least you will be able to talk things over. You don’t want to be waiting possibly another 12 weeks, and wishing you’d spoken to him while you had the chance. I’m sure you will be called to a follow-up anyway, if anything abnormal is found.