The MRI is more to find out if any other things could be causing a problem, or if bones or structures are a problem.
You're right about the position part of it-
They usually won't clearly show a soft tissue compression - unless it is the special MRA{?} but those are few and far between.
The results depends on the skill of the rad/tech/dr that reads the films too.
Much of the testing is to rule out and narrow down the possible causes of your symptoms.
I think some have said that they were able to be in the certain positions during their MRI - but I can't remember who.