I asked Dr.Godley similar question yesterday during the webcast.
His answer was (not quoting verbatim of course) - "The Doppler is tricky. If not done from every possible angle and in every possible head position it might not reveal anything. I think venography is the only sure way to test."
The reason I asked was because my own "I am not so sure they will find anything" Doppler test is scheduled for this upcoming Monday.