That is fascinating on the Open MRI scanners. With my spinal injury I have spent plenty of time being scanned both with the MRI and a Bone scan. Ironically I found the Bone Scan which was open more of an ordeal that the MRI. The MRI I found very noisy even with all the ear defenders , I had a "silence of the lambs" style mask placed over my head so they could get a good look at my neck which was very claustrophobic the only way I could deal with it was to keep my eyes closed all of the time. As for falling asleep I found it too noisy and would be worried I would move and end up being in there even longer.
I hope the scan went well.