I just got back from the CT scanning session where they also make the mask. It was a little uncomfortable---less so once I asked if they could put a pillow under my knees so it wouldn't be so hard on my lower back. It did feel a little scary with the pressure of the mask on top and also the plastic head/neck support thing they use was digging into the back of my head on one side--I felt like I was getting a headache by the end of it all.

Anyway, I made it through OK and I'm sure I will get more used to it as time goes on. I appreciate everyone telling me what to expect here--that made it much more bearable.