I've had two in the machine like that.
The first one was a head/neck scan so I went into the tube head first with my head at the closed end. I didn't care for that much but I did okay. It was lit up inside (very white) and they had placed mirrors in there so I could see out the bottom.
My second scan was of my lower back. I went in feet first this time but the whole way in. My head was at the opening though and that was a lot better. Again, the inside of the tube was white and it had lights in it. They also gave me this little air filled thing I could squeeze and that, surprisingly, helped a lot. After about 15 minutes, the banging became kind of rhythmic and I felt a little relaxed.