Film is forever.
The first sequence is a testament to Friedkin's directing ability as we really feel Regan's pain as she's put through this battery of tests. Apparently, it's taken from the book and sets up a theological point that maybe it's the people around the girl that are the target, not the girl. Like I said, I'll take their word for it because the flick had put me to sleep by that point. I woke up right after that, but they assured me it wasn't worth going back to. It happens at the tail end of a conversation between Burstyn's character and her assistant.