Debbie,

yes it looks like a doughnut. Re segment. It (the detection) goes in stages. It moves you, then you wait... then it moves you. ... I just count.... that is how I came up with 140-180. It is just boring that is all. Nothing bad is happening to you in the machine. The only thing you MIGHT feel if they inject you with a contrast agent (for the CT portion). That just feels like you get warm all over, not really unpleasant.
As Jim said they will inject you with 18F-glucose after which you sit still for 45 minutes or so. FYI, the glucose the inject you is radioactive so do not freak out. It will decay quickly and not be in your system very long. But this is what they need to see where the glucose goes..... Also follow the instructions you get to the letter. The last thing you want is a false positive which creates a lot of anxiety. Compared to IMRT it is a piece of cake.
Markus