Re: C-Scan question
I'm in NY and they don't do the CT scan in both positions anymore -- they are able to get all views while lying on your back. Find out if the place you went to does it this way.
As for your other question, yes, conditions in your sinuses can change all the time. I have had far too many sinus scans over the years and they differ frequently. Try to get one when you're feeling realtively well, so that it will show any chronic problems, not just an infection. You can develop polyps at any time, too.