Why do you have to hold your breath during CAT scans and X-Rays?

Don't worry about it. The person doing the CAT scan and the x-rays will say "Hold still and hold your breath." These machines are sort of like a camera. If you are taking a picture of someone and they move then you are going to get a blurred image. It's important you listen to them because if not they'll have to do it all over again. It's painless and all you have to do is follow their directions.