A cysto is just sticking a needle into the bladder through the abdominal wall to get urine. If the dog is small and/or the bladder is hard to find, doing it guided by u/s is a good option. I've actually watched it before and Ellie has had quite a few. No big deal...
Getting urine off of a pad isn't considered free catch. It's considered table top collection and is the least sterile way. Free catch would be following them with a dish, etc.