Cats using my garden as a litter box?

Last year I had a 4'X8' raised bed vegetable garden that we grew tomatoes, lettuce, carrots, and cucumbers in. We really enjoyed it and would like to do it again this year. The problem is that I've noticed the neighborhood cats in there a few times and when I looked today it appears that they have been digging. I'm assuming the cats are using my garden as their litter box. I don't want to start all over from scratch again but I don't want to suffer any adverse health effects from the cat waste. Can I dig out the top few inches of dirt and add some new dirt and till it? Or am I screwed with this bed now that the cats have used it?