Getting ruby to wait for return from command line call

Question

I'm using the backtick method to make an ImageMagick call on each file in a directory. It looks like 10 or so images get processed and then it freezes. Forgive my ignorance but is Ruby trying to make all these calls serially without waiting for a return? How should I be doing this?

I would just use ImageMagick to directly batch process but I need to manipulate some strings with ruby before adding them to the image.