Why? Is your patient one-sided? Lacking a posterior iliac crest?
Why the hell would you go for the anterior iliac crest? There are so many things you can mess up in the neighborhood if you don't get it right on the first shot that frankly, you'd be a ***** to even try it without image guidance.
If you can't get a posterior iliac biopsy, send them to IR and let them figure out where to get a sample.