Yep, depending on what they're looking for, the time restraints, and if you have a history of reactions to contrast medications. I've had Barium, and I've had injections, and I've had no contrast at all in an emergency situation. A very versatile machine, lol.
I think they get better pictures with contrasts, and depending on what parts of the body they're looking at, they use different contrasts.
Once I was getting a barium x-ray before surgery and they weren't getting the images they wanted so they rushed me down to the CT scan and got them.
Actually, I think all the imaging machines can be done with or without.