Doc says i have loose body in knee. but won't do an mri. what is this?

Consult your doctor: A loose body in the joint is usually a small fragment inside the joint. It may come from a piece of cartilage or something else. It's not "loose" in the sense that it won't rattle around the joint; it's just not attached to the bones. Perhaps your dr saw this on a knee xray & thinks an MRI won't change how he/she treats you, thus no reason to get one. You should be able to ask him for sure.