- It is important for doctors to know how much fat a person has in his or her body. Machines called DXAs measure how much fat mass and fat-free mass a person has. Researchers in this study have a new DXA machine, and they want to know if it measures body fat as well as the old machine. They also want to see how body fat readings from the new DXA compare to readings from a magnetic resonance imaging (MRI) scan.