Cross Calibration of Two Dual Energy Densitometers and Comparison of Regional Adipose Tissue Measurements by iDXA and MRI

Background: \- 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. Objectives: \- To see if two DXA machines give the same results when measuring body fat. Also, to know whether the DXA and MRI give the same results in measuring visceral and subcutaneous fat. Eligibility: \- People 18 years and older who weigh less than 350 pounds. Design: * Participants will be screened with a medical history and physical exam. Women will have a pregnancy test. Researchers will make sure participants are able to have an MRI if they volunteered to. * Participants will have their body fat measured by each DXA scanner. They will lie quietly on a padded table while being scanned. The scan uses X-rays to take pictures of the body. Both scans will be done on the same morning and will take 10-15 minutes on both machines. * Some participants also will have their body fat measured with MRI. They will lie in a long, tube-shaped scanner. The machine uses strong magnetic fields and radio waves to take pictures of internal organs. Participants will sign an additional consent form for this procedure.