The MRI protocol consists of intravenous injection of 0.4 ml/kg dose of gadopentetate dimeglumine 2- [Gd-DTPA2-] contrast agent (Magnevist, Bayer) 90 minutes prior to imaging. Due to the need to image within a relatively narrow window of time, each hip of each subject must be imaged on separate days. The subject will be asked to walk for 10 minutes after receiving the contrast agent injection to facilitate equilibration of contrast agent into the hip joint.