For the 6 healthy volunteers, only an MRI of the right knee will be required at 4 time points - baseline, 6 months, 12 months, and 18 months. The control volunteers will be used as a measure of reproducibility. The control volunteers will not perform any physical performance testing or study questionnaires. The purpose of the six control subjects is to track the reproducibility and physiologic variation in T1p and T2 values in a normal knee. These subjects will complete imaging at four time points, spaced six months apart. The results will help evaluate the expected range of differences in knees without a traumatic injury. This information will assist in the interpretation of values from the patients with ACL injuries as to if any observed variation is outside the expected magnitude for a healthy individual. The control subjects will also help monitor for any changes in the scanner reliability at our site and compared to the other two clinical sites. Phantoms will be scanned each month to aid in this process, and control subjects will add and help ensure that data collection is constant over time.