Researchers will recruit and retain patients with acute ACL tears within 28 days of injury, and follow them longitudinally at 6 weeks, 6 months, and 12 months. Standard of Care MR imaging will be captured at baseline, then a quantitative morphological MRI and quantitative T1ρ and T2 imaging of the injured and contra-lateral knees will be taken within 7 days of baseline, and again at 6 months, and 1 year post-injury or post-surgery. Scans will not take place immediately after surgery because we are not attempting to study the immediate effect of ACL reconstruction. The physical performance testing will not be acquired until 6 months follow-up because patients will have unstable knees.