Diagnostic ultrasound performance: The physician performing the ultrasound will be blinded to the results of the x-ray and MRI. The physician performing the ultrasound will be informed of the bone in question (ex. Tibia) and will be able to perform a focused history and exam. The bone in which acute stress fracture is suspected will then be scanned throughout its length in two orthogonal views. The criteria for diagnosing acute stress fracture on ultrasound include displaying 2 out of 3 of the following: hypoechoic periosteal elevation of cortical bone, visible cortical disruption, hyperemia surrounding the periosteal lesion on power Doppler. The ultrasound physician will then be asked to make a determination whether the ultrasound study is positive for acute stress fracture or negative/indeterminate.