Methods: In this double-blind, prospective clinical study, subjects (age 14 years and up) suspected to have an acute stress fracture of the lower extremity will be recruited from the Sports Medicine clinic at the University of Virginia Health System in the Department of Physical Medicine & Rehabilitation. Subjects will undergo the traditional diagnostic algorithm including screening x-ray as part of standard care. Subjects will then undergo a confirmatory MRI of the region of concern if the initial x-ray was negative as part of standard care. Any subject who does not require an MRI for clinical purposes (initial X-ray was positive) will have one completed for research purposes. All subjects will also undergo diagnostic ultrasound performed by a separate, blinded physician competent in diagnostic ultrasound for research purposes. A statistician in the Department of Public Health at the University of Virginia will be performing statistical analysis during data analysis. Findings will be analyzed using a McNemar chi-square test to evaluate for significant differences between the sensitivities of ultrasound and x-ray.