PROJECT SUMMARY More than 10% of the 15 million Americans who experience skeletal fractures each year suffer from impaired healing. The large incidence of impaired healing results in poor quality of life and high total cost of care for pa- tients. To enhance fracture healing, various regenerative medicine based approaches are being developed, but their development rates are slow due to the limitations of current imaging techniques. For example, vasculariza- tion, a key step in bone healing, is assessed using endpoint methods requiring sacrifice/destruction of animals, leading to high inter-subject variability, time and cost. Our objective is to establish diffuse optical tomography (DOT) and correlation tomography (DCT) as non-invasive, longitudinal research tools to monitor vascularization and predict bone fracture healing. DOT and DCT, 3D deep-tissue hemodynamic imaging techniques, are ideal for frequent longitudinal monitoring due to low cost and the use of non-ionizing light. In addition, DOT and DCT are easily scalable between preclinical and clinical applications. Together, these methods report multiple microvas- cular parameters: total hemoglobin concentration and oxygen saturation with DOT and blood flow with DCT. We hypothesize that DOT and DCT provide accurate quantification of regional blood volume, oxygenation and flow in bone and surrounding soft tissues, which can serve as surrogate markers for the quality of bone healing. Three specific aims are designed to test the hypotheses: Aim 1: Construct a prediction model for bone fracture treatment efficacy using an animal bone fracture model, Aim 2: Directly compare DOT and DCT with histomorphometry, fluorescent microsphere technique and dynamic-contrast-enhanced magnetic resonance imaging in a murine fracture model, Aim 3: Construct clinical prediction model for nonunions in proximal fifth metatarsal fractures based on DOT and DCT measurements. Successful completion of these aims will result in great advances in regenerative medicine and musculoskeletal research by providing a comprehensive and innovative non-invasive longitudinal monitoring tool. In addition, the proposed work lays the foundation for clinical translation of DOT and DCT for bone vascular monitoring by direct comparison with traditional vascularization assessment techniques and by constructing a clinical prediction model for impaired healing in fractures.