After traumatic spinal cord injury (SCI), significant deterioration of bone tissue is seen in most patients. This continues throughout life and about 50% of chronic SCI patients will sustain a low-impact or spontaneous fracture at some point, typically occurring around the knee. Traditionally, the pathogenesis of osteoporosis after SCI has been focused on mechanical unloading and its effect on bone quantity, but advances in the field of skeletal biology have revealed the involvement of a much broader array of bone physiology—in particular the critical roles that the structural, physiological, and chemical properties of bone contribute to bone health. Evaluating the multi-scale changes that occur in bone tissue components such as the organic matrix and water, which together occupy approximately 60% of bone by volume, can provide critical information on chronic SCI risks including osteoporotic fracture, as well as the anemia and immune dysfunction associated with the “acquired bone marrow failure syndrome.” Unfortunately, such crucial information on bone structure, vascularity, and chemical properties is all but inaccessible using the standard imaging modalities available in clinics today. Magnetic resonance imaging (MRI), particularly ultrashort echo time (UTE) techniques, continues to gain interest for the investigation of numerous structural and physiological properties of bone. The development and application of quantitative UTE MRI techniques to measure multi-scale features of bone, including macro- and micro-scale structure (UTE), micro-scale vascularity (UTE double echo steady state [UTE-DESS]), and molecular-scale chemical properties (UTE acido-chemical exchange saturation transfer [UTE-acidoCEST]), would represent a revolutionary step forward in the care of SCI patients. The goal of this proposal is to establish a panel of non-invasive MRI biomarkers tailored for bone quality assessment in SCI and test its effectiveness on a novel chronic SCI rat model. In the first Aim, cadaveric knee samples from SCI and healthy donors will be used to optimize a panel of novel, quantitative MRI techniques for fast and accurate volumetric evaluation of bone quality and health. The first hypothesis is that multi-scale bone quality and health measures can be reliably assessed using the new techniques. The second hypothesis is that the new biomarkers will be highly correlated with reference standards, including bone structure, composition, biomechanical properties, vascular density, and pH measures. In the second Aim, a longitudinal rat model with chronic, complete T8 SCI will be used and investigations will focus on validating the new MRI biomarker panel to assess bone recovery after induction of functional locomotion through manipulation of activity level in propriospinal neurons (via Designer Receptors Exclusively Activated by Designer Drugs) with and without sympathetic inhibition. The third hypothesis is that the biomarker panel will be highly correlate...