Non-Invasive Assessment of Structural and Mechanical Micro-Environmental Changes During Long Bone Regeneration Using a Multi-Modal and Multi-Parametric Ultrasound Imaging Technique in a Segmental Tibial Defect Sheep Model In Vivo

Objective: The underlying regeneration process of bony defects often exhibits multifaceted nature, which may not be completely characterized by imaging methods currently available to the clinic. In this paper, we present the first longitudinal study to use multi-modal and multi-parametric ultrasound (US) imaging to assess bone regeneration in situations of segmental defects. Methods: We derived two imaging markers from the proposed US imaging technique: the new-bone bulk volume and fibrovascular connective tissue area and computed their global and local statistics in a subject-specific manner. Results: From a cohort of 5 sheep treated with baseline tissue engineered construct, the distance (mm) between surface reconstructions from multi-view 3-D US and CT was 0.30 ± 0.67 (60 days post implantation) and 0.22± 0.43 (120 days post implantation). From US elastography (USE), we discovered a new contrast mechanism between the soft tissue and fibrovascular connective tissue in axial normal strain elastograms and corroborated it using endpoint histology. From two sheep, we detected negative and positive correlations between the fibrovascular connective tissue area at 60 days post shell implantation and the area of bone mass that continued to form after 60 days post shell implantation. Conclusion: Based on our results, it is feasible to use the proposed multi-modal and multi-parametric US imaging technique to assess structural and mechanical micro-environmental changes in the sheep animal model. Significance: In the future, 3-D US and USE may become important quantitative tools for bone fracture healing diagnosis and prognosis.