Ultra-High Field Magnetic Resonance Imaging (MRI) Characterisation of the Functional Anatomy and 3D Structural Arrangement of Equine Superficial Digital Flexor Tendons

The equine superficial digital flexor tendon (SDFT) shares many functional similarities with the human Achilles tendon and serves as an energy storing tendon model in re-search. SDF tendinopathies account for a high proportion of injuries, particularly in racing Thoroughbreds. This thesis investigates the multi-level three-dimensional ar-rangement of the SDFT fascicles and the associated interfascicular matrix (IFM) with the hypothesis that it is possible to accurately describe the tendon fascicular arrangement based on magnetic resonance imaging (MRI) at 9.4 T combined with mathematical image analysis. A protocol utilizing the high soft tissue contrast of the ultrashort echo-time T2* weighted 3D-FISP MRI sequence was developed to facilitate the non-destructive structural assessment of a continuous SDFT segment. Based on consecutive high-resolution transverse MR images, the IFM spatial distribution and tendon vascularisation was analysed in SDFT samples of young (1-4 years) and aged (18-25 years) Thoroughbreds and Irish Sport Horses with unknown training history. A computer vision and deep learning approach was further used for training of a 3D U-Net tendon fascicle reconstruction model. Biomechanical testing with mechanical load applied to entire SDFT samples was performed under MR imaging control. Additionally, age related changes including interfascicular-chondroid like bodies (ICBs) as well as clinically apparent tendinopathy were characterised. Image analysis identified a higher quantity of the IFM at the proximal aspect of the tendon. The developed 3D U-Net model facilitates tendon fascicle segmentation and tracking along the length of the tendon. Variations in the thickness of the IFM and the volume of tendon fascicles can be observed as the tendon is exposed to different loading conditions. ICBs were detected in varying number, size and distribution in the SDFT of aged horses only. SDF tendinopathy was characterised by a loss of IFM architecture with irregularly hyperintense tissue appearance as well as alterations in the tendon shape. In conclusion, the protocol provided for ultra-high field MRI facilitates the detailed visualisation of the tendon structural components and serves as a basis for analysis of a larger sample size with comparison of the fascicular pattern in different tendon types.