Prediction of Rotator Cuff Muscle Fibre Orientations Using a Population-Averaged Atlas Generated With Anatomical and Diffusion-Weighted Magnetic Resonance Images.

Measurements of muscle architecture are crucial for understanding muscle function but are often difficult to obtain in human muscles in vivo. This study aimed to create population-averaged atlases of human rotator cuff muscle shape and muscle fibre orientations from anatomical magnetic resonance images (MRI) and diffusion-weighted images (DWI) and to utilise these atlases to predict muscle fibre orientations from anatomical MRI data alone. An image registration framework was applied to coregister anatomical MRI and DWI data of 11 male and 9 female subjects into sex-specific common spaces, forming the basis for the atlases. The accuracy of registration was quantified using Dice coefficients, angular correlation coefficients (ACCs) and angular differences. The same metrics were used to assess the capability of the atlases to predict fibre orientations for subjects not included in the atlas construction, via leave-one-out cross-validation. The results showed that individual male and female image data were accurately registered into their respective atlas spaces, with high Dice coefficients (0.888 ± 0.002 for males, 0.856 ± 0.021 for females) and consistent angular alignment as evidenced by the ACCs and angular differences. Predicted fibre orientations for out-of-sample subjects closely matched those derived from DWI images, exhibiting improved smoothness and coverage (ACC: 0.909 ± 0.011 for males, 0.942 ± 0.011 for females; angular difference: 13.8° ± 1.3° for males, 11.2° ± 1.2° for females). These findings demonstrate that population-averaged atlases enhance muscle architecture reconstructions and enable the accurate prediction of muscle fibre orientations using only anatomical MRI scans in younger individuals without shoulder injuries.