The use of contrast-enhanced micro- CT for studying the forelimb musculature of the sparrowhawk

Background. Gross dissection is a widespread method for studying animal anatomy, despite being highly destructive and time-consuming. X-ray computed tomography (CT) has been shown to be a non-destructive alternative for studying anatomical structures. However, in the past it has been limited to only being able to visualise mineralised tissues. In recent years, morphologists have started to use traditional X-ray contrast agents to allow the visualisation of soft tissue elements in the CT context. The aim of this project is to assess the ability of contrast-enhanced micro-CT (μCT) to reconstruct a three-dimensional (3D) model of the musculoskeletal system of the bird wing and to quantify muscle geometry and any systematic changes due to shrinkage. Methods. A 3% iodine-based buffered formalin solution with a two-week staining period was used to visualise the wing myology of the sparrowhawk (Accipiter nisus). μCT scans of the wing were taken over the staining period until full penetration of the forelimb musculature by iodine was reached. A 3D model was reconstructed by manually segmenting out the individual elements of the avian wing with the help of 3D visualisation software. Results. Different patterns of contrast are observed over the duration of the staining treatment with the best results occurring after 25 days of staining. The staining process made it possible to visualise and identify different elements of the soft tissue of the internal wing. Finally, a 3D reconstruction of the musculoskeletal system of the sparrowhawk wing is presented and numerical data of muscle geometry is compared to values obtained by dissection. Discussion. Contrast-enhanced μCT allows the visualisation and identification of the wing myology of birds, including the smaller muscles in the hand, and provides a non-destructive way for quantifying muscle architecture with an accuracy of 96.2%. By combining it with 3D visualisation techniques it is possible to study the individual muscles of the forelimb in their original position and 3D design, which can be the basis of further biomechanical analysis. Because the stain can be optionally washed out post analysis this technique provides a means of obtaining quantitative muscle data from museum specimens non-destructively.