Visualization and quantification of facial muscles with 9.4T MRI-DTI in common marmosets (Callithrix jacchus)

The traditional dissections of the specialized mimetic muscles that produce facial expressions have revealed important insights into the evolution of muscles and their function in human and other mammals' social interactions. However, in small species, the task of manually visualizing and analysing musculature is challenging; even in larger species and muscles, digital anatomy methods, such as DiceCT and MRI-DTI, have been growing in popularity recently, providing detailed new insights into the structure and morphology of muscles. The current work presents for the first time a 9.4T MRI-DTI visualization and analysis of the complete facial musculature used for facial expression in a primate species, in this case a very small species hard to analyse with traditional dissection, the common marmoset. In addition, a quantification and comparison of overall, individual and bilateral muscles was performed in the light of functional anatomy, followed by a critical analysis of this novel method for the study of facial anatomy. Twenty-two muscles were tracked, described, quantified and compared in three individuals. This work demonstrates the feasibility of MRI-DTI as a relatively novel method to digitally visualize the structure of facial muscles in a small species. In addition, this method is able to quantify varied muscle parameters for comparisons between individuals, muscles and hemifaces.