Validity of freehand three-dimensional ultrasound system in measurement of three-dimensional surface shape of shoulder muscles

Accurate measurement of muscle morphology is crucial for assessing skeletal muscle capacity. Although the freehand three-dimensional ultrasound (3DUS) system is a promising technique for assessing muscle morphology, its accuracy has been validated mainly in terms of volume by examining lower limb muscles. The purpose of this study was to validate 3DUS in the measurements of 3D surface shape and volume by comparing them with MRI measurements while ensuring the reproducibility of participant posture by focusing on the shoulder muscles. The supraspinatus, infraspinatus, and posterior deltoid muscles of 10 healthy males were scanned using 3DUS and MRI while secured by an immobilization support customized for each participant. A 3D surface model of each muscle was created from the 3DUS and MRI methods, and the agreement between them was assessed. For the muscle volume, the mean difference between the two models was within 0.51 cm3 for all muscles. For the surface shape, the distances between the closest points of the two models was calculated for every point on the 3DUS surface model. The results showed that the median (third quartile) of the distances was less than 1.21 mm (1.89 mm) for all muscles. These results suggest that, given the above error is permitted, 3DUS can be used as an alternative to MRI in measuring volume and surface shape, even for the shoulder muscles.