Fig 1. Construction of virtual ultrasound images.

(a) Posterior view of the leg. The medial and lateral heads of the gastrocnemius muscle have been outlined. Also shown are the ultrasound transducer (grey box), its corresponding image plane (yellow) and the intersection between the image plane and the medial gastrocnemius surface (red curve). The definitions of transducer ‘tilt’ and ‘rotation’ are indicated by the curved arrows. (b) Sites on the skin at which the virtual transducer was located. (c) Detail of the medial gastrocnemius model (mediolateral view) with fascicles that were reconstructed using DTI data (represented as black lines). (d) Example of a real ultrasound image obtained at the approximate location indicated in (a). The image is orientated so the distal end of the muscle is to the right. The proximal (superficial) and distal (deep) aponeuroses appear as two distinct, nearly horizontal white lines on the image. The fascicles appear as oblique striations with a clear directional pattern. To obtain this image, two ultrasound transducers were rigidly fixed together to form a composite image of 110 × 40 mm with a 18 mm gap (black region) between the images. Note that we do not use real ultrasound images in this study; the image is only used here to show the similarity of real and virtual images. (e) A virtual ultrasound image reconstructed from the same location in the muscle. Virtual images were reconstructed from MRI and DTI data. The aponeuroses (red lines) are calculated as the intersection of the muscle surface with the image plane and the fascicles (white lines) as the part of all 3D fascicles within 2.5 mm of the image plane.