Abstract
Ultrasound imaging is often used to measure muscle fascicle lengths and pennation angles in human muscles in vivo. Theoretically the most accurate measurements are made when the transducer is oriented so that the image plane aligns with muscle fascicles and, for measurements of pennation, when the image plane also intersects the aponeuroses perpendicularly. However this orientation is difficult to achieve and usually there is some degree of misalignment. Here, we used simulated ultrasound images based on three-dimensional models of the human medial gastrocnemius, derived from magnetic resonance and diffusion tensor images, to describe the relationship between transducer orientation and measurement errors. With the transducer oriented perpendicular to the surface of the leg, the error in measurement of fascicle lengths was about 0.4 mm per degree of misalignment of the ultrasound image with the muscle fascicles. If the transducer is then tipped by 20°, the error increases to 1.1 mm per degree of misalignment. For a given degree of misalignment of muscle fascicles with the image plane, the smallest absolute error in fascicle length measurements occurs when the transducer is held perpendicular to the surface of the leg. Misalignment of the transducer with the fascicles may cause fascicle length measurements to be underestimated or overestimated. Contrary to widely held beliefs, it is shown that pennation angles are always overestimated if the image is not perpendicular to the aponeurosis, even when the image is perfectly aligned with the fascicles. An analytical explanation is provided for this finding.
Highlights
The length and pennation of muscle fascicles influence the capacity of skeletal muscles to produce force [1,2,3]
By comparing to 3D measurements of fascicle lengths and pennation angles from diffusion tensor images, we found that fascicle length measurements from ultrasound images are unbiased but imprecise [13]
Ultrasound image-based measurements of muscle fascicle length and pennation of the medial gastrocnemius have least error when the ultrasound image is aligned with the muscle fascicles
Summary
The length and pennation of muscle fascicles influence the capacity of skeletal muscles to produce force [1,2,3]. Biomechanists and muscle physiologists often measure muscle fascicle length and pennation [4,5,6,7]. Ultrasonography is widely used to measure muscle fascicle length and pennation in human muscles in vivo [8,9]. With conventional ultrasonography the ultrasound transducer is PLOS ONE | DOI:10.1371/journal.pone.0157273. Transducer Alignment Errors in Ultrasound Measurements of Muscle Architecture positioned on the skin overlying the muscle, generating a two-dimensional (2D) image of a slice through the muscle. Fascicle lengths and pennation angles can be measured from the images using manual [5] or semi-automated methods [10,11]
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