Validation of 2-dimensional ultrasound imaging protocols for measuring the masseter muscle

To determine the most repeatable ultrasound imaging protocols for assessing the masseter muscle, evaluating repeatability across different scan locations, muscle contraction states, and operator positions.

Ten adult volunteers underwent ultrasound scans, including three axial (superior/middle/inferior) and three longitudinal (anterior/middle/posterior) views on the left and right sides and in the relaxed and clenched states. Each subject was scanned twice, roughly 15 minutes apart. Muscle borders were manually traced on all images. Muscle thickness was calculated using custom software. Agreement between scans was assessed using the Bland-Altman plot.

(1) High intra- and inter-rater reliability (intraclass correlation coefficient = 0.99 and intraclass correlation coefficient = 0.98, respectively) for muscle thickness measurement. (2) Thickness at the image center is similar to the average muscle thickness. (3) The longitudinal middle location had the greatest repeatability, regardless of muscle state. (4) Scans at both relaxed and clenched state are repeatable, with relaxed scans yielding slightly greater repeatability. (5) Operator position showed negligible effect on repeatability. (6) Location of the maximum thickness increase in the clenched state varied between subjects.

The study identified longitudinal middle location as the most repeatable ultrasound transducer position to measure masseter muscle, regardless of muscle contraction state or the operator position.