A comparison of human masseter muscle thickness measured by ultrasonography and magnetic resonance imaging.

Non-invasive imaging techniques such as computerized tomography, magnetic resonance imaging (MRI), and ultrasonography enable measurements of the cross-section and thickness of human jaw muscles in vivo, providing an indication of the maximal force a muscle can exert. In 15 adult Caucasian men the thickness of the masseter muscle was registered bilaterally on three different levels by ultrasonography. Scans were made on the contracted and relaxed muscle. A comparison was then made with measurements from serial MRI scans, using univariate analysis of variance for repeated measurements and Pearson's correlation coefficients. Variances of the repeated measurements were calculated for the different scanning levels and the different muscle conditions and tested for homogeneity. For both the ultrasound and MRI measurements there was no difference in thickness between the left and right muscle. The registration level with highest reproducibility was halfway between the origin and insertion. Measurements from the contracted muscle were more reproducible than those from the relaxed muscle. The relaxed muscle thickness measured by ultrasonography was smaller than that measured by MRI. The correlation between ultrasound and MRI was significant for the upper and middle level of scanning (p < 0.001). The highest correlation was found between MRI (relaxed) and ultrasound (contracted) at the middle level (R = 0.83, p < 10(-6)). The conclusion is that ultrasonography is an accurate and reproducible method for measuring the thickness of the masseter in vivo. It allows for large-scale longitudinal study of changes in jaw-muscle thickness during growth in relation to change in biomechanical properties of masticatory muscles.