A COMPARISON OF HUMAN MASSETER MUSCLE THICKNESS MEASURED BY ULTRASONOGRAPHY AND MAGNETIC-RESONANCE-IMAGING

Non-invasive imaging techniques such as computerized tomography, magne tic resonance imaging (MRI), and ultrasonography enable measurements o f the cross-section and thickness of human jaw muscles in vivo, provid ing an indication of the maximal force a muscle can exert. In 15 adult Caucasian men the thickness of the masseter muscle was registered bil aterally 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 vari ance for repeated measurements and Pearson's correlation coefficients. Variances of the repeated measurements were calculated for the differ ent scanning levels and the different muscle conditions and tested for homogeneity. For both the ultrasound and MRI measurements there was n o difference in thickness between the left and right muscle. The regis tration level with highest reproducibility was halfway between the ori gin and insertion. Measurements from the contracted muscle were more r eproducible than those from the relaxed muscle. The relaxed muscle thi ckness measured by ultrasonography was smaller than that measured by M RI. The correlation between ultrasound and MRI was significant for the upper and middle level of scanning (p < 0.001). The highest correlati on was found between MRI (relaxed) and ultrasound (contracted) at the middle level (R = 0.83, p < 10(-6)). The conclusion is that ultrasonog raphy is an accurate and reproducible method for measuring the thickne ss of the masseter in vivo. It allows for large-scale longitudinal stu dy of changes in jaw-muscle thickness during growth in relation to cha nge in biomechanical properties of masticatory muscles.