[Anatomical study with MRI of a small muscle: the masseter].

Magnetic resonance imaging (MRI) provides functional information in an anatomic presentation allowing to distinguish soft tissues with high sensitivity. The goal of this study was to investigate the normal anatomy of the major masticatory muscle, the masseter, both at rest or during contraction by using three dimensional (3D) MRI. Eighteen subjects aged from 19 to 28 years, all in good health, were studied. Several experiments were first realized on phantoms to test the 3D-MRI technique. After reconstruction and segmentation processing, 3D acquisition, enabled obtaining data on the masseter anatomy. The normal anatomical position of the masseter was reported to the skin plan as the mean internal distance (7.9 +/- 0.42 mm) and external distance (15.2 +/- 0.41 mm). While there was no difference between internal distance, for sex or side, the external distance was significantly (p = 0.02) shorter in male (7.7 +/- 0.5 mm) than in female (8.8 +/- 0.4 mm) for both sides. The mean volume for all subjects and both sides (20.3 +/- 1.1 cm3) did not change significantly between rest and exercise. The masseter volume was significantly (p < 0.00001) greater in male (24.2 +/- 2.0 cm3) than in female (16.4 +/- 3.6 cm3) groups. These physiological references may be useful for further MRI investigations of masticatory system pathologies.