Evaluation of T₂ values and apparent diffusion coefficient of the masseter muscle by clenching

Objective: The aim of this study was to evaluate the changes in T ₂ values and apparent diffusion coefficient (ADC) in the masseter muscle by clenching in healthy volunteers. Methods: 37 volunteers were enrolled in the study. We measured bite force using pressuresensitive paper and a T ₂ map. The ADC map was obtained at rest, during clenching, immediately after and 5 min after clenching. The spin-echo sequence was used to calculate T₂, and single-shot spin-echo echo planar imaging was used to calculate the ADC. The motion-probing gradients (MPGs) were applied separately along the posterior-to-anterior (PA), right-to-left (RL) and superior-to-inferior (SI) directions, with b values of 0, 300 and 600 smm ^-2 in each direction. ADC-PA, ADC-RL, and ADC-SI values were obtained, and we calculated the ADC-iso for the mean diffusivity. Results: There were no significant differences between the stronger and weaker sides of bite force before, during or 5 min after clenching for T₂ and ADC. The bite force had little effect on these parameters; thus, we used the average of the two sides for the following analyses. Time course analysis of ADC-iso, ADC-PA, ADC-RL and ADC-SI demonstrated a marked increase after clenching and a rapid decrease immediately after clenching, although they did not completely return to the initial values; however, the change in ADC-RL was significantly greater than those in ADC-PA or ADC-SI (P < 0.001 each). The changes in T₂ were similar to those of ADC, although not as marked. Conclusions: ADC (especially ADC-RL) was altered by contraction of the masseter muscle.