title: Assessment of the sequential change of the masseter muscle by clenching: a quantitative analysis of T1, T2, and the signal intensity of the balanced steady-state free precession.
abstract: BACKGROUND
  The persistent muscle contractions during clenching are thought to cause some temporomandibular disorders. However, no report has so far evaluated the effect of clenching on the masticatory muscles by magnetic resonance imaging (MRI).
  
  
  OBJECTIVE
  To investigate the effect of clenching with maximum voluntary contraction on the T(1), T(2), and signal intensity (SI) of the balanced fast field-echo (b FFE) of the masseter muscle.
  
  
  METHODS
  A total of 11 volunteers participated. Multi-echo spin-echo echo-planar imaging was used for T(2) measurements, and multi-shot Look-Locker sequence for T(1) measurements. The Look-Locker sequence has been used for fast T(1) mapping and this method has been applied for the imaging of various tissues. In addition, the b FFE was used due to the high temporal resolution. These three sequences lasted for 10 min and the participants were instructed to clench from 60 s to 80 s after the start of the data acquisition. T(2), T(1), and SI were normalized compared to pre-clenching values.
  
  
  RESULTS
  T(2) decreased by clenching, which reflected a decrease of tissue perfusion due to the mechanical pressure. It increased rapidly after the clenching (peak value, 1.11+/-0.03; peak time, 16.8+/-7.6 s after the clenching), which corresponded to the reactive hyperemia and later, it gradually returned to the initial values (half period, 2.22+/-0.84 min). The change in the SI of the b FFE was triphasic and similar to that of T(2) clenching. T(1) increased after the cessation of the clenching and later gradually decreased during the recovery periods. However, the change of T(1) was quite different from that of T(2), with a lower peak value (1.04+/-0.02), a later peak time (36.0+/-28.0 s), and a longer half period (4.76+/-3.40 min) (P<0.0001, 0.0066, 0.02, respectively).
  
  
  CONCLUSIONS
  The change in T(2) was triphasic and we considered that it predominantly reflected the tissue perfusion.
authors.affiliation: Department of Oral and Maxillofacial Radiology, Faculty of Dental Science, Kyushu University, Maidashi 3-1-1, Higashi-ku, Fukuoka, Japan. chikui@rad.dent.kyushu-u.ac.jp
authors.forename: Toru
authors.initials: T
authors.lastname: Chikui
authors.forename: Tomoko
authors.initials: T
authors.lastname: Shiraishi
authors.forename: Kenji
authors.initials: K
authors.lastname: Tokumori
authors.forename: Daisuke
authors.initials: D
authors.lastname: Inatomi
authors.forename: Masamitsu
authors.initials: M
authors.lastname: Hatakenaka
authors.forename: Kenji
authors.initials: K
authors.lastname: Yuasa
authors.forename: Kazunori
authors.initials: K
authors.lastname: Yoshiura
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D007568:Jaw / Q000033:anatomy & histology; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008406:Masseter Muscle / Q000033:anatomy & histology*; D008875:Middle Aged; D009119:Muscle Contraction