masticatory muscles were evaluated using electromyography ( emg ) , , , near - infrared spectroscopy ( nirs ) , and so on , for purpose of evaluation of muscle activities and blood oxygen kinetics . al - saleh et al . performed systematic review of two studies ( 74 patients ) that were collected using mesh terms of tmd , jaw pain and emg , and stated that the sensitivity of emg in diagnosing tmd was low ( 0.250.40 ) . in the recent years , studies using mri and sonography in patients with masticatory muscle myalgia have frequently been reported , and the newly developed technologies have been introduced , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , . muscles after treatment in those patients also have been evaluated using mri and sonography , , , , , , , , , , . this article reviews recently published studies focusing on the mri and sonographic diagnosis of masticatory muscle myalgia in temporomandibular disorder ( tmd ) patients , and shows the intramuscular state , blood flow , and muscle hardness . the mri and sonographic evaluations in relation to the efficacy of various treatments are also discussed . literatures published within the last 15 years were searched from the pubmed database using the following mesh terms : ( 1 ) ( mri or sonography ) and masticatory muscle pain ; ( 2 ) ( mri or sonography ) and masticatory muscle pain and treatment . original papers and review articles published in english and japanese were adopted . selection criteria were mri and sonographic features in diagnosis of masticatory muscle myalgia and in evaluation of muscle after therapy . the number of literature that met the mesh terms of ( 1 ) and the above - mentioned criteria was 34 , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , . most mri studies in tmd patients evaluated intra - articular changes , such as disk displacement , , , , , , and mri studies evaluating the masticatory muscles were a few . zanoteli et al . evaluated changes in the signal intensities of the masticatory muscles in 15 patients with myotonic dystrophy on t1- and t2-weighted images , and diagnosed the presence of a high signal intensity area ( fatty replacement ) as abnormal . the former revealed a difference in masseter muscle metabolism between 11 dolichofacial and 11 brachyfacial subjects , and the latter showed an increase in masseter muscle metabolism secondary to heat pack use in 12 healthy subjects . the most common index for evaluation of masticatory muscles in the sonography studies was muscle thickness ( fig . 1 ) , , , , , , , , , , , , . ariji et al . showed that the masseter muscle thickness in 25 tmd patients was greater than that of 30 healthy volunteers . . showed that the masseter muscle thickness on sonography and the masseter muscle activity on electromyography were positively correlated with occlusal force in 19 tmd patients . a next focusing index was the appearance of the intramuscular echogenic bands , which correspond to the internal fascia ( fig . masseter muscle appearance was classified into three types based on the internal echogenic bands , and it was revealed that the masseter muscles in 25 tmd patients frequently displayed thickened bands or reduction in the number of bands . in recent years , sonoelastography has been used to evaluate the skeletal muscles , , , . the principal of sonoelastography can be divided into two types : strain and shear wave elastography . hardness on strain elastography is displayed as a relative value compared to the reference ( fig . shear wave elastography is based on the propagation velocity of the shear wave generated by acoustic radiation force . hardness on shear wave elastography is displayed as the shear wave speed or young 's modulus ( fig . 4 ) . studies regarding the masseter muscle hardness on strain elastography recently have been reported , . the ei was originally developed for sonography machine and software , and defined as the strain values of each area compared with the average strain value ( ei  =  1 ) of the whole area of interest . the eis of the softer and harder areas than the average ei were assigned as 01 and 16 , respectively . ei ratio of the masseter muscle was evaluated compared with the reference tissue , such as the subcutaneous fat or coupling agent ( fig . evaluated the difference in the masseter muscle hardness between eight tmd patients and 35 healthy volunteers using the ei ratios of the masseter muscle against the subcutaneous fat . as a result , there was a significant difference in the ei ratios between the symptomatic and asymptomatic sides in tmd patients . the ei ratio of the symptomatic side in tmd patients was greater than that seen in the healthy volunteers . adopted a coupling agent with a known young 's modulus as a reference in order to be generally available to ei ratio , and measured the ei ratio of the masseter muscle against the coupling agent in 25 healthy volunteers . arda et al . reported that the average young 's moduli of the masseter muscles in 127 healthy volunteers was 10.4    3.7  kpa . measured the average young 's moduli of the masseter muscles in 30 healthy volunteers using the other sonoelastographic device , and reported that it was 42.8    5.56 . badea et al . found no significant difference in the shear wave velocities of the masseter muscles between 25 healthy volunteers and 13 tmd patients . based on the hypothesis that masseter muscle myalgia is partially caused by edematous changes in the masseter muscles , the following studies investigated the water content in the masseter muscles during jaw clenching using mri : nagayama et al . adopted magnetization transfer contrast ( mtc ) imaging , and revealed that the magnetization transfer ratios of masticatory muscles in 50 tmd patients were significantly lower than in 28 healthy volunteers . they concluded that mtc imaging strongly reflected edematous changes in masseter muscle , possibly related to masseter muscle myalgia . shiraishi et al . examined changes in the diffusion parameters of the masseter muscles in 11 healthy volunteers during jaw opening and clenching on diffusion - tensor imaging . assessed edematous changes in the masseter muscles of 10 healthy volunteers caused by low - level static contraction on t2-weighted interactive decomposition of water and fat with echo asymmetry and least - square estimation ( ideal ) water images . a similar study was performed using sonoelastography , which showed that the thickness and hardness of the masseter muscles increased immediately after contraction . when changes in the masseter muscles by contraction were compared using mri and sonoelastography , the soft area ratio of muscle on sonoelastography increased in correlation with increases in water signal on mri . tmd patients with masticatory muscle myalgia were treated by splint therapy , , , medication , , , physical therapy , ,   and so on . as an index for evaluating the therapeutic efficacy , the clinical findings , including pain intensity ( visual analog scale ) , , pressure pain threshold , , , degree of mouth opening , , and degree of jaw function impairment , , , were analyzed . muscle activity was also evaluated using electromyography , , , . to assess the efficacy of massage therapy on the skeletal muscles , evaluated the change in masseter muscle hardness after massage therapy using a muscle hardness meter , and confirmed that the hardness decreased after massage treatment . as to the evaluation by mri and sonography , the number of literature that met to the mesh terms of 2 and the previously - mentioned criteria was 11 , , , , , , , , , , . of them , all 3 mri studies evaluated changes in disc position after therapy , and did not evaluate changes in the muscles , , . in studies using sonography , an useful index for therapeutic efficacy is the thickness and intramuscular echo appearance of the masseter muscle following treatment ( figure 1 , figure 2 ) , , , , , , , . a stabilization splint therapy is considered as a therapy of weak recommended level ( grade 2c ) for patients with masticatory muscle myalgia , according to the japanese society 's temporomandibular joint evidence - based clinical guidelines . a systematic review on the efficacy of stabilization splints showed that the use of a stabilization splint aided patients with masticatory muscle myalgia , . however , a stabilization splint did not appear to yield a better outcome than a soft splint , a non - occluding palatal splint , physical therapy , or body acupuncture . evaluated the changes in the masseter muscle thickness in 35 patients with myofascial pain , and reported that a reduced thickness was found . bertram et al . evaluated the changes in the masseter muscle thickness in 29 patients and reported that the asymmetry index of thickness decreased , . a few studies performed randomized controlled trials of massage therapy for masticator muscle myalgia , , . gomes et al . compared the efficacies among massage therapy , splint therapy , and the combination therapy ( n  =  15 patients in each group ) , and showed that the combination of two therapies demonstrated a greater improvement although no significant difference among the groups . compared the efficacies between 4 weeks and 6 weeks of physical therapies ( n  =  24 patients in each group ) , and craane et al . compared the efficacies between physical therapy ( n  =  23 ) and control group ( n  =  26 ) . the physical therapy including massage showed improvement in pain and jaw function , but the degree of improvement was not significant between the groups . in order to adapt of massage therapy to masticatory muscle pain , an oral rehabilitation robot was previously developed . investigated the safety , a suitable treatment regimen , and the efficacy of masseter and temporal muscle massage treatment using this oral rehabilitation robot ( fig . suitable massage was delivered at a pressure of 10  n for 16  min . the standard treatment regimen was five sessions , spaced 2 weeks apart for a 9.5-week duration . as to the sonographic evaluation of the masseter muscles after massage therapy , the thickness decreased in all of 41 patients , , and fine echogenic bands began to appear in 93.3% of the therapy - effective 26 patients . ,   in particular , the ei ratio in the effective - therapy group significantly decreased in early period of treatment , and therefore , the ei ratio may be an effective tool for predicting therapeutic efficacy . the number of literature that met the mesh terms of ( 1 ) and the above - mentioned criteria was 34 , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , . most mri studies in tmd patients evaluated intra - articular changes , such as disk displacement , , , , , , and mri studies evaluating the masticatory muscles were a few . zanoteli et al . evaluated changes in the signal intensities of the masticatory muscles in 15 patients with myotonic dystrophy on t1- and t2-weighted images , and diagnosed the presence of a high signal intensity area ( fatty replacement ) as abnormal . the former revealed a difference in masseter muscle metabolism between 11 dolichofacial and 11 brachyfacial subjects , and the latter showed an increase in masseter muscle metabolism secondary to heat pack use in 12 healthy subjects . the most common index for evaluation of masticatory muscles in the sonography studies was muscle thickness ( fig . 1 ) , , , , , , , , , , , , . ariji et al . showed that the masseter muscle thickness in 25 tmd patients was greater than that of 30 healthy volunteers . . showed that the masseter muscle thickness on sonography and the masseter muscle activity on electromyography were positively correlated with occlusal force in 19 tmd patients . a next focusing index was the appearance of the intramuscular echogenic bands , which correspond to the internal fascia ( fig . masseter muscle appearance was classified into three types based on the internal echogenic bands , and it was revealed that the masseter muscles in 25 tmd patients frequently displayed thickened bands or reduction in the number of bands . in recent years , sonoelastography has been used to evaluate the skeletal muscles , , , . the principal of sonoelastography can be divided into two types : strain and shear wave elastography . hardness on strain elastography is displayed as a relative value compared to the reference ( fig . shear wave elastography is based on the propagation velocity of the shear wave generated by acoustic radiation force . hardness on shear wave elastography is displayed as the shear wave speed or young 's modulus ( fig . 4 ) . studies regarding the masseter muscle hardness on strain elastography recently have been reported , . the ei was originally developed for sonography machine and software , and defined as the strain values of each area compared with the average strain value ( ei  =  1 ) of the whole area of interest . the eis of the softer and harder areas than the average ei were assigned as 01 and 16 , respectively . ei ratio of the masseter muscle was evaluated compared with the reference tissue , such as the subcutaneous fat or coupling agent ( fig . evaluated the difference in the masseter muscle hardness between eight tmd patients and 35 healthy volunteers using the ei ratios of the masseter muscle against the subcutaneous fat . as a result , there was a significant difference in the ei ratios between the symptomatic and asymptomatic sides in tmd patients . the ei ratio of the symptomatic side in tmd patients was greater than that seen in the healthy volunteers . adopted a coupling agent with a known young 's modulus as a reference in order to be generally available to ei ratio , and measured the ei ratio of the masseter muscle against the coupling agent in 25 healthy volunteers . arda et al . reported that the average young 's moduli of the masseter muscles in 127 healthy volunteers was 10.4    3.7  kpa . measured the average young 's moduli of the masseter muscles in 30 healthy volunteers using the other sonoelastographic device , and reported that it was 42.8    5.56 . badea et al . found no significant difference in the shear wave velocities of the masseter muscles between 25 healthy volunteers and 13 tmd patients . based on the hypothesis that masseter muscle myalgia is partially caused by edematous changes in the masseter muscles , the following studies investigated the water content in the masseter muscles during jaw clenching using mri : nagayama et al . adopted magnetization transfer contrast ( mtc ) imaging , and revealed that the magnetization transfer ratios of masticatory muscles in 50 tmd patients were significantly lower than in 28 healthy volunteers . they concluded that mtc imaging strongly reflected edematous changes in masseter muscle , possibly related to masseter muscle myalgia . shiraishi et al . examined changes in the diffusion parameters of the masseter muscles in 11 healthy volunteers during jaw opening and clenching on diffusion - tensor imaging . assessed edematous changes in the masseter muscles of 10 healthy volunteers caused by low - level static contraction on t2-weighted interactive decomposition of water and fat with echo asymmetry and least - square estimation ( ideal ) water images . a similar study was performed using sonoelastography , which showed that the thickness and hardness of the masseter muscles increased immediately after contraction . when changes in the masseter muscles by contraction were compared using mri and sonoelastography , the soft area ratio of muscle on sonoelastography increased in correlation with increases in water signal on mri . tmd patients with masticatory muscle myalgia were treated by splint therapy , , , medication , , , physical therapy , ,   and so on . as an index for evaluating the therapeutic efficacy , the clinical findings , including pain intensity ( visual analog scale ) , , pressure pain threshold , , , degree of mouth opening , , and degree of jaw function impairment , , , were analyzed . muscle activity was also evaluated using electromyography , , , . to assess the efficacy of massage therapy on the skeletal muscles , evaluated the change in masseter muscle hardness after massage therapy using a muscle hardness meter , and confirmed that the hardness decreased after massage treatment . as to the evaluation by mri and sonography , the number of literature that met to the mesh terms of 2 and the previously - mentioned criteria was 11 , , , , , , , , , , . these were studies on the evaluation after the splint or massage therapies . of them , all 3 mri studies evaluated changes in disc position after therapy , and did not evaluate changes in the muscles , , . in studies using sonography , an useful index for therapeutic efficacy is the thickness and intramuscular echo appearance of the masseter muscle following treatment ( figure 1 , figure 2 ) , , , , , , , . a stabilization splint therapy is considered as a therapy of weak recommended level ( grade 2c ) for patients with masticatory muscle myalgia , according to the japanese society 's temporomandibular joint evidence - based clinical guidelines . a systematic review on the efficacy of stabilization splints showed that the use of a stabilization splint aided patients with masticatory muscle myalgia , . however , a stabilization splint did not appear to yield a better outcome than a soft splint , a non - occluding palatal splint , physical therapy , or body acupuncture . following stabilization splint therapy , aldemir et al . evaluated the changes in the masseter muscle thickness in 35 patients with myofascial pain , and reported that a reduced thickness was found . bertram et al . evaluated the changes in the masseter muscle thickness in 29 patients and reported that the asymmetry index of thickness decreased , . . found no change , whereas sasaki et al . reported a change in intramuscular echo types in 12 of 40 patients . a few studies performed randomized controlled trials of massage therapy for masticator muscle myalgia , , . gomes et al . compared the efficacies among massage therapy , splint therapy , and the combination therapy ( n  =  15 patients in each group ) , and showed that the combination of two therapies demonstrated a greater improvement although no significant difference among the groups . de laat et al . compared the efficacies between 4 weeks and 6 weeks of physical therapies ( n  =  24 patients in each group ) , and craane et al . compared the efficacies between physical therapy ( n  =  23 ) and control group ( n  =  26 ) . the physical therapy including massage showed improvement in pain and jaw function , but the degree of improvement was not significant between the groups . in order to adapt of massage therapy to masticatory muscle pain , an oral rehabilitation robot was previously developed . investigated the safety , a suitable treatment regimen , and the efficacy of masseter and temporal muscle massage treatment using this oral rehabilitation robot ( fig . suitable massage was delivered at a pressure of 10  n for 16  min . the standard treatment regimen was five sessions , spaced 2 weeks apart for a 9.5-week duration . as to the sonographic evaluation of the masseter muscles after massage therapy , the thickness decreased in all of 41 patients , , and fine echogenic bands began to appear in 93.3% of the therapy - effective 26 patients . ariji et al . also evaluated changes in the masseter muscle hardness using sonoelastography ( fig . ,   in particular , the ei ratio in the effective - therapy group significantly decreased in early period of treatment , and therefore , the ei ratio may be an effective tool for predicting therapeutic efficacy . this review provides a summary of the literature published within the last 15 years investigating the mri and sonographic diagnosis of masticatory muscle myalgia in tmd patients . with the recent advances in imaging technology , edematous changes and muscle hardening in patients with masticatory muscle myalgia evaluation of therapeutic efficacy should be based not only on the clinical findings , but also on imaging findings . sonography will enable the clinician to select the appropriate treatment modalities for managing masticatory muscle myalgia . this study was partially supported by a grant - in - aid for scientific research ( c ) ( 23592785 ) from the japan society for the promotion of science .
summarythis article reviews recently published studies investigating the mri and sonographic diagnosis of masticatory muscle myalgia in temporomandibular disorder patients . 
 the mri and sonographic features of muscle after treatment are also discussed . 
 literature published within the last 15 years was obtained from the pubmed database using the following mesh terms : magnetic resonance imaging ( mri ) or sonography , masticatory muscle pain , and treatment . 
 mri and sonography enable accurate visualization and evaluation of the masticatory muscles , thereby increasing our understanding of pathology and cause of pain associated with these muscles . although therapeutic efficacy is often evaluated based on clinical findings , mr and sonographic imaging studies 
 may also be valuable .