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RESEARCH REPORT |
1 Division of Aging and Geriatric Dentistry, Tohoku University Graduate School of Dentistry, 4-1 Seiryo-machi, Aoba-ku, Sendai, 980-8575, Japan; and
2 Division of Nuclear Medicine, Cyclotron and Radioisotope Center, Tohoku University, 2-1 Aoba, Aramaki, Aoba-ku, Sendai, 980-8578, Japan
* corresponding author, makoto-w{at}mail.tains.tohoku.ac.jp
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: positron emission tomography • 18F-fluoro-D-deoxyglucose • lateral excursion • masticatory muscle
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Recently, positron emission tomography and 18F-fluoro-deoxy-glucose (FDG-PET) have been used to evaluate muscle activity by measuring intramuscular variation in glucose metabolism within exercising skeletal muscles (Fujimoto et al., 1996; Tashiro et al., 1999; Pappas et al., 2001; Oi et al., 2003). A previous study has shown that the FDG-PET can simultaneously quantify the overall activity of individual masticatory muscles during chewing, and without any physical invasion (Rikimaru et al., 2001, 2002), which is not achievable with conventional electromyography.
The present study was designed to quantify the overall activity of lateral and medial pterygoid muscles and masseter and temporal muscles by FDG-PET, and to compare their contributions to the lateral jaw movement, in which the lateral pterygoid muscle has been shown to play an important role (Murray et al., 2001).
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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PET Imaging
Subjects were instructed to avoid eating and drinking for at least 4 hrs before the start of the experiment. Each subject received an injection of 39.2 ± 5.2 MBq of 18F-FDG into the cubital vein, following a quiet 10-minute sitting period. Just after the injection, each subject was asked to start repetition of leftward jaw motion to the canine edge-to-edge position, and rightward jaw motion back to the midline, at a frequency of 1.0 Hz (paced by a metronome) and without exerting a biting force. This continued for 30 min while the subject was in a sitting position, monitored by an examiner. All subjects were asked to practice the repetitive jaw movement before the 30-minute exercise period. A period of 30 min was required for sufficient accumulation of FDG in masticatory muscles to allow for analysis of the PET images. The PET images were acquired after completion of the exercise.
PET scanning was performed by a SET-2400W three-dimensional PET scanner (Shimadzu Co., Kyoto, Japan) with a full-width half-maximum spatial resolution of 7.9 mm (axial) and 3.9 mm (trans-axial) at the center of the 20.0-cm axial field of view (Fujiwara et al., 1997). Thirty-five min after injection of 18F-FDG, head and neck images were acquired for 15 min, while the subject was in a supine position. Tissue attenuation of annihilation gamma rays was corrected by post-injection transmission scans in a rotating 68Ge/68Ga source. Acquired three-dimensional raw data were reconstructed into 94 slices by means of an ‘ordered subsets expectation maximization’ algorithm on a XP-1000 workstation (Digital Equipment Corporation, Boston, MA, USA) (Hudson and Larkin, 1994).
MRI imaging
T1-weighted magnetic resonance images (MRI) of the head and neck of each subject were obtained with a spoiled gradient echo sequence after the PET study. The equipment used was a SIGNA MR/i 1.5T (General Electric Medical Systems, Waukesha, WI, USA).
Image Analysis
Image processing and regions-of-interest analysis were carried out with PET-MRI Tools (Varga, 2000) integrated with the "Automated Image Registration" algorithm (Woods et al., 1993) on Matlab (Math Works, Natick, MA, USA). We re-aligned MR images to corresponding PET images using the "Automated Image Registration" method to determine the location and structure of the masticatory muscles in the PET images. This PET-MRI registration method has most commonly been used in brain PET studies, since it can add anatomical information to functional PET images by alignment of a corresponding MR image with the objective PET image. Regions-of-interest were manually drawn over every masticatory muscle on all slices of registered MR images, and these were applied to the corresponding PET images (Fig. 1
). The mean value of the radioactivity per voxel in each region-of-interest was automatically measured with PET-MRI Tools and normalized for both the amount injected and the subject’s body weight, to give a standardized uptake ratio as an indicator of muscle activity, according to the following formula (Kubota et al., 1985; Strauss and Conti, 1991; Schomburg et al., 1996):
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The standardized uptake ratio is a relative semi-quantitative value of radioactivity in regions-of-interest, which can be understood as the concentration of radiotracer in the region-of-interest divided by the concentration of injected dose distributed throughout the mass of the body. The standardized uptake ratio for each masticatory muscle is expressed as a mean ± standard deviation.
Statistical Analysis
The statistical significance of differences among all masticatory muscles was examined with a one-factor analysis of variance and the Games/Howell method. In addition, a comparison between the contralateral and ipsilateral activities of each muscle was performed with a Student’s t test. A p value of less than 0.05 was considered to be significant.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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) (B, C, D, E, G, H, I, K), and the standardized uptake ratio of the contralateral medial pterygoid muscle was the second largest among all masticatory muscles in seven subjects (A, B, C, D, E, H, K). The standardized uptake ratios of the contralateral lateral and medial pterygoid muscles were higher than those of the bilateral masseter and temporal muscles in all subjects, and the standardized uptake ratios of all pterygoid muscles were higher than those of all bilateral masseter and temporal muscles in nine subjects (A, B, C, E, F, G, I, J, K).
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). Statistical analysis revealed significantly higher mean standardized uptake ratios (p < 0.05 by the Games/Howell method) in the contralateral lateral and medial pterygoid muscles, compared with the bilateral masseter muscles.
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). The mean standardized uptake ratio of the contralateral medial pterygoid muscle was also higher than that of its ipsilateral counterpart, but the values were not significantly different. There was little difference between the mean standardized uptake ratios of the ipsilateral and contralateral masseter and temporal muscles.
FDG Accumulation in PET-MRI Fusion Images
The contralateral lateral pterygoid muscle of subject C showed the highest standardized uptake ratio of all the subjects (Table). In this subject, obvious accumulation of FDG was observed in the contralateral lateral and medial pterygoid muscles, the brain, and the sublingual gland, but was not clearly detected in the bilateral masseter and temporal muscles (Fig. 3). FDG accumulation in the brain and the sublingual gland was observed in all subjects.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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presents a typical example of such an image, showing FDG accumulation in the contralateral lateral and medial pterygoid muscles during lateral excursion. FDG accumulation in these muscles indicates that they have high activity during lateral excursion, since FDG has previously been reported to be accumulated in the brain and sublingual gland, but not in any masticatory muscles under resting conditions (Jabour et al., 1993). In the present study, we propose that FDG-PET can be applied to quantify the overall activity of every masticatory muscle during lateral excursion.
PET-MRI Registration
We applied, for the first time, the "Automated Image Registration" method (Woods et al., 1993) to the analysis of skeletal muscles, based on the assumption that the relative physical relationship between the brain and each masticatory muscle is unchanged when the mandible is held in an occlusal position. The "Automated Image Registration" algorithm is an established program that allows for accurate automated registration of three-dimensional images of the brain mathematically across various imaging modalities. It was difficult to define regions-of-interest tracing the anatomical outline of the masseter, temporal, and ipsilateral lateral and medial pterygoid muscles using PET images only, because of low FDG accumulation in these muscles. To overcome this problem, we applied the PET-MRI registration method and defined accurate regions-of-interest over each masticatory muscle on the anatomical MR images registered to the corresponding PET images (Fig. 1). Hence, the PET-MRI registration method is beneficial for quantification of the overall activity of all masticatory muscles.
Overall Activity of Each Masticatory Muscle
FDG-PET facilitates analysis of the activity of each masticatory muscle during various jaw movements within a certain period of time, based on quantitation of glucose metabolism, whereas conventional electro-myography examines instantaneous muscle activity based on electrical signals from the muscles. The present results show that the activities of the pterygoid muscles, and particularly the contralateral pterygoid muscles, are higher than those of the masseter and temporal muscles during lateral excursion.
Lateral and Medial Pterygoid Muscles
Analysis of the quantitative data for glucose metabolism corresponding to muscle activity confirms the results of previous electromyographic studies that have indicated that the lateral and medial pterygoid muscles are involved in horizontal jaw movements (Hannam and Wood, 1981; Gibbs et al., 1984; Widmalm et al., 1987; Murray et al., 2001, 2004), further suggesting that the contralateral lateral pterygoid muscle plays the most important role during lateral excursion, and that the contralateral medial pterygoid muscle is of secondary importance. Previous studies have showed that intercuspal clenching initiates greater activity of the medial pterygoid muscles when the bite force is directed anteriorly, compared with vertical intercuspal clenching (Wood, 1986), which suggests that the medial pterygoid muscles pull the mandible forward. The present study suggests that the high activity of the contralateral medial pterygoid muscle represents the generation of a forward force to pull on the contralateral angle of the mandible during lateral excursion.
Masseter and Temporal Muscles
The activity of the masseter and temporal muscles were lower compared with those of the pterygoid muscles, and there was little difference between the ipsilateral and contralateral activity of these muscles. These results do not concur with previous electromyographic reports of the activities of the ipsilateral temporal muscle and the contralateral masseter muscle during lateral excursion (Vitti and Basmajian, 1977; Gibbs et al., 1984). The present results were obtained by quantification of the overall masticatory activity in multiple paths of leftward jaw motion with rightward motion to the midline over a 30-minute period, whereas the previous electromyographic studies may have recorded instantaneous partial activity of the masseter and temporal muscles in a single path of jaw motion. Therefore, the conditions used in the present study resulted in little difference between the ipsilateral and contralateral activities of the masseter and temporal muscles. Comparison of the overall masticatory activity suggests that the contributions of the masseter and temporal muscles to lateral excursions without bite force are small, compared with those of the pterygoid muscles.
Prospects
The present study shows that the use of FDG-PET allows for quantification of the overall activity of every masticatory muscle and the evaluation of the relative contribution of each masticatory muscle to any particular jaw movement. A time-based measurement of motor unit activities by FDG-PET is impossible because of its lower time and spatial resolutions, compared with electromyography. However, FDG-PET allows for comparison of the total workload in various muscles during a certain period of time on a common scale, which indicates glucose metabolism per unit volume, and therefore this method has potential clinical application in the diagnosis of muscle disorders caused by large loading. Development of novel PET-tracers may provide a new perspective for the analysis of muscle activity and for clinical applications.
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ACKNOWLEDGMENTS |
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Received October 19, 2004; Last revision September 2, 2005; Accepted September 29, 2005
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REFERENCES |
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