Technical noteSimultaneous acquisition of magnetic resonance elastography of the supraspinatus and the trapezius muscles
Introduction
Rotator cuff tears are among the most common injuries of the shoulder [1] and usually initiate at the anterosuperior portion of the supraspinatus muscle and extend posteriorly [2]. The rotator cuff is susceptible to repetitive damages due to eccentric contraction in the deceleration phase during throwing and overhead hand use, and eventually rotator cuff tear occurs [3]. Eccentric contraction leads to stiff and sore muscles within a day or two of the activity, leading to a prolonged reduction in force generation capacity [4]. In the most initial mild stage of muscle damages (i.e., Type I muscle injury), one of main symptoms is muscle firmness or an increase in muscle tone with minimal changes in MRI findings [5]. After a tear of the supraspinatus tendon, the musculotendinous unit retracts, atrophies, undergoes fatty infiltration, and loses elasticity [6]. Even after successful rotator cuff repair, these muscle changes were irreversible [7,8].
Thus, eccentric contraction induces muscle damages such as sarcomere disruption, and mechanical properties such as muscle stiffness and force are altered [4,[9], [10], [11]]. Therefore, quantitative evaluation of stiffness of the supraspinatus muscle could assist the diagnosis of muscle pathologies and assess effects of treatments of rotator cuff tears. Clinically, palpation is used to diagnose abnormal stiffness changes in skeletal muscles [12,13]. However, palpation depends on the examiner's clinical skills for muscle identification, and it is impossible to detect stiffness changes in deep tissues by palpation. Since the supraspinous muscle is located under the trapezius muscle, it is difficult to differentiate stiffness of the supraspinatus and trapezius muscles by palpation. Therefore, a new technique is required to detect rotator cuff abnormalities before rotator cuff tears occur.
Magnetic resonance elastography (MRE) is a phase-contrast technique that applies an oscillating motion encoding gradient (MEG) to detect tissue wave displacements introduced by external vibration [14]. The displacement image (wave image) is then used to reconstruct the mechanical properties of the tissue via inversion algorithms [15]. MRE allows measurement of mechanical properties of both superficial and deep tissues only if vibrations reach these tissues. For vibration to reach the tissues, the vibration system should be adequately designed. Over the years, several vibration systems have been developed; for example, the electromechanical, piezo-electric-stack and pneumatic vibration [16]. Among them, a pneumatic vibration system is often used for MRE in clinical practice or situation [[17], [18], [19]]. This system is easily available in high magnetic field and has high penetration ability if the wave transducer (vibration pad) adheres tenaciously to the skin. Therefore, it is important for the MRE to appropriately design the vibration pad according to the imaging object. Vibration frequency is also an important factor to determine penetration ability of the vibration. Higher frequencies have shorter wavelengths and better resolution of the stiffness, but lower penetration ability. Therefore, it is necessary to adjust vibration frequency according to size of an imaging object and its depth from the body surface.
Our previous study developed an MRE technique that can be applied to the supraspinatus muscle [20]. Chen et al. [21] performed MRE of the trapezius muscle in order to evaluate myofascial taut band, a contracted or shortened muscle fiber band with increased muscle tone. However, as far as we know, no previous studies have applied MRE simultaneously to the supraspinatus and trapezius muscles. Simultaneous MRE of the supraspinatus and trapezius muscles would allow not only more accurate and objective assessment of stiffness of the supraspinatus muscles but also identification of the muscle with higher stiffness, either the supraspinatus or trapezius muscles. In addition, simultaneous MRE of the supraspinatus and trapezius muscles can reduce examination time greatly, because both acquisition and setting times of a vibration pad can be reduced. The purpose of this study was to develop an MRE technique to be applied to the supraspinatus and trapezius muscles simultaneously. We used a conventional gradient-echo type multi-echo MR sequence [22] in order to conduct an MRE of the supraspinatus and trapezius muscles in a conventional magnetic resonance imaging (MRI). To perform the simultaneous MRE of the supraspinatus and trapezius muscles, it is necessary to supply vibrations to both muscles efficiently. We have previously demonstrated that MRE can be applied to the supraspinatus muscle and found the best excitation location [20,23]. Hence, this study developed a technique for simultaneous MRE of the supraspinatus and trapezius muscles by adjusting the shape of a vibration pad and vibration frequency.
Section snippets
Participants
Five healthy volunteers (5 men, mean age: 22.0 ± 1.22 years, age range: 20–23 years, mean BMI: 21.6 ± 1.87) with no history of skeletal muscle disease, shoulder trauma, and recent/present shoulder pain were enrolled in this MRE study. We instructed the volunteers to relax during imaging and to lightly bend the elbow joint by 30° and put the hand on the abdomen in order to keep the body position constant among volunteers and relax the supraspinatus and trapezius muscles. All studies were
Results
Fig. 5 shows mean quality scores in the supraspinatus and trapezius muscles in each frequency. The value of Cohen's kappa concordance coefficient was almost perfect (k = 0.82; CI 95% 0.71; 0.93). In the supraspinatus muscle, the mean quality score was high at the all frequencies and was >4 for frequencies >75 Hz. In the trapezius muscle, the mean quality score was high at 50 and 75 Hz (3.7 and 4.1). However, it was decreased in higher frequencies and the scores were <3 for frequencies over
Discussion
The results demonstrated that our MRE technique enables simultaneous acquisition of MRE information of the supraspinatus and trapezius muscles using a conventional MRI. In this study, clarity of the wave images across the vibration frequencies was different between the supraspinatus and trapezius muscles. Anatomical locations of these muscles suggest that the waves would propagate more easily in the trapezius muscle that is located closer to the surface than the supraspinatus muscle. However,
Conclusion
We demonstrated that our MRE techniques can simultaneously assess stiffness of the supraspinatus and trapezius muscles using a conventional MRI. The vibration frequency of 75 Hz in our technique was suitable to obtain the clear wave images of propagating shear waves in both muscles. This technique may provide a new technical means to detect rotator cuff abnormalities.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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