Simultaneous acquisition of magnetic resonance elastography of the supraspinatus and the trapezius muscles

Highlights

• Our technique permits simultaneous MRE of the supraspinatus and trapezius muscles.

• Optimal vibration frequency for simultaneous MRE of these muscles is 75 Hz.

• Our technique provides a new means for early detection of shoulder abnormalities.

Abstract

We developed a Magnetic Resonance elastography (MRE) technique using a conventional magnetic resonance imaging (MRI), which allows a simultaneous elastography of the supraspinatus and trapezius muscles, by designing a new wave transducer (vibration pad) and optimizing the mechanical vibration frequency. Five healthy volunteers underwent an MRE. In order to transmit the mechanical vibration (pneumatic vibration) to the supraspinatus and trapezius muscles, a new vibration pad was designed using a three-dimensional (3D) printer. The vibration pad was placed on the skin 2 cm medial and 2 cm cephalad the deltoid tubercle. MRE acquisition was performed with a multi-slice gradient-echo type multi-echo MR sequence, which allows MREs even in a conventional MRI; two oblique axial images of the supraspinatus and trapezius muscles were obtained simultaneously. Vibration frequencies were set at 50–150 Hz, with a 25 Hz step. Wave image quality in each frequency was analyzed using a phase-to-noise ratio (PNR) and clarity of propagating wave that was assessed by two readers qualitatively. In the supraspinatus muscle, the wave images were of good quality especially at frequencies >75 Hz. In the trapezius muscle, the wave images were of better quality at low frequencies (50 and 75 Hz) compared with high frequencies (100–150 Hz). The PNR of both muscles were higher at low frequencies. The mean stiffness in the trapezius muscle (7.26 ± 2.13 kPa at 75 Hz) was larger than those in the supraspinatus muscle (4.16 ± 0.50 kPa at 75 Hz). The results demonstrated that our MRE technique allows simultaneous assessment of the stiffness in the supraspinatus and trapezius muscles using a conventional MRI, and that optimal vibration frequency for simultaneous MRE of these muscles is 75 Hz. This technique provides a new means for early detection of abnormality in the shoulder.

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.