Application of the novel estimation method by shear wave elastography using vibrator to human skeletal muscle

Objective measurement of muscle elasticity is crucial for diagnosing various diseases associated with skeletal muscle stiffness and assessing treatment efficacy. Current methods, such as manual palpation and tests like ramp-and-hold, pendulum, and dynamometry, provide limited information and cannot isolate individual muscle properties. Ultrasound strain elastography offers a semi-quantitative approach but is limited by its dependence on examiner skill and inability to directly measure Young's modulus. Shear wave elastography, using either acoustic radiation force impulses or external mechanical vibrations, provides a more quantitative assessment of tissue stiffness based on shear wave propagation speed. However, acoustic radiation force impulse-based systems are expensive and require high frame-rate ultrasound equipment. This study focuses on the development and validation of a cost-effective alternative using external mechanical vibration, specifically investigating the reproducibility and reliability of a novel estimation method for measuring shear wave velocity in human skeletal muscle. This approach leverages color Doppler shear wave imaging, a novel wavefront imaging method, enabling real-time observation of shear wave propagation to improve measurement accuracy. The improved accuracy reduces reliance on complex wave propagation interpretations and potential issues caused by wave reflection and refraction from tissue boundaries.

Existing methods for assessing muscle stiffness have limitations. Manual palpation is subjective, while methods like ramp-and-hold tests and dynamometry involve surrounding tissues, making it difficult to isolate muscle properties. Ultrasound strain elastography provides a semi-quantitative measure but is operator-dependent and doesn't directly measure Young's modulus. Shear wave elastography using acoustic radiation force impulse (ARFI) is more quantitative but expensive due to the requirement for high frame rate ultrasound systems. External mechanical vibration offers a potentially cost-effective alternative for shear wave induction, and recent advancements in this area have shown promise. However, challenges remain in visualizing shear wave wavefronts in real time, which is crucial for accurate velocity estimation. The researchers here address this challenge by utilizing color Doppler shear wave imaging, which allows for direct and real-time observation of shear wavefronts. Existing literature highlights the varied reliability and reproducibility of different shear wave elastography techniques for muscle stiffness assessment.

The study used an experimental system incorporating an ultrasound scanner (Toshiba Xario SSA-660A) with a novel estimation method for shear wave elastography. A linear vibration motor (designed for electric toothbrushes) generated shear waves, with displacement measured using a microelectromechanical system accelerometer. Color flow imaging on the ultrasound scanner was recorded and analyzed to reconstruct shear wave velocity maps using Fourier analysis and directional filtering. The study included 14 healthy young male volunteers. Shear wave velocities were measured in konjac and agar phantom gels and in six major skeletal muscles (biceps brachii, flexor carpi radialis, semitendinosus, biceps femoris, medial gastrocnemius, and tibialis anterior) twice daily for two days. Intra-day, day-to-day, and inter-operator reliabilities were assessed using coefficient of variation (CV) and intra-class correlation coefficient (ICC). Agar gels of varying concentrations were used to verify the method's effectiveness against known elasticity values. The probe and vibrator positions were carefully standardized relative to anatomical landmarks to ensure consistent shear wave propagation direction along muscle fibers. Measurements focused on the middle portion of the muscles, guided by standard electromyography electrode placement recommendations. Statistical analysis involved Shapiro-Wilk tests for normality, paired t-tests for comparisons, and Pearson's correlation analysis for evaluating the relationship between agar concentration and shear wave velocity.

The novel estimation method demonstrated excellent reproducibility and reliability in both phantom gels and skeletal muscle. In konjac phantom jelly, the coefficient of variation (CV) for shear wave velocity was low (0.38%-1.38%). Agar gel measurements showed a strong positive correlation (r=0.99) between agar concentration and shear wave velocity, with good intra-day, day-to-day, and inter-operator reliabilities (CV < 7.7%, ICC > 0.96). Skeletal muscle measurements exhibited good intra-day (CV < 9.8%, ICC > 0.92) and day-to-day (CV < 9.6%, ICC > 0.90) reliabilities across all six muscles examined. Shear wave velocities measured in the biceps brachii were comparable to those reported in a previous study using external vibration, though differences in experimental parameters make direct comparison challenging. The study's findings on reliability are comparable to some, but not all, previous studies utilizing shear wave elastography techniques in resting muscles. The good reliability was attributed to careful standardization of scanning site, imaging parameters, and the positioning of the probe and vibrator to ensure uniform shear wave propagation along muscle fibers.

The findings demonstrate the excellent reliability and reproducibility of the novel vibration-based shear wave elastography method for measuring skeletal muscle stiffness. The good reliability achieved highlights the importance of standardized measurement protocols, including careful probe and vibrator positioning to ensure consistent shear wave propagation. The direct visualization of shear wave wavefronts using color Doppler imaging contributes to the method’s enhanced accuracy and reliability compared to techniques relying on indirect estimation of wave propagation. The method's cost-effectiveness compared to ARFI-based systems makes it a promising tool for widespread clinical application, potentially benefitting patients with various conditions involving muscle stiffness, such as myofascial pain syndrome, neck and shoulder pain, and low back pain. The ability to accurately assess muscle elasticity could aid in diagnosis, treatment planning, and monitoring of treatment efficacy.

This study confirms the adequate reproducibility and reliability of a novel estimation method for vibration-based shear wave elastography in measuring skeletal muscle stiffness. The method's cost-effectiveness and good performance suggest its potential as a valuable tool for evaluating muscle stiffness in clinical and research settings. Further studies are needed to validate the method's reliability across diverse patient populations and clinical conditions.

The study had several limitations. First, it lacked comparison with other quantitative methods, although previous studies have shown consistency with other instruments. Second, participants were asymptomatic young males, limiting generalizability to other populations. Third, only the middle portion of muscles was examined, and only major surface muscles were included. Further research should investigate other muscle areas and types, potentially modifying shear wave frequency to optimize measurements in deeper tissues.