Ultrafast ultrasound imaging for muscle onset mapping during its contraction

Abstract

Available methods for studying muscle dynamics, including electromyography (EMG), mechanomyography (MMG) and M-mode ultrasound, have limitation in spatial resolution. B-mode ultrasound has the advantage of visualising the heterogeneity within the muscle, such as muscle rupture, calcification, and trigger point. Using ultrafast ultrasound for muscle dynamics assessment gives us information on how these local defects affects muscle contraction. This study first developed a novel method and protocol of two-dimensional mapping of muscle motion onset using ultrafast ultrasound imaging, i.e. sono-mechano-myo-graphy (SMMG). Then this new system was used for the following three investigations. In the first part of this study, the developed method was compared with the EMG, MMG and force outputs of tibialis anterior (TA) muscle during ankle dorsiflexion at different percentages of maximum voluntary contraction (MVC) force in 14 healthy young adults. Significant differences for all pairwise comparisons of onsets were identified, except between SMMG and MMG. The EMG onset significantly led SMMG, MMG and force onsets by 40.0 ± 1.7 ms (p < 0.001), 43.1 ± 5.2 ms (p < 0.005) and 73.0 ± 4.5 ms (p < 0.001), respectively. The results also demonstrated that voluntary muscle motion started earlier at the middle aponeurosis than those muscles located closed to skin surface and at deeper regions when viewing longitudinally (p < 0.001). No significant difference was found for the onset time measured by SMMG between different MVC levels. In the second part of the study, the developed method was applied to study the gender difference in electromechanical delay in ankle dorsiflexion of the tibialis anterior (TA) muscle in 20 healthy young adults using electrical stimulation. Shear-wave elastography (SWE) was used to evaluate stiffness of the muscle. In longitudinal view, muscle motion started earlier at the middle aponeurosis than the location close to skin surface (p < 0.01). Significant differences in muscle onset time observed using SMMG was found between genders, with females leading males by 7.39 ms (p < 0.001, 95% CI: 3.80 - 10.98 ms). Female subjects had significantly lower elasticity than males by 43.63 kPa (p < 0.05, 95% CI: 7.71 - 79.55 kPa). Simple linear regression showed a positive correlation between the onset time and the shear-wave elasticity (p < 0.001, R2 = 0.80). In the third part of the study, six different methods for detecting the onset time in SMMG signal were evaluated for detecting the stimulated muscle onset time using ultrafast ultrasound recording. The six algorithms were first compared with manual detection on a randomly selected set of data, and then applied to a trial of the stimulated muscle contraction case. Results showed that each algorithm had their advantages and limitations. Change point detection looking for the change in mean of a signal performed best overall for the muscle onset from SMMG signals. However, searching for a more optimized onset detection method for SMMG signals remains a topic for future study. This novel technology can potentially provide new insights in future studies of neuromuscular diseases, such as multiple sclerosis and muscular dystrophy.