Aims: To investigate the feasibility of quantitative analysis of muscle stiffness by Acoustic Radiation Force Impulse (ARFI) imaging, to obtain the reference values at relaxing and contraction position of gastrocnemius medialis (GCM) and to evaluate the inter-observer and intra-observer reliabilities of shear wave velocities measurements in healthy skeletal muscles.
Material and methods: The stiffness of the left GCM muscle of 15 healthy volunteers was measured by ARFI elastography in transverse scan while the ankle was in the relaxed position and in 30 degrees of plantar flexion with the submaximal isometric contraction. All subjects were examined by two experienced physiatrists with four years of experience. Reliability of ARFI measurements was assessed by means of the intraclass correlation coefficient (ICC). Interobserver and intra-observer reliabilities were statistically analyzed.
Results: The mean shear wave velocity (SWV) of GCM at plantar flexion position with submaximal isometric contraction was significantly higher than that at relaxed position (first physiatrists: plantar flexion with submaximal isometric contraction 2.41±1.09 m/s, relaxed 0.84±0.28 m/s, second physiatrist: plantar flexion with submaximal isometric contraction 2.49±0.94m/s, relaxed 0.83±0.21), p<0.05. The inter-observer reliability was excellent for ankle-plantar flexion with submaximal isometric contraction (ICC=0.968), and good for relaxed position (ICC=0.891) respectively. The intra-observer reliability for ankle-plantar flexion with submaximal isometric contraction were excellent (ICC: first physiatrist 0.98 and second physiatrist 0.96) and that for relaxed position (ICC: first physiatrist 0.98 and second physiatrist 0.90) were also excellent. There was no significant correlation between SWV and body mass index.
Conclusions: ARFI elastography is a reliable imaging modality for quantifying the stiffness of contracting muscles. Additionally, the characterization of pathological soft tissues by ARFI elastography would be a promising clinical practice for patients with musculoskeletal issues.