The effect of unit, depth, and probe load on the reliability of muscle shear wave elastography: Variables affecting reliability of SWE

Purpose: There is currently no standardized method for muscle shear wave elastography (SWE). The objective of this study was to investigate the effect of unit of measurement, depth, and probe load on the reliability of muscle SWE. Methods: The vastus lateralis, biceps femoris, biceps brachii, and abductor digiti minimi muscles were scanned on 20 healthy participants. The SWE readings were measured in shear wave velocity (m/s) and Young's modulus (kPa). Three acquisitions of varying depths were acquired from vastus lateralis. Minimal probe load was compared with the use of a standoff gel layer. Three repeated measurements were acquired to assess reliability using intraclass correlations (ICC). Results: The mean elasticity varied across muscle groups and ranged from 1.54 m/s for biceps femoris to 2.55 m/s for abductor digiti minimi (difference = 1.01 m/s [95% confidence interval, CI = 0.92, 1.10]). Reporting readings in meters per second resulted in higher ICC of 0.83 (0.65, 0.93) in comparison to 0.77 (0.52, 0.90) for kilopascal for the vastus lateralis muscle only. Variance increased proportionally with depth reaching 0.17 (equivalent to ±0.82 m/s) at 6 cm. Using a standoff gel decreased ICC to 0.63 (0.20, 0.84) despite similar mean elasticity readings to minimal probe load. Conclusions: Different acquisition and technical factors may significantly affect the reliability of SWE in skeletal muscles. Readings acquired in the unit of shear wave velocity (m/s) from depths less than 4 cm using a minimal probe load without a standoff gel yielded the best reliability.