SWE of Muscles in Spasticity: Assessment of Passive Stiffness and Positioning
Rationale for Using SWE in Spasticity
According to EFSUMB (Săftoiu et al., 2019), SWE for assessing muscle stiffness in various neurological conditions—Parkinson's disease, chronic stroke, cerebral palsy, multiple sclerosis, Duchenne muscular dystrophy—is a reliable quantitative imaging method applicable for diagnosis, treatment decision-making, and follow-up. The method can serve as an alternative to electromyography.
According to ACR-SRU (2024), SWE is a quantitative method for determining tissue stiffness: the stiffer the tissue, the faster the shear wave propagates through it. Color maps contain quantitative units—m/s or kPa. Softer tissues deform more than stiffer ones.
Limitation on Reference Values
EFSUMB (2019) explicitly states: despite the availability of muscle studies, results are limited and reference values have not been established [327, 333, 336, 338]. Specific stiffness thresholds for assessing passive stiffness of spastic muscle are not provided in the source fragments [clarify].
Positioning and Methodology
A detailed positioning protocol specifically for muscles in spasticity is absent in the source fragments [clarify]. According to general SWE principles (Griffith, 2025), the study is performed in the longitudinal plane relative to muscle/tendon fibers; in the example of the Achilles tendon, normal stiff tissue is displayed as 'red' (497 kPa), while severe tendinosis with softer heterogeneous tissue is displayed as 'blue' (133 kPa).
In quantitative assessment, stiffness is expressed through Young's modulus: E (in kPa) = 3ρc², where ρ is the tissue density (assumed to be 1000 kg/m³), c is the shear wave speed in m/s (Berg & Leung, 2019). Speed is higher in stiffer tissue.
Advantages of the Method
| Parameter | SWE |
|---|---|
| Operator Dependence | Less operator-dependent than strain elastography (Griffith, 2025) |
| Type of Measurement | Qualitative and quantitative |
| Units | m/s or kPa |
| Operator Pressure | Not required (low-frequency pulse is electronically delivered) |
According to Griffith (2025), in inflammatory myopathies, strain elastography (SE) demonstrated increased stiffness of involved muscles with significant correlations with histological data.
Frequently asked questions
Are reference values for muscle stiffness in SWE established?
No. EFSUMB (2019) indicates that muscle results are limited and reference values have not been established.
Why is SWE preferred over strain elastography for muscles?
SWE is less operator-dependent and allows for both qualitative and quantitative assessment (Griffith, 2025).
In which neurological conditions is muscle SWE recognized as reliable?
Parkinson's disease, chronic stroke, cerebral palsy, multiple sclerosis, Duchenne muscular dystrophy (EFSUMB, 2019).
How is stiffness calculated in SWE?
Through Young's modulus: E (kPa) = 3ρc², where ρ = 1000 kg/m³, c is the shear wave speed in m/s (Berg & Leung, 2019).
Can SWE replace electromyography?
According to EFSUMB (2019), SWE can be an alternative to electromyography for diagnosis, treatment decisions, and monitoring.