Characterisation of soft tissue viscous and elastic properties using ultrasound elastography and rheological models: validation and applications in plantar soft tissue assessment

Objective: The mechanical behaviour of soft tissue is influenced by its elastic and viscous 22 characteristics. Therefore, the aim of this study was to develop a protocol to characterise the 23 viscoelastic properties of soft tissues based on ultrasound elastography data. Approach: 24 Plantar soft tissue was chosen as the tissue of interest, and gelatine-phantoms replicating its 25 mechanical properties were manufactured for validation of the protocol. Both plantar soft 26 tissue and the phantom were scanned using Reverberant shear wave elastography at 400-600 27 Hz. Shear wave speed (SWS) was estimated using the US particle velocity data. The 28 viscoelastic parameters were extracted by fitting the shear wave dispersion data to the 29 Young’s modulus as a function of frequency derived from the constitutive equations of the 30 eight rheological models (four classic and their fractional-derivative versions). Furthermore, 31 stress-time functions derived from the eight rheological models were fitted to the phantom 32 stress-relaxation data. Main results: The viscoelastic parameters estimated using 33 elastography data based on the fractional-derivative (FD) models were closer to those 34 quantified using the mechanical test. In addition, the FD-Maxwell and FD-Kelvin-Voigt 35 models can more effectively replicate the viscoelastic behaviour of the plantar soft tissue with minimum number of model parameters (