The adoption of multiscale approaches by the biomechanical community has caused a major improvement in quality in the mechanical characterization of soft tissues. The recent developments in elastography techniques are enabling in vivo and non-invasive quantification of tissues’ mechanical properties. Elastic changes in a tissue are associated with a broad spectrum of pathologies, which stems from the tissue microstructure, histology and biochemistry. This knowledge is combined with research evidence to provide a powerful diagnostic range of highly prevalent pathologies, from birth and labor disorders (prematurity, induction failures, etc.), to solid tumors (e.g., prostate, cervix, breast, melanoma) and liver fibrosis, just to name a few. This review aims to elucidate the potential of viscous and nonlinear elastic parameters as conceivable diagnostic mechanical biomarkers. First, by providing an insight into the classic role of soft tissue microstructure in linear elasticity; secondly, by understanding how viscosity and nonlinearity could enhance the current diagnosis in elastography; and finally, by compounding preliminary investigations of those elastography parameters within different technologies. In conclusion, evidence of the diagnostic capability of elastic parameters beyond linear stiffness is gaining momentum as a result of the technological and imaging developments in the field of biomechanics.

Pregnancy involves a gradual change in tissue consistency, where, as gestational age increases a drop in stiffness is noticeable. The extracellular matrix (ECM) of the cervix whose behavior is controlled by the collagen structure is reconfigured, due to hormonal and protein related factors that degraded it. There is an increase in cellular and water content. The collagen maintains a stable percentage, although the synthesis-solubility interaction causes a transformation in the network increasing its diameter and waviness, which destabilizes the mechanical integrity. Further understanding of the remodeling of the cervix becomes a key element in the progress against preterm birth. However, since the woman is in a very delicate state, the available information is scattered. The aim is to provide a framework where the variation of the histological properties is linked with the individual biomechanical evolution of the constituents; therefore a multiscale approach to the problem can be made. The results of different authors were reviewed to obtain values of each component depending on the gestational age, and then they were combined with the mechanical development of each variable obtained from the outputs of numerical simulations of another author. An additive function is proposed, in which the mechanical contribution of each constituent is added to a reference value established for a nonpregnant state. Results show how biochemistry models the mechanical behavior of the tissue through the histology and morphology of the ECM.

The reconstruction of viscous properties of soft tissues, and more specifically, of cervical tissue is a challenging problem. In this paper, a new method is proposed to reconstruct the viscoelastic parameters of cervical tissue-mimicking phantoms by a Torsional Wave Elastography (TWE) technique. The reconstruction method, based on a Probabilistic Inverse Problem (PIP) approach, is presented and experimentally validated against Shear Wave Elastography (SWE). The anatomy of the cervical tissue has been mimicked by means of a two-layer gelatine phantom that simulates the epithelial and connective layers. Five ad hoc oil-in-gelatine phantoms were fabricated at different proportion to test the new reconstruction technique. The PIP approach was used for reconstructing the Kelvin-Voigt (KV) viscoelastic parameters by comparing the measurements obtained from the TWE technique with the synthetic signals from a Finite Difference Time Domain (FDTD) KV wave propagation model. Additionally, SWE tests were realized in order to characterize the viscoelastic properties of each batch of gelatine. Finally, validation was carried out by comparing the KV parameters inferred from the PIP with those reconstructed from the shear wave dispersion curve obtained from the SWE measurements. In order to test the degree of agreement between both techniques, a Student’s T-test and a Pearson’s correlation study were performed. The results indicate that the proposed method is able to reconstruct the KV viscoelastic properties of the cervical tissue, for both the epithelial and connective layers, as well as the thickness of the first layer with acceptable accuracy.