Assessing the mechanical properties of anisotropic soft tissues using guided wave elastography: Inverse method and numerical experiments

Real-time ultrasound shear wave elastography using a local phase gradient

BACKGROUND AND OBJECTIVE

Current approaches for ultrasound spectral elastography make use of block processing, resulting in long computational times. This work describes a real-time, robust, and quantitative imaging modality to map the elastic and viscoelastic properties of soft tissues using ultrasound.

METHODS

This elastographic technique relies on the spectral estimation of the shear-wave phase speed by combining a local phase-gradient method and angular filtering. We first apply directional filtering in the spatio-temporal frequency domain for providing one-way, smooth, and harmonic displacement maps in the frequency range of interest. Thanks to this, we can apply a simple, fast, and local phase gradient approach to obtain the axial and lateral components of the wavevector, which are linked to phase velocity and soft-tissue elasticity and viscoelasticity. The technique is validated numerically and experimentally using a 7.6 MHz ultrasound probe, tested in calibrated soft-tissue phantoms and ex vivo liver tissues. The method is compared with state-of-the-art spectral methods.

RESULTS

The technique significantly reduces the computation time, e.g., the reconstruction time for a 155 × 315-pixel phase-velocity map was 0.16 s, while local-phase velocity-imaging techniques was 156.73 s for 2D implementation and 13.56 s for the 1D version, a reduction between two and three orders of magnitude, while showing a similar accuracy and resolution than standard methods.

CONCLUSIONS

This approach eliminates the need for block processing that may limit the spatial resolution and computational time of the velocity map. In this way, the phase gradient elastography method is revealed as an efficient and robust approach for real-time spectral elastography.

In Vivo Optical Coherence Elastography Unveils Spatial Variation of Human Corneal Stiffness

OBJECTIVE

The mechanical properties of corneal tissues play a crucial role in determining corneal shape and have significant implications in vision care. This study aimed to address the challenge of obtaining accurate in vivo data for the human cornea.

METHODS

We have developed a high-frequency optical coherence elastography (OCE) technique using shear-like antisymmetric (A0)-mode Lamb waves at frequencies above 10 kHz.

RESULTS

By incorporating an anisotropic, nonlinear constitutive model and utilizing the acoustoelastic theory, we gained quantitative insights into the influence of corneal tension on wave speeds and elastic moduli. Our study revealed significant spatial variations in the shear modulus of the corneal stroma on healthy subjects for the first time. Over an age span from 21 to 34 (N = 6), the central corneas exhibited a mean shear modulus of 87 kPa, while the corneal periphery showed a significant decrease to 44 kPa. The central cornea's shear modulus decreases with age with a slope of -19 +/- 8 kPa per decade, whereas the periphery showed non-significant age dependence. The limbus demonstrated an increased shear modulus exceeding 100 kPa. We obtained wave displacement profiles that are consistent with highly anisotropic corneal tissues.

CONCLUSION

Our approach enabled precise measurement of corneal tissue elastic moduli in situ with high precision (<7%) and high spatial resolution (<1 mm). Our results revealed significant stiffness variation from the central to peripheral corneas.

SIGNIFICANCE

The high-frequency OCE technique holds promise for biomechanical evaluation in clinical settings, providing valuable information for refractive surgeries, degenerative disorder diagnoses, and intraocular pressure assessments.

Shear wave imaging the active constitutive parameters of living muscles

Shear wave elastography (SWE) of human skeletal muscles allows for measurement of muscle elastic properties in vivo and has important applications in sports medicine and for the diagnosis and treatment of muscle-related diseases. Existing methods of SWE for skeletal muscles rely on the passive constitutive theory and have so far been unable to provide constitutive parameters describing muscle active behavior. In the present paper, we overcome this limitation by proposing a SWE method for quantitative inference of active constitutive parameters of skeletal muscles in vivo. To this end, we investigate the wave motion in a skeletal muscle described by a constitutive model in which muscle active behavior has been defined by an active parameter. An analytical solution relating shear wave velocities to both passive and active material parameters of muscles is derived, based upon which an inverse approach has been developed to evaluate these parameters. To demonstrate the usefulness of the reported method, in vivo experiments were carried out on 10 volunteers to obtain constitutive parameters, particularly those describing active deformation behaviors of living muscles. The results reveal that the active material parameter of skeletal muscles varies with warm-up, fatigue and rest. STATEMENT OF SIGNIFICANCE: Existing shear wave elastography methods are limited to imaging the passive parameters of muscles. This limitation is addressed in the present paper by developing a method to image the active constitutive parameter of living muscles using shear waves. We derived an analytical solution demonstrating the relationship between constitutive parameters of living muscles and shear waves. Relying on the analytical solution, we proposed an inverse method to infer active parameter of skeletal muscles. We performed in vivo experiments to demonstrate the usefulness of the theory and method; the quantitative variation of the active parameter with muscle states such as warm-up, fatigue and rest has been reported for the first time.

In situ measurement of the stiffness increase in the posterior sclera after UV-riboflavin crosslinking by optical coherence elastography

Scleral crosslinking may provide a way to prevent or treat myopia by stiffening scleral tissues. The ability to measure the stiffness of scleral tissues in situ pre and post scleral crosslinking would be useful but has not been established. Here, we tested the feasibility of optical coherence elastography (OCE) to measure shear modulus of scleral tissues and evaluate the impact of crosslinking on different posterior scleral regions using ex vivo porcine eyes as a model. From measured elastic wave speeds at 6 - 16 kHz, we obtained out-of-plane shear modulus value of 0.71 ± 0.12 MPa (n = 20) for normal porcine scleral tissues. After riboflavin-assisted UV crosslinking, the shear modulus increased to 1.50 ± 0.39 MPa (n = 20). This 2-fold change was consistent with the increase of static Young's modulus from 5.5 ± 1.1 MPa to 9.3 ± 1.9 MPa after crosslinking, which we measured using conventional uniaxial extensometry on tissue stripes. OCE revealed regional stiffness differences across the temporal, nasal, and deeper posterior sclera. Our results show the potential of OCE as a noninvasive tool to evaluate the effect of scleral crosslinking.

Multi-Point Nanoindentation Method to Determine Mechanical Anisotropy in Nanofibrillar Thin Films

Biomaterials with outstanding mechanical properties, including spider silk, wood, and cartilage, often feature an oriented nanofibrillar structure. The orientation of nanofibrils gives rise to a significant mechanical anisotropy, which is extremely challenging to characterize, especially for microscopically small or inhomogeneous samples. Here, a technique utilizing atomic force microscope indentation at multiple points combined with finite element analysis to sample the mechanical anisotropy of a thin film in a microscopically small area is reported. The system studied here is the tape-like silk of the Chilean recluse spider, which entirely consists of strictly oriented nanofibrils giving rise to a large mechanical anisotropy. The most detailed directional nanoscale structure-property characterization of spider silk to date is presented, revealing the tensile and transverse elastic moduli as 9 and 1 GPa, respectively, and the binding strength between silk nanofibrils as 159 ± 13 MPa. Furthermore, based on this binding strength, the nanofibrils' surface energy is derived as 37 mJ m^-2 , and concludes that van der Waals forces play a decisive role in interfibrillar binding. Due to its versatility, this technique has many potential applications, including early disease diagnostics, as underlying pathological conditions can alter the local mechanical properties of tissues.

Non-invasive measurement of the internal pressure of a pressurized biological compartment using Lamb waves

In this study, we propose a mechanical analysis for estimating the internal pressure of a finitely deformed spherical compartment from Lamb wave measurements. The proposed analysis produces a dispersion relation associating Lamb wave speed with pressure using limited material parameters (only a strain stiffening term). The analysis was validated on ultrasound bladder vibrometry (UBV) experiments collected from 9 ex vivo porcine bladders before and after formalin cross-linking. Estimated pressures were compared with pressures measured directly by a pressure transducer. The proposed analysis proved broadly effective at estimating pressure from UBV based Lamb wave without calibration as demonstrated by the observed concordance between estimated and measured pressures (Lins CCC = 0.82 (0.66-0.91). Theoretical limitations and potential refinements to improve the accuracy and generality of the approach are discussed.

Molecular mechanisms underlying the extreme mechanical anisotropy of the flaviviral exoribonuclease-resistant RNAs (xrRNAs)

Mechanical anisotropy is an essential property for many biomolecules to assume their structures, functions and applications, however, the mechanisms for their direction-dependent mechanical responses remain elusive. Herein, by using a single-molecule nanopore sensing technique, we explore the mechanisms of directional mechanical stability of the xrRNA1 RNA from ZIKA virus (ZIKV), which forms a complex ring-like architecture. We reveal extreme mechanical anisotropy in ZIKV xrRNA1 which highly depends on Mg²⁺ and the key tertiary interactions. The absence of Mg²⁺ and disruption of the key tertiary interactions strongly affect the structural integrity and attenuate mechanical anisotropy. The significance of ring structures in RNA mechanical anisotropy is further supported by steered molecular dynamics simulations in combination with force distribution analysis. We anticipate the ring structures can be used as key elements to build RNA-based nanostructures with controllable mechanical anisotropy for biomaterial and biomedical applications.