Evaluating elastic properties of heterogeneous soft tissue by surface acoustic waves detected by phase-sensitive optical coherence tomography

Abstract

The combined use of surface acoustic wave (SAW) and phase-sensitive optical coherence tomography (PhS-OCT) is useful to evaluate the elasticity of layered biological tissues, such as normal skin. However, the pathological tissue is often originated locally, leading to the alternation of mechanical properties along both axial and lateral directions. We present a feasibility study on whether the SAW technique is sensitive to detect the alternation of mechanical property along the lateral direction within tissue, which is important for clinical utility of this technique to localize diseased tissue. Experiments are carried out on purposely designed tissue phantoms and ex vivo chicken breast samples, simulating the localized change of elasticity. A PhS-OCT system is employed not only to provide the ultra-high sensitive measurement of the generated surface waves on the tissue surface, but also to provide the real time imaging of the tissue to assist the elasticity evaluation of the heterogeneous tissue. The experimental results demonstrate that with PhS-OCT used as a pressure sensor, the SAW is highly sensitive to the elasticity change of the specimen in both vertical and lateral directions with a sensing depth of ∼5 mm with our current system setup, thus promising its useful clinical applications where the quantitative elasticity of localized skin diseases is needed to aid in diagnosis and treatment.

Quantitative elasticity measurement of urinary bladder wall using laser-induced surface acoustic waves

“…It can be calculated from the distance divided by the time delay as measured from the SAW energy peak of envelop of the SAW signal [46,47]. Group velocity carry the general information about the material elasticity in time domain [35,[46][47][48][49][50]. Increasing in the bladder content will significantly increase the SAW group velocity, which reflect increase in the stiffness of all layers of bladder wall.…”

“…However, the propagation of SAWs in a heterogeneous medium (i.e., layered materials or structures) shows a unique dispersive behavior [38][39][40][41], meaning that the different frequency components travel at different phase velocities within the material. The phase velocity (C R ) depends on the elastic and geometric properties of the material or structure through all the layers it penetrates [32][33][34][35][36][37][38]. In a single, isotropic homogeneous layer, the surface wave phase velocity can be approximated as:…”

“…In the laser-induced SAW method, the deformation experienced by the target sample is less than 1 µm (typically ~100 nm) [37], which ensures linear behavior of the soft tissue. Our previous works proved that SAWs can be used to accurately evaluate the mechanical properties of layered viscoelastic soft tissues such as those in the skin, liver, and cornea [32][33][34][35][36][37]. On the other hand, optical coherence tomography (OCT) is a promising noninvasive, noncontact imaging technique capable of providing microstructural information on tissue with high resolution.…”

“…Wang et al was the first to describe the application of laserinduced SAWs to clinical dental disease diagnosis [30,31]. Our group pioneered the use of shaker-and laser-induced SAWs to evaluate the Young's modulus of heterogeneous soft tissue and phantoms [32][33][34][35][36][37]. In the laser-induced SAW method, the deformation experienced by the target sample is less than 1 µm (typically ~100 nm) [37], which ensures linear behavior of the soft tissue.…”

Optical coherence elastography – OCT at work in tissue biomechanics [Invited]

“…Such quantification requires application of either time-consuming FEM, or other analytical models, which are all constrained by specific assumptions [45], but may provide a more accurate and complete biomechanical assessment. Spectral decomposition of the elastic waves can also provide depth-resolved information due to the wavelength dependence of elastic wave penetration into the sample [70,71,117,189]. Additionally, it allows for assessment of mechanical properties well beyond the OCT imaging depth, since the penetration depth of Rayleigh waves can reach many millimeters and even centimeters, depending on the wavelength [190].…”