Measurement of surface acoustic waves in high-frequency ultrasound: Preliminary results

Abstract

Skin lesions change elastic properties near the surface. In the last decades, several non-invasive elastography techniques have been developed for detecting the mechanical properties of tissue. In particular, harmonic elastography is characterized for inducing shear wave propagation by an external vibrator in order to estimate shear modulus. However, near the boundary region, propagation is governed by surface acoustic waves (SAW). This paper combines crawling waves elastography with a high-frequency ultrasound (HFUS) system for the estimation of the SAW-to-shear compensation factor when ultrasound (US) gel is used as coupling interface. Experiments explore the SAWspeed in a homogeneous phantom with a solid-water interface in order to corroborate theoretical findings. Subsequently, experiments in a solid-US gel interface are conducted in order to find the correct compensation factor. Preliminary results suggest that SAW propagation can be detected using HFUS, and shear velocity maps can be generated by applying the estimated empirical correction factor. This study will potentially avoid the underestimation of shear modulus when using SAW-based HFUS elastography which is promising for the better diagnosis of skin diseases.

Ultrasonic surface acoustic wave elastography: A review of basic theories, technical developments, and medical applications

“…The foundation of the crawling-wave-based elastography is to generate the slowly moving interference pattern from two SAWs propagating in opposite directions. 44,45 This technique assumes the linear interference of the surface waves. Because the crawling wave speed is related to its source surface wave speed, which is determined by the wave propagation medium's mechanical properties, it can be used for elastography purposes.…”

“…As shown in Figure 9, to generate a crawling wave on a tissue surface, a signal generator or wave generator (Tektronix AFG3252) produces two separate signals with a minimal frequency difference (0.3-0.5 Hz). 44,45 Then, each signal is passed to a digital amplifier (Type 2718, Bruel and Kjaer). The amplified signals are sent to the two different vibration sources or mini-shakers (Type 4810, Bruel and Kjaer).…”

“…It is a technique where surface wave speed is calculated from a dynamic wave interference of two surface waves propagating in opposite directions. 44,45 Recently, the Scholte-wave-based SAW elastography technique was proposed to generate SAWs at a liquid-tissue interface of target tissues and perform a comprehensive elastic characterization of both surficial tissue and deep tissue simultaneously. 46 As an emerging imaging method, ultrasonic SAW elastography is in its critical stage of development: several techniques have been proposed and validated, further development is in process, related clinical studies have just started, and potential medical applications are awaiting exploration.…”

Viscoelastic Characterization of in vivo Human Dermis Using High-Frequency Ultrasonic Crawling Wave Elastography

“…However, skin tissue has a semi-infinite boundary condition. Therefore, surface acoustic waves guided by the skin surface are the most dominant perturbation and, assuming the incompressibility condition of the tissue, it propagates at the speed of [16] 𝑐 𝑠𝑤 (𝜔) ≈ 0.95 ⋅ 𝑐 𝑠 (𝜔)…”

“…where c sw (ω) is the surface wave speed. The calculation of c sw (ω) from the crawling wave interference pattern was conducted using a phase-based estimator proposed in [16]. Considering Δk<<k (which occurs when Δω<<ω), the spatial phase extracted from the oscillatory term of (1) reads…”