A Scholte wave approach for ultrasonic surface acoustic wave elastography

Abstract

Background: Pathological changes in tissues are often related to changes in tissue mechanical properties, making elastography an important tool for medical applications. Among the existing elastography methods, ultrasound elastography is of great interest due to the inherent advantages of ultrasound imaging technology, such as low cost, portability, safety, and wide availability. Although ultrasonic shear wave elastography, as a platform technology, can potentially quantify the elasticity of tissue at any depth, its current implementation cannot assess superficial tissue but can only image deep tissue.

Purpose: To address this challenge, we proposed an ultrasonic Scholte-wave-based approach for imaging the elasticity of superficial tissue.

Methods: The feasibility of the proposed technique was tested using a gelatin phantom with a cylindrical inclusion. To generate Scholte wave in the superficial region of the phantom, we proposed a new experimental configuration in which a liquid layer was introduced between an ultrasound imaging transducer and the tissue-mimicking phantom. We utilized an acoustic radiation force impulse to excite the tissue-mimicking phantom, analyzed the properties of the generated Scholte waves, and applied the waves for elasticity imaging.

Results: In this study, we first reported the observation that Scholte (surface) waves and shear (bulk) waves were simultaneously generated, and they propagated in the superficial and deeper regions of the phantom, respectively. Then, we presented some important properties of the generated Scholte waves. For a 5w/v% gelatin phantom, the generated Scholte waves have a speed of around 0.9 m/s, a frequency of about 186 Hz, and thus a wavelength of about 4.8 mm. The speed ratio between the simultaneously generated Scholte wave and shear wave is about 0.717, which is 15% lower than the theoretical expectation. And we further demonstrated the feasibility of Scholte wave as a mechanism for imaging superficial tissue elasticity. Together with the simultaneously generated shear wave, the Scholte wave was shown to be able to quantitatively image both the background and the cylindrical inclusion (4 mm in diameter) of the tissue-mimicking gelatin phantom.

Conclusions: This work shows that the elasticity of superficial tissue can be evaluated by utilizing the generated Scholte wave alone, and it also shows that a comprehensive elasticity imaging of the tissue extending from the superficial to deep regions can be achieved by combining the proposed Scholte wave technique and the conventional shear wave technique.

Keywords: Scholte wave; acoustic radiation force impulse; elastography; shear wave; shear wave elasticity imaging; surface acoustic wave.

FIGURE 1

Comparison of two elastography configurations for wave generation. (a) The configuration used in the conventional shear wave elastography. (b) The configuration proposed for elastography based on Scholte (surface) wave, in which a layer of liquid is introduced between the ultrasound transducer and the target tissue (gelatin phantom here). In both cases, acoustic radiation force was applied along the center line of the phantom, as shown by the red arrow. The two regions marked by dashed rectangles are chosen to demonstrate the generated waves.

FIGURE 2

A typical axial displacement profile of the generated Scholte wave for demonstrating the definition of the depth penetration of the Scholte wave in the solid medium.

FIGURE 3

The proposed method for converting Scholte wave speed to its corresponding shear wave speed. The Scholte wave region, shear wave region, and transition region in an axial speed profile are distinguished first. The wave speeds in the Scholte wave region and transition region are then converted to their shear wave counterparts by multiplying its conversion coefficient.

FIGURE 4

Snapshots of the waves generated in the regions marked in Figure 1 at 1, 6, and 11 ms after the acoustic radiation force excitation. (a)-(c) correspond to the shear wave elastography in Figure 1(a). (d)-(f) correspond to the proposed Scholte-wave-based method in Figure 1(b).

FIGURE 5

B-mode images of the homogeneous phantom and the snapshots of the waves generated inside them at 6 ms after the acoustic radiation force excitation. (a)-(c) correspond to the cases with a water layer thickness of 0.4, 1.1, and 4.3 mm, respectively.

FIGURE 6

Comparison of the normalized Scholte wave speeds in the gelation phantom using ultrasound gel as a coupling medium with those using water as the coupling medium. The Scholte wave speeds were normalized with respect to the shear wave speed measured in the same experiments.

FIGURE 7

The intensity field of the acoustic beam for exciting Scholte waves with different thicknesses of the coupling water layer.

FIGURE 8

Comparison of the axial displacement profiles of the Scholte waves generated with different thicknesses of coupling water layer at 6 ms after the acoustic radiation force excitation. All these axial displacement profiles were obtained from the same lateral location as indicated by the axial dashed lines in the right sub-figure of Figure 5(c).

FIGURE 9

The maximum amplitudes of the axial displacement profiles in Figure 8 show the effect of the water layer thickness on the energy level of the generated Scholte waves.

FIGURE 10

The penetration depths of the generated Scholte waves with different water layer thicknesses.

FIGURE 11

(a) A typical waveform of the generated Scholte wave in the superficial position of the phantom. (b) The spectrum of the waveform in (a).

FIGURE 12

Representative lateral displacement profiles of the displacement map with a 2.1 mm coupling water layer captured 6 ms after the acoustic radiation force excitation. The lateral profiles obtained at 2.4, 3.4, and 4.4 mm away from the transducer surface show different cases in terms of the relative amplitudes of the Scholte wave and shear wave.

FIGURE 13

(a) B-mode image of the gelatin phantom with a cylindrical inclusion. The circle indicates the location of the inclusion; the two rectangles are the regions of interest (ROI) chosen for wave speed evaluation. (b) The wave speed map of the left ROI in (a). The dashed line shows the location of the axial speed profile used in Figure 5(a). (c) The wave speed map of the right ROI in (a). The upper and lower rectangles are the ROIs chosen in the Scholte wave region and shear wave region, respectively, for statistical analysis. (d)-(e) Shear wave speed maps converted from (b)-(c). (f)-(g) Shear modulus maps computed from the speed maps in (d)-(e). In (b), (d), and (f), the location of the inclusion was also marked the dashed circle as in (a).