Ex Vivo Evaluation of Mechanical Anisotropic Tissues with High-Frequency Ultrasound Shear Wave Elastography

Abstract

The use of imaging devices to assess directional mechanics of tissues is highly desirable. This is because the directional mechanics depend on fiber orientation, and altered directional mechanics are closely related to the pathological status of tissues. However, measuring directional mechanics in tissues with high-stiffness is challenging due to the difficulty of generating localized displacement in these tissues using acoustic radiation force, a general method for generating displacement in ultrasound-based elastography. In addition, common ultrasound probes do not provide rotational function, which makes the measurement of directional mechanics inaccurate and unreliable. Therefore, we developed a high-frequency ultrasound mechanical wave elastography system that can accommodate a wide range of tissue stiffness and is also equipped with a motorized rotation stage for precise imaging of directional mechanics. A mechanical shaker was applied to the elastography system to measure tissues with high-stiffness. Phantom and ex vivo experiments were performed. In the phantom experiments, the lateral and axial resolution of the system were determined to be 144 μm and 168 μm, respectively. In the ex vivo experiments, we used swine heart and cartilage, both of which are considered stiff. The elastography system allows us to acquire the directional mechanics with high angular resolution in the heart and cartilage. The results demonstrate that the developed elastography system is capable of imaging a wide range of tissues and has high angular resolution. Therefore, this system might be useful for the diagnostics of mechanically anisotropic tissues via ex vivo tests.

Keywords: cartilage experiment; elastography; heart experiment; high-frequency ultrasound; mechanical anisotropy; mechanical wave imaging.

Figure 1

Experimental setup for the mechanical wave imaging system.

Figure 2

Characteristics of the 38 MHz lithium niobate ultrasound transducer at the focus of ultrasound beam. (a) Photographic image of the transducer, (b) pulse-echo characteristics of the transducer, (c) lateral beam profile, and (d) axial beam profile.

Figure 3

The results of mechanical wave elastography in homogeneous phantom. (a) Wave speed for various excitation frequencies and (b) frequency spectrum on excitation frequency of 750 Hz.

Figure 4

B-mode image and its corresponding wave speed image for heterogeneous phantoms. The red dotted line indicates the boundary between two phantoms. (a) B-mode image: side by side phantom, (b) elastography: side by side phantom, (c) B-mode image: layered phantom, and (d) elastography: layered phantom.

Figure 5

Axial and lateral resolution determination for mechanical wave elastography. (a) Lateral direction and (b) axial direction.

Figure 6

Heart image: directional dependence of the heart at the middle myocardium. The slope of the red line indicates the wave speed. (a) Data acquisition, (b) displacement map: circumferential direction, (c) displacement map: longitudinal direction, (d) heart image: 0 mm depth, and (e) heart image: 360${}^{\circ }$ (A depth of 0 mm does not mean the surface, but the point at which the ultrasound transducer started measuring inside of the tissue).

Figure 7

Boxplot for analyzing the wave speed depending on both location and layer. Red line: median, Upper black line: maximum, Lower black line: minimum, Top of the blue box: first quartile (25 %), Bottom of the blue box: third quartile (75 %), Red cross mark: outliers. (a) Epicardium, (b) Myocardium, and (c) Endocardium (L: longitudinal, C: circumferential).

Figure 7

Boxplot for analyzing the wave speed depending on both location and layer. Red line: median, Upper black line: maximum, Lower black line: minimum, Top of the blue box: first quartile (25 %), Bottom of the blue box: third quartile (75 %), Red cross mark: outliers. (a) Epicardium, (b) Myocardium, and (c) Endocardium (L: longitudinal, C: circumferential).

Figure 8

Cartilage image: directional dependence of the cartilage. The slope of the red line indicates the wave speed. (a) Cartilage image, (b) displacement map: circumferential direction, (c) displacement map: longitudinal direction, (d) cartilage image1: 0 mm depth, (e) cartilage image2: 0 mm depth, (f) cartilage image1: 180${}^{\circ }$, and (g) cartilage image2: 180${}^{\circ }$ (A depth of 0 mm does not mean the surface, but the point at which the ultrasound transducer started measuring inside of the tissue).