Acoustic radiation force-based elasticity imaging methods

Abstract

Conventional diagnostic ultrasound images portray differences in the acoustic properties of soft tissues, whereas ultrasound-based elasticity images portray differences in the elastic properties of soft tissues (i.e. stiffness, viscosity). The benefit of elasticity imaging lies in the fact that many soft tissues can share similar ultrasonic echogenicities, but may have different mechanical properties that can be used to clearly visualize normal anatomy and delineate pathological lesions. Acoustic radiation force-based elasticity imaging methods use acoustic radiation force to transiently deform soft tissues, and the dynamic displacement response of those tissues is measured ultrasonically and is used to estimate the tissue's mechanical properties. Both qualitative images and quantitative elasticity metrics can be reconstructed from these measured data, providing complimentary information to both diagnose and longitudinally monitor disease progression. Recently, acoustic radiation force-based elasticity imaging techniques have moved from the laboratory to the clinical setting, where clinicians are beginning to characterize tissue stiffness as a diagnostic metric, and commercial implementations of radiation force-based ultrasonic elasticity imaging are beginning to appear on the commercial market. This article provides an overview of acoustic radiation force-based elasticity imaging, including a review of the relevant soft tissue material properties, a review of radiation force-based methods that have been proposed for elasticity imaging, and a discussion of current research and commercial realizations of radiation force based-elasticity imaging technologies.

Keywords: acoustic radiation force; elasticity; shear wave; stiffness; ultrasound.

Figure 1.

Isocontours of acoustic radiation force distribution from a focused linear array in media with two different acoustic attenuation coefficients (0.7 and 2.0 dB cm⁻¹ MHz⁻¹). Red indicates peak radiation force magnitudes with an overall lower peak magnitude in the more attenuating medium. Notice that the peak of the radiation force magnitude occurs at the focus for the lower attenuating material (0.7 dB cm⁻¹ MHz⁻¹), while in the more attenuating medium (2.0 dB cm⁻¹ MHz⁻¹) the radiation force is distributed more evenly throughout the near field, without a strong focal point gain.

Figure 2.

Examples of shear wave propagation represented as isocontours of displacement at different times after impulsive (i.e. <1ms) ARFI excitation in a three-dimensional finite-element simulation of a purely elastic medium with a Young's modulus of 4 kPa and an acoustic attenuation coefficient of 0.7 dB cm⁻¹ MHz⁻¹. The 0 ms isocontour image portrays the radiation force region of excitation (ROE), and the central axis of this displacement profile is the location used to generate qualitative ARFI images as shown in figure 3. The plot in the upper right shows the displacement-through-time profiles at the axial focal depth of the radiation force excitation at three different lateral positions (indicated by the arrows in the isocontour images). Blue, 0 mm; red, 1.5 mm; green, 3 mm.

Figure 3.

The top row of images shows a metastatic melanoma mass in an otherwise healthy liver background. The mass appears as a hypoechoic region in the B-mode image (a); in the corresponding ARFI image (b), the malignant mass does not displace as much as the background liver tissue and can be interpreted to be stiffer than the liver tissue. This mass is also identified as a region of reduced opacity on the corresponding computed tomographic (CT) image, indicated with an arrow (c). The images in the bottom row show B-mode (d) and ARFI displacement (e) images of a hepatocellular carcinoma in a fibrotic liver. In the ARFI image, the mass appears more compliant (i.e. displaces more) than the stiffer, diseased liver tissue. The corresponding CT image for this hepatocellular carcinoma is shown in (f), with the lesion indicated with an arrow. The greyscale bars in the ARFI images represent displacement in micrometres. (Reproduced with permission from Physics in Medicine and Biology [32]).

Figure 4.

Liver stiffness, as characterized using transient radiation force excitations and shear wave speed quantification with the RANSAC method, as a function of biopsy-proven fibrosis stage in patients being evaluated for non-alcoholic fatty liver disease. Choosing a shear stiffness threshold of 4.24 kPa allowed F3–F4 fibrosis stages (advanced fibrosis and cirrhosis) to be distinguished from mild to no fibrosis (F0–F2) with 90% sensitivity and specificity (AUC = 0.90) [57].

Figure 5.

Characterization of a 5 mm grade III infiltrating ductal carcinoma using SuperSonic Imaging and an L74 linear imaging array (centre frequency 5 MHz). The B-mode image (a) shows a slightly hypoechoic region that is clearly delineated as stiff (red) in the corresponding shear wave velocimetry map (b). (Reproduced with permission from Ultrasound in Medicine and Biology [60]).

Figure 6.

Matched (a) ARFI and (b) SWEI images of a calibrated elasticity phantom with a 20 mm diameter stiff spherical inclusion. The images were generated from the same dataset, which was obtained with a 4 MHz abdominal imaging array, using parallel receiving beam-forming techniques to monitor the tissue response to each excitation throughout the entire field of view. A total of 88 excitation pulses were located at two focal depths (50 and 60 mm), with a beam spacing of 1 mm. The ARFI image portrays normalized displacement at 0.7 ms after each excitation, whereas the SWEI image portrays reconstructed shear wave speed. The lesion contrast is 0.37 and 0.71 for the ARFI and SWEI images, respectively, and the edge resolution (20–80%) is 1.2 mm (ARFI) and 5.0 mm (SWEI) in the plots from a depth of 50 mm, shown in the bottom row.