Introduction to ultrasound elastography

Abstract

For centuries tissue palpation has been an important diagnostic tool. During palpation, tumors are felt as tissues harder than the surrounding tissues. The significance of palpation is related to the relationship between mechanical properties of different tissue lesions. The assessment of tissue stiffness through palpation is based on the fact that mechanical properties of tissues are changing as a result of various diseases. A higher tissue stiffness translates into a higher elasticity modulus. In the 90's, ultrasonography was extended by the option of examining the stiffness of tissue by estimating the difference in backscattering of ultrasound in compressed and non-compressed tissue. This modality is referred to as the static, compression elastography and is based on tracking the deformation of tissue subjected to the slowly varying compression through the recording of the backscattered echoes. The displacement is estimated using the methods of cross-correlation between consecutive ultrasonic lines of examined tissue, so calculating the degree of similarity of ultrasonic echoes acquired from tissue before and after the compression was applied. The next step in the development of ultrasound palpation was to apply the local remote tissue compression by using the acoustic radiation force generated through the special beam forming of the ultrasonic beam probing the tissue. The acoustic radiation force causes a slight deformation the tissue thereby forming a shear wave propagating in the tissue at different speeds dependent on the stiffness of the tissue. Shear wave elastography, carries great hopes in the field of quantitative imaging of tissue lesions. This article describes the physical basis of both elastographic methods: compression elastography and shear wave elastography.

Keywords: dynamic sonoelastography; elastography; static sonoelastography; ultrasonography.

Fig. 1

A change in the shape of material under external force F, strain ɛ = ΔL/L

Fig. 2

The principle of estimation compression-induced tissue strain. The time segments of echoes indicated with rectangles correspond (on the left edge) to echoes from before compression (A) and (on the right curve) to echoes after compression (B). The echoes in the upper rectangles are very similar (only deeper located), whereas the lower rectangles contain echoes that are significantly altered due to deformation. Unchanged echo signals come from stiff tissue areas, whereas changed echo signals come from soft tissue areas. Strain is determined based on comparison of pre- and post-compression echoes. The higher the similarity, and thus the correlation coefficient for the echoes, the stiffer the tissue. The position of maxima in the cross-correlation function is an estimate of echo time shift between both lines.

Fig. 3

Evaluation of a focal lesion in the breast based on static elastography using breathing motion. BIRADS category 4 lesion (Breast Imaging-Reporting and Data System) confirmed by histopathological findings as invasive ductal carcinoma G1. In the B-mode (left), the lesion appears as hypoechoic with irregular margin and shape, with the dominance of lateral-lateral size over the anterior-posterior size. An enhancement (increased brightness due to wave propagation through the tissue with reduced attenuating effects). In the elastogram (on the right), the entire lesion and the adjacent tissues are blue-coded (the blue encodes non-deformable tissues) – Tsukuba score 5. The deformable adipose tissue (an arrow) located in the preglandular area is coded in red, yellow and green.

Fig. 4

BIRADS category 5 lesion confirmed by postoperative histopathological findings as carcinoma mixtum (G2) in static elastography using breathing motion. In the B-mode (left) the lesion appears as hypoechoic with irregular, angular edges and irregular shape, with a hyperechoic rim around the lesion and with the dominance of lateral-lateral size over the anterior-posterior size. In the elastogram (on the right), the entire lesion is blue-coded (the blue encodes non-deformable tissues) – Tsukuba score 4.

Fig. 5

Radiation force generated by a long push pulse causes tissue dislocation and generates a spherical shear wave. The wave disappears with depth d (time t) as a result of attenuation (increasingly brighter wavefronts in the drawing)

Fig. 6

Shear wave generated by acoustic radiation force for subsequent push pulse locations. The inclination of the wavefront is proportional to the velocity of the shear wave, which depends on medium stiffness

Fig. 7

A solid-liquid lesion with microcalcifications, postoperatively classified as classic papillary carcinoma, is visible in the lower center of the left thyroid lobe in the longitudinal section. SWE elasticity image (the top image) shows three regions of interest (ROIs): „+” refers to the area comprising the largest portion of lesion, „x” refers to a 2-milimeter area from the stiffest part of lesion, „λ” refers to an area of the surrounding tissues with normal thyroid parenchyma. The maximum E value is 152.6 kPa in the ROI of the stiffest part of the lesion (coded in red – tissues with Young's modulus E > 80 kPa), and 20.9 kPa in the surrounding tissues.

Fig. 8

A solid hypoechoic lesion with microcalcifications (B-mode), classified as papillary carcinoma in the postoperative assessment, located in the upper pole of the right thyroid lobe. SWE elasticity image (the top image) shows maximum E values of 174.7 kPa in the 2 mm ROI in the stiffest part of the lesion and 54.8 kPa in the surrounding tissues.