Shear wave elastography for breast masses is highly reproducible

Abstract

Objectives: To evaluate intra- and interobserver reproducibility of shear wave elastography (SWE) for breast masses.

Methods: For intraobserver reproducibility, each observer obtained three consecutive SWE images of 758 masses that were visible on ultrasound. 144 (19%) were malignant. Weighted kappa was used to assess the agreement of qualitative elastographic features; the reliability of quantitative measurements was assessed by intraclass correlation coefficients (ICC). For the interobserver reproducibility, a blinded observer reviewed images and agreement on features was determined.

Results: Mean age was 50 years; mean mass size was 13 mm. Qualitatively, SWE images were at least reasonably similar for 666/758 (87.9%). Intraclass correlation for SWE diameter, area and perimeter was almost perfect (ICC ≥ 0.94). Intraobserver reliability for maximum and mean elasticity was almost perfect (ICC = 0.84 and 0.87) and was substantial for the ratio of mass-to-fat elasticity (ICC = 0.77). Interobserver agreement was moderate for SWE homogeneity (κ = 0.57), substantial for qualitative colour assessment of maximum elasticity (κ = 0.66), fair for SWE shape (κ = 0.40), fair for B-mode mass margins (κ = 0.38), and moderate for B-mode mass shape (κ = 0.58), orientation (κ = 0.53) and BI-RADS assessment (κ = 0.59).

Conclusions: SWE is highly reproducible for assessing elastographic features of breast masses within and across observers. SWE interpretation is at least as consistent as that of BI-RADS ultrasound B-mode features.

Key points: • Shear wave ultrasound elastography can measure the stiffness of breast tissue • It provides a qualitatively and quantitatively interpretable colour-coded map of tissue stiffness • Intraobserver reproducibility of SWE is almost perfect while intraobserver reproducibility of SWE proved to be moderate to substantial • The most reproducible SWE features between observers were SWE image homogeneity and maximum elasticity.

Figures

Fig. 1

Three sequentially acquired Shearwave Elastography (SWE™) images of a breast cancer showing the small changes registered over time. The repeatability in this case was scored as “all images very similar”. The upper portion of each image shows the tissue elasticity as a colour overlay where red represents the highest stiffness and blue represents the lowest stiffness. The lower image shows the registered B-mode image

Fig. 2

A 52-year-old woman with invasive ductal carcinoma. Shear wave elastography shows high stiffness values around the lesion (red tints in the upper frame). One ROI is in the stiffest part of the lesion (arrow) and another is in the adjacent subcutaneous fat (arrowhead). The quantitative values for the two ROIs are shown in kPa in the panel on the left: the lesion value is given first (here, mean 6.3 m/s), followed by the value in fat (here 1.2 m/s). This was available on the prototype RUBI system. The commercially available system, Aixplorer, can display the elasticity scale in kPa or m/s

Fig. 3

Colour scale used by the blinded observer. The SWE scale corresponds to that in the colour bar of the RUBI image shown in Fig. 2 but it has been divided into bands as indicated. The blinded reader chose the range corresponding to the stiffest portion of the mass and/or surrounding tissue on the colour overlay. 0 kPa = 0 m/s, 0-36 kPa = 0-3.5 m/s, 36-72 kPa = 3.5-4.9 m/s, 72-108 kPa = 4.9-6.0 m/s, 108-144 kPa = 6.0-6.9 m/s, 144-180 kPa =6.9-7.7 m/s