## 3 Results
### Difference in \(c_{s}\) Estimates between Machines In Vivo  
A total of 128 elastograms were acquired, 64 for each machine. Ultrasound imaging revealed that one volunteer had small dense breasts comprising of mainly parenchymal tissue, this volunteer had a maximum imaging depth (skin to pectoral muscles) of approximately 2 cm. The other three volunteers had breasts of various sizes with differing amounts of adipose and parenchymal tissues, the maximum imaging depths in these volunteers were greater than 4 cm. Artefacts of ill-defined vertical bands of higher \(c_{s}\) than the average, appearing at approximately regularly spaced lateral intervals (as in Figure 3a, were observed in 100% of Aixplorer elastograms. In two Aixplorer images, artefacts covered the entire skin and adipose regions, and \(c_{s}\) estimates from regions without artefacts could not be obtained. In five Aixplorer elastograms, the elastogram-overlay box was incorrectly positioned below the outermost layer of the skin and \(c_{s}\) in the skin could not be obtained. A gel stand-off was clearly visualised on 96% of Aixplorer images and 100% of S2000 images. In two volunteers, the B-mode image did not clearly differentiate adipose tissue from the parenchyma. In these volunteers, \(c_{s}\) estimates were obtained in skin and parenchyma only. Figure 4a illustrates that shear-wave speed estimates were significantly greater when measured using Aixplorer (avoiding artefacts) compared to S2000 in all tissue types (\(p\)-values \(<\) 0.001). Symmetric percentage differences in median \(c_{s}\) estimates between machines were 40.0%, 50.0% and 51.4% for skin, adipose and parenchyma tissues, respectively. The variance of Aixplorer mean \(c_{s}\) estimates was also statistically significantly greater than the variance of S2000 estimates (\(p\)-values \(<\) 0.001) in all tissues. The median and IQR of the mean \(c_{s}\) estimates were significantly less when measured excluding artefacts in the skin and adipose (\(p\)-values \(<\) 0.001) and gave better agreement with S2000 \(c_{s}\) estimates. All subsequent Aixplorer data presented were determined using \(c_{s}\) estimates measured avoiding artefacts. Pearson correlation coefficients between \(c_{s}\) estimates measured using individual elastograms acquired using S2000 and Aixplorer in the skin, adipose and parenchyma were 0.25, 0.15 and 0.33 and were not statistically significant (\(p\) > 0.05). When \(c_{s}\) estimates were averaged over the whole breast (eight elastograms) Pearson correlation coefficients were 0.16 (\(p\) = 0.5), 0.75 (\(p\) = 0.057) and 0.75 (\(p\) \(<<\) 0.01), respectively. All data are tabulated in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Background** & **Inclusion 1** & **Inclusion 2** & **Inclusion 3** & **Inclusion 4** & **Mean (SD)** \\ \hline  & & S2000: \(c_{s}\) estimates (ms\({}^{-1}\)) & & & & \\ \hline Observer 1 & 2.55 & 1.78 & 2.50 & 3.51 & 4.17 & \\ Observer 2 & 2.47 & 1.89 & 2.40 & 3.40 & 4.11 & \\ Observer 3 & 2.55 & 1.90 & 2.49 & 3.53 & 4.11 & \\ Mean & 2.50 & 1.90 & 2.50 & 3.50 & 4.10 & \\ CV & 0.02 & 0.04 & 0.02 & 0.02 & 0.01 & \\ \hline  & & & &Figure 4: Showing: (**a**) Shear-wave speed (\(c_{\mathrm{s}}\)) estimates for skin, adipose and parenchyma using Aixplorer (excluding artefacts) and S2000, and; (**b**) \(c_{\mathrm{s}}\) estimates for skin, adipose and parenchyma measured using Aixplorer including and excluding artefacts. Median is illustrated by the central horizontal bar; the horizontal box edges show the IQR and the whiskers (dashed lines) show the range. Range is defined by adjacent values, which are the 25 th or 75 th quartile plus or minus the IQR multiplied by 1.5. Opposing triangles illustrate the 95% confidence interval on the median \(c_{\mathrm{s}}\) and ‘+’ indicates an outlier (any value outside the adjacent values). *** indicates where machine \(c_{\mathrm{s}}\) estimates were significantly different with \(p\)-value < 0.001.