## 4 Discussion  
This study assessed the reliability, variability of measurements and validity of semi-quantitative elastography SR calculation under optimal conditions (controlling the 90\({}^{\circ}\) transducer position and pressure by using as reference a cylindrical mimicking vessel) considering the experience level of the examiners. In general, reliability, variability of measurements and validity of semi-quantitative elastography SR calculation were acceptable. Elastography could be one of the most important technological breakthroughs in the field of ultrasound imaging (since development of Doppler imaging or Panoramic US) including the main advantages of US compared to other imaging techniques (e.g., low cost, short examination time, noninvasiveness and accessibility) [16]. After an extensive literature search, several recently published studies were found that assessed the reliability estimates for elastography in different soft tissues structures, Ref. [17] including nerves [18], muscles [19], tendons [20,21] and arteries [22]. Features were assessed with quantitative elastography (including transverse velocity, transverse stiffness, cranio-candal velocity and cranio-caudal stiffness, showing a poor repeatability of measurements with wide limits of agreement) and semi-quantitative elastography assessing SRs [8,9]. Previous phantom studies [8,11] reported that qualitative, semi-quantitative and quantitative data collected with strain and shear wave elastography can classify the targets as harder or softer than backgrounds properly. However, SRs were more accurate in shear wave elastography compared to strain. Aligned with this study, our results showed central areas of the images to be more accurate, reliable and stable than lateral areas for calculating SR. In addition, the influence of the examiner experience was controversial [11,13]. We found lower SR inter-examiner reliability and validity for novice examiners once the region of interest was focused on lateral areas. Two possible reasons explaining these phenomena could be the higher number of cross-sound waves in the middle of the transducer, yielding a greater accuracy in the greyscale and elastography quality and a more uniform pressure. There is also evidence supporting better SR accuracy and CVs (CV = 0.08-0.65 for SR ranging 1.57-2.47) when the stiffness difference between a target and a control point is big, whereas CVs are higher in small differences (CV = 1.22-1.7 for SR ranging 0.40-0.60) [8]. \begin{table}
\begin{tabular}{c c c c c} \hline  & **Examiner 1** & **Examiner 2** & **Examiner 3** & **Examiner 4** \\  & **(Experienced)** & **(Experienced)** & **(Novice)** & **(Novice)** \\  & **(n = 50 Images)** & **(n = 50 Images)** & **(n = 50 Images)** \\ \hline \multicolumn{5}{c}{_Lateral areas of the image_} \\ \hline SR obtained & 0.96 & 0.98 & 1.09 *.\({}^{\dagger}\) & 1.10 *.\({}^{\dagger}\) \\ Error & 0.03 \(\pm\) 0.14 & 0.02 \(\pm\) 0.17 & 0.09 \(\pm\) 0.17 & 0.10 \(\pm\) 0.15 \\ Absolute Error & 0.11 \(\pm\) 0.09 & 0.14 \(\pm\) 0.08 & 0.15 \(\pm\) 0.12 & 0.14 \(\pm\) 0.06 \\ Coefficient of Variation (\%) & 0.14 & 0.17 & 0.15 & 0.13 \\ Percent Error (\%) & 11.80 & 14.28 & 15.16 & 12.72 \\ Mean Error of Measurements & & 0.14 \(\pm\) 0.05 \({}^{\star}\) & & \\ \hline \multicolumn{5}{c}{_Central areas of the image_} \\ \hline SR obtained & 0.97 & 0.97 & 0.96 & 0.97 \\ Error & 0.02 \(\pm\) 0.05 & 0.03 \(\pm\) 0.06 & 0.03 \(\pm\) 0.06 & 0.04 \(\pm\) 0.06 \\ Absolute Error & 0.04 \(\pm\) 0.03 & 0.05 \(\pm\) 0.03 & 0.05 \(\pm\) 0.05 & 0.06 \(\pm\) 0.03 \\ Coefficient of Variation (\%) & 0.05 & 0.06 & 0.06 & 0.06 \\ Percent Error (\%) & 4.89 & 5.15 & 5.52 & 6.18 \\ Mean Error of Measurements & & 0.05 \(\pm\) 0.02 \({}^{\star}\) & & \\ \hline \multicolumn{5}{c}{\({}^{\star}\) Significant differences between lateral areas and central areas (\(p<0.001\)); \({}^{\ddagger}\) Significant differences with reference SR were found (\(p<0.001\)).} \\ \end{tabular}
\end{table}