# Intra-System Reliability Assessment of 2-Dimensional Shear Wave Elastography
## 4 Discussion  
Inter-system variability is a known issue in ultrasound elastography research and may preclude the meaningful comparison of measurements performed on different systems, even when the same phantom [9; 18] or patient [19; 39] is imaged. As a result, many investigators would agree that standardisation of imaging protocols on a single system must first be done before it is applied clinically [8; 10; 21; 33; 40]. This work provides an important analysis of the sources of intra-system variability on a modern 2D-SWE commercial system. To the author's knowledge, this is the first work to investigate these factors on the popular Canon Aplio i-series (Canon Medical, Otawara-shi, Japan), and provides clinically relevant SWV ranges where the system performs reliably and highlights factors that can introduce variability. The influence of sample depth on elastography measurements has previously been reported [12; 15; 17; 18] suggesting a trend towards lower SWV values with increasing acquisition depth [15; 17]. As a result, recommendations have been made to include the depth of the target organ or lesion into any standardized protocol for SWE [2]. The results \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Target** & **Depth** & \
\begin{tabular}{c} **Repeatability** \\ **Coefficient** \\ \end{tabular} &
\begin{tabular}{c} **Coefficient of** \\ **Variation** \\ \end{tabular} \\ \hline Type I (10 kPa) & 3 cm & 0.06–0.07 m/s & 1.5–1.8\% \\  & 6 cm & 0.21–0.24 m/s & 5.5–5.7\% \\ \hline Type II (17 kPa) & 3 cm & 0.07–0.08 m/s & 1.3–1.4\% \\  & 6 cm & 0.21–0.22 m/s & 3.9–4.0\% \\ \hline Background (25 kPa) & 3 cm & 0.31–0.55 m/s & 4.1–6.5\% \\  & 6 cm & 0.67–0.81 m/s & 5.5–10.4\% \\ \hline Type III (47 kPa) & 3 cm & 0.53–0.68 m/s & 5.3–7.3\% \\  & 6 cm & 0.80–0.86 m/s & 9.2–11.0\% \\ \hline Type IV (85 kPa) & 3 cm & 1.23–1.93 m/s & 10.2–15.2\% \\  & 6 cm & 0.82–1.13 m/s & 7.8–11.7\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary table of calculated repeatability coefficient (RC) and coefficient of variation (CV) of the different elasticity targets at two different depths. The RC and CV are displayed as min-max range. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Target** & **5 mm ROI** & **10 mm ROI** & **Significance** \\ \hline Type I (10 kPa) & 1.41 \(\pm\) 0.04 m/s & 1.41 \(\pm\) 0.04 m/s & \(p\) = 0.266 \\ \hline Type II (17 kPa) & 1.96 \(\pm\) 0.06 m/s & 1.95 \(\pm\) 0.04 m/s & \(p\) = 0.396 \\ \hline Background (25 kPa) & 2.37 \(\pm\) 0.08 m/s & 2.3 \(\pm\) 0.2 m/s & \(p\) = 0.105 \\ \hline Type III (47 kPa) & 3.2 \(\pm\) 0.4 m/s & 3.2 \(\pm\) 0.4 m/s & \(p\) = 0.776 \\ \hline Type IV (85 kPa) & 4.3 \(\pm\) 0.5 m/s & 3.9 \(\pm\) 0.6 m/s & \(p\) < 0.0001 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mean SWV and SD of each target combining both depths (3 cm and 6 cm) comparing two different sampling sizes 5 mm vs. 10 mm round ROI. in this study concur with previous data and support this recommendation, with many of the phantom targets displaying lower mean SWV at 6 cm compared to 3 cm. However, it is important to recognise that there is a greater underestimation of SWV with increasing target stiffness. Further, our study was able to demonstrate good agreement between the two softer targets (10 kPa and 17 kPa) reporting no significant difference between the mean SWV derived from the two depths. Imaging depth results in a systematic bias and a subsequent underestimate of SWVs with increasing depth, and the literature provides several explanations for this discrepancy. SWV is viscosity and frequency dependant [41], and measurement errors may be related to tissue viscoelasticity. In our study, using a phantom with low viscosity the cause of depth dependant bias is not likely due to viscosity dispersion. SWV is calculated via a time-of-flight (TOF) estimation using several tracking beams to measure the arrival time of the shear wave at locations away from the central axis of excitation [42].