# Intra-System Reliability Assessment of 2-Dimensional Shear Wave Elastography
## 4 Discussion
Geometric errors resulting from focusing the acoustic force pulse at different depths and a bias caused by the lateral range of the tracking beams have been elegantly described in more technical papers [43]. Transducer shape also may contribute to this bias, with a larger bias detected using curvilinear shaped probes, like the one used in this study, compared to a linear shaped transducer [43]. Random errors reducing measurement precision caused by the attenuation of the shear wave tracking pulses have also been implicated in these variations [9]. Increasing depth results in increased signal jitter and noise causing an underestimation of the measured SWV due to averaging [44]. These types of random errors have also been reported to be more prevalent when imaging stiffer targets, with experiments showing that TOF uncertainty increases with increasing stiffness, resulting in less precise repeated measurements [42]. Our experiment confirms this is still applicable to a commercial scanner, demonstrating greater variability of repeated measurements of stiffer versus softer targets. Therefore, we recommend guidelines involving 2D-SWE should also consider the relative stiffness of the structure being assessed, and not simply its depth, and indicate higher reliability of SWV measurements can be achieved when imaging softer structures in the range of 1.0-2.0 m/s. This range would be most relevant to clinical investigations involving the liver [2] and placenta [8]. As observed in similar studies, the inter-observer agreement was high between all operators in the study [11; 12; 45; 46]. This result is consistent with other studies demonstrating high inter-observer agreement of 2D-SWE on phantom studies [12] and clinical studies [11]. The experience of the operator is likely to play a greater role in the clinical setting. In practical terms, the elastography phantom provides a convenient platform to scan and it proved relatively easy to identify each target and acquire repeated measurements even for someone new to the technique. In the clinical environment, however, achieving standardised imaging planes due to the variability of human anatomy is likely to be more difficult. Measurement repeatability is an important consideration for any quantitative measurement system [16]. Repeated measurement calculations in this study were comparable to reports in other studies albeit on different systems [12; 13; 39]. This study achieved very low RC and CV values when imaging the 10 kPa and 17 kPa targets, indicating excellent repeatability. However, increasing target stiffness is a significant source of measurement variability, as discussed above. Such a range of values would likely make it difficult to interpret the results and to detect differences between normal and abnormal tissues when imaging tissues that produce high SWV values. An analysis of the mean SWV between the sample ROI size of 5 mm and 10 mm demonstrated overall good reliability, with only the 85 kPa target demonstrating a significant difference between means. Greater variability and subsequent averaging of a larger ROI explains why there are lower values for the 10 mm ROI compared to the 5 mm ROI. Published guidelines relating to liver elastography promote the use of a 10 mm or larger ROI [2; 21], however, this may not be practical for smaller structures or organs in which there is a larger variability in the B-mode characteristics. One example could be the placenta, which is a large organ, but often develops in various configurations, such as variable thickness and may display areas of grey-scale heterogeneity such as placental lakes [47; 48; 49]. Therefore, to evaluate such an organ with ultrasound elastography a larger number of smaller ROIs may be required to provide a better assessment of its overall elastic properties. There are several limitations to this study. It is important to recognise that this study aimed to investigate which factors contribute most to intra-system variability.