## 4 Discussion  
In this paper, we presented \(SR\) measurement in a tissue-mimicking phantom and in resected tissue, where the stress was applied by pushing gently with the probe itself, focusing on the measurement variability. We also presented data from EUS elastography of focal pancreatic lesions where arterial pulsations, particularly from the aorta, acted as an internal stress source. We showed that the IQR increased with application to more complex anatomical structures and inability to control the stress source to a level that surpasses the median \(SR\) value. The mean value of the IQR/median increased 8.6 times from the phantom (0.07) to resected tissue (0.62) and 28.3 times with endosonographic application (2.04). When applying strain elastography in an endoscopic application, the strain is dependent on endogenic stress sources, such as aortic pulsation and respiratory movements. These sources may vary considerably between patients and can hardly be standardized. Also, the availability of stable reference tissue that should be subject to similar stress as the lesion, may be hard to find in this application. Moreover, further differences may be caused by variable levels of pre-compression and applied stress caused by the endoscope. Since soft tissues have non-linear elastic properties, the tissue will appear harder with more applied stress or increased pre-compression [5; 16]. An endoscopic application is also challenging because the probe is inserted into the gastrointestinal cavity and cannot be controlled directly by the observer's hands, as is the case with external strain imaging ultrasonography. [PERSON] et al. performed a meta-analysis of EUS elastography in pancreatic lesions evaluated qualitatively with a visual score, strain histograms, \(SR\), contrast enhanced EUS, and EUS FNA. They identified a large range in \(SR\) cut-off values based on previous published cut-off values or the ROC curves (3.05 to 24.82). This caused large heterogeneity for the specificity, and three-eighths of the studies were identified as outliers. After removing the outliers, the evaluation for identification of malignant lesions based on qualitative visual scores and strain histograms outperformed the \(SR\)-based evaluation (sensitivity: 0.94, specificity: 0.54, diagnostic odds ratio (OR): 29.42 [17]. However, strain elastography and \(SR\)-based assessment of rectal tumors using a dedicated radial rectal probe, which allows induction of strain by rapid water-inflation of a balloon around the US probe, was used to improve patient selection for organ-sparing treatment compared to standard multidisciplinary assessment [18]. In our experience, the variability in \(SR\) measurements increases when we transitioned from application with good access to relevant reference tissue, which allows for hand-eye coordination of inflicted stress, made possible by the real-time strain-feedback to the examiner on the screen. Selection of the reference area for comparison with a focal lesion also introduces variability that may influence the resulting \(SR\) more than the strain variability of the lesion itself, as illustrated in Figure 5. To avoid the variability induced by the reference area, some researchers recorded the strain values in the lesion of interest using the Strain Histogram (SH) function and found that it performed equally as well as the \(SR\) in some applications. However, this is dependent on a near standardized application of stress to the lesions of interest. The SH function displays the distribution of the different colors representing different strain intervals in a 256 scale, from no strain (0) to maximum strain (256), as set by the scanner software. From a histogram, quantifying the distribution of recorded strains it is also possible to evaluate if the lesion and reference tissue is homogeneous or heterogeneous by using kurtosis and range. [PERSON] et al.