# Shear Wave Elastography Implementation on a Portable Research Ultrasound System: Initial Results
## 4 Discussion
### Phantom Experiments  
In this study, two sets of phantom experiments were performed to evaluate the system performance and compare the three different push beam generation methods. In the homogeneous part of the study, all of the techniques performed similarly, even though there were significant differences between the methods, especially in terms of the energy that was transmitted by a single shear wave source. A closer look at the acquired SWS maps exposed variations in the SWS estimates, depending on the pixel position. FCUSE showed increased SWS values near the center, while these were near the sides of the FOV during the FCUSE-SSI method. The SWS estimation accuracy could depend on the local shear wave energy, as stronger displacement signals could correlate better. Because the total energy that was utilized by each technique was the same, the shear wave energy distribution, as defined by Equation (8), resulted in a different energy distribution in each technique (Figure 16). The regions of reduced energy on the push beam axis or at depths below 30 mm could be clearly seen, which was possibly the result of diffraction and attenuation. The local energy was especially high in the regions that were covered by shear waves from multiple sources. During the FCUSE method, the energy was distributed close to the focal points, which were positioned shallower than 30 mm (as in the transmit plan). This was the result of the increased \(F_{\#}\) value of 2.4. In the SSI and FCUSE-SSI techniques, the energy was distributed over a large part of the FOV, although the energy levels were not as high as those in the FCUSE method. Figure 16: The shear wave energy distributions within the FOVs for the various beam generation methods: (**a**) FCUSE; (**b**) SSI; (**c**) FCUSE-SSI. Note the different color maps for each image. The reconstructed SWS maps and their quality metrics confirmed that all three methods exhibited similar performances and that none of them were generally superior over the others. This was not unexpected, since the total electrical energy that was used for the push excitation was the same in all cases. The differences between the techniques could, in most cases, be explained by the differences between the shear wave energy distributions within the ROIs. Due to the lack of consistency and small differences between the quality metrics of the analyzed methods, only a simplified comparison could be conducted in this study. For inclusion SNR, the FCUSE method performed favourably compared to the other methods, showing the best results in seven out of nine measurements; however, it suffered from poor background SNR. The SSI method outperformed the other techniques in the homogeneous experiments and with regard to background bias in the heterogeneous experiments, for which it had the lowest bias in eight out of nine measurements. Finally, the FCUSE-SSI method performed well in terms of background SNR and CNR but suffered from poor inclusion SNR. It must also be noted that the comparison was not completely fair as the SSI method was the only technique that used three acquisitions to reconstruct a single SWS map, while the other techniques only used one. The push excitation energy was the same, but the SSI method required three tracking sequences instead of one and, as a result, produced a dataset for processing that was three times larger. Overall, the techniques appeared to be complimentary and could be used in conjunction with a series of acquisitions to achieve a synergy effect. One striking result was the bias that was observed in all of the estimates. The pixel-wise SWS estimate distribution for each homogeneous phantom image (Figure 17) confirmed that in each technique, almost all pixels had values that were biased higher than the nominal value. The bias observed that was in the heterogeneous phantom experiments was even higher, ranging from \(-30.4\%\) (SSI; \(6.5\,\mathrm{mm}\) type III inclusion) to \(+23.8\%\) (FCUSE-SSI; \(6.5\,\mathrm{mm}\) type II inclusion).