## 3 Results  
Figure 7 shows the vertical and horizontal strain results at peak systolic phase (\(t=0.3\) s) for six distinct elevational slices for the three different methods. The reference results based on the axial strain as obtained with probe A and the axial strain obtained with probe B are presented in the left column. The compounding results and zero-degree single probe results are shown in the center and right column, respectively. As indicated by the axial probe direction arrows and described in the methods section, the reference results originate from displacement estimations using solely information along the ultrasound beam direction for both probes. Clear resemblance is shown between the reference strains and the compounded strains, although the strains obtained by the compounding method appear noisier in some regions. This in contrast to the zero-degree lateral component, which depicts  
Figure 6: Flowchart indicating the in-line processes performed from the acquisitions to strain estimation to compare the single-probe compounding versus the single-probe zero-degree performance in terms of minimal (compressive) and maximum (tensile) 3D principal strains. Please note that this flowchart describes the situation with respect to probe A. A similar flowchart with respect to probe B was also applied and can be obtained by interchanging A and B.  
a highly non-uniform strain distribution with many strain clipping regions delivering unreliable lateral strain distributions. Figure 8 depicts the elevational strain distributions at peak systolic pressure, for the reference, compounding, and zero-degree method for both probes. All strain distributions show compressive and tensile strains along the longitudinal direction of the vessel wall at the same longitudinal positions. Strains of up to 10% are observed at the external carotid artery for both probe orientations. Intra-probe strain distributions indicate a high similarity between the different methods. Inter-probe comparison, however, reveals a rotation in the strain distribution orientation at the inlet side (communis) and the outlet side (external CA). Figure 9 shows the strain distribution over time for one cardiac cycle for the vertical, horizontal, and longitudinal direction for the reference, compounding and zero degree methods for probe A and B. For the vertical strains of probe A and the horizontal strain of probe B, all lines almost perfectly  
Figure 8: Longitudinal strain estimates originating from both probes at peak systolic pressure. Figure 7: Vertical and horizontal strain estimates of the bifurcation in 3D at maximum systolic pressure. Please note that for visualization purposes, only 6 distinctive transverse slices are depicted at a 3 times larger elevational scale. coincide. The 5 th and 95 th percentile horizontal strain of the zero-degree method show extreme strain values of \(<\)\(-\)15% and \(>\)20% at peak systolic pressure compared to the reference (\(--\)5%, \(\sim\)8%) strain, respectively. The compounding method outperforms the zero-degree method, which shows a high resemblance to the reference strain distribution for the complete cardiac cycle. For the longitudinal strain distribution, all three methods perform almost equally. Similar trends can be observed for probe B with respect to the reference of probe A. Table 3 lists the absolute median strain and the 5 th and 95 th percentile strain values for both probes to indicate the strain range at two different time points in the cardiac cycle. For the vertical component of probe A and the horizontal component of probe B, the percentile strains shows almost no difference at maximum systolic pressure for all methods. Please note that these strains originate from axial (Figure 5) displacement estimations for both probes. Also, small residual median strains were measured for these corresponding orientations.