# Comparison of Manual versus Semi-Automatic Segmentations of the Stenotic Carotid Artery Bifurcation
## 2 Materials and Methods
### Evaluation  
The reason behind using the calculated diameter instead of the area of the cross-sections for evaluation is that the diameter can be normalized by a unit of the voxel length. The first step was a qualitative analysis, which helps visualize the diameter difference curve normalized by the voxel difference (DDV curves) of all patients in each subgroup (in Table 2). The second step was the quantitative analysis of the geometrical parameters obtained by the semi-automatic and the manual methods in each patient group. Pearson correlation coefficients were calculated for the minimum diameter D\({}_{\text{min}}\) in the respective groups to compare the values of the semi-automatic and manual segmentations. Evaluation of D\({}_{\text{min}}\) was not applicable if the excluded external carotid artery was included in the most stenotic segment (CS-050, CS-004, CS-025, CS-029). In Table 3, the group-averaged D\({}_{\text{min}}\) values are presented. Correlations with the calcification metrics were also calculated. Here, patients with a calcification score of 0 (non-calcified soft plaque) were excluded from the calcification score correlation analysis as the calcification correlation evaluation was not applicable if the calcification score was 0 (CS-010, CS-031, CS-036). Here we want to remark  
Figure 2: Steps of data preparation. (**a**) segmentation with both techniques (top VascuCap-semi-automatic segmentation by medical examiner, bottom Slicer—manual segmentation by non-medical examiner), (**b**) cross-section extraction and (**c**) diameter difference calculation along the centerline. The diameters are given in mm on the diagram. The dark blue line shows the lumen diameter segmented by Slicer, the dark red line shows the lumen diameter segmented by VascuCap and the green line shows the difference. ICA: internal carotid artery, CCA: common carotid artery.  
that in the calculation of the average voxel differences, small segments, where the external carotid artery cut-out (at the bifurcation point) in VascuCap introduced a non-applicable difference were not taken into account. In the third step, the effect of the calcification severity on the difference in each segmentation group with the two methods was correlated. Table 4 summarizes the effect of calcification on the variance of segmentation between examiners, methods and homogeneous patient groups. Significance was calculated with paired Student t-test and the significance level was set at 5%.  
CFD simulations were performed on two patients to demonstrate how subtle differences in geometry can affect the hemodynamic flow field. Each patient was simulated with both geometries generated by the semi-automatic and manual segmentation. In Figure 5 the locations of the data extraction are visualized in the middle, while on the right, diagrams of velocity profiles along a normalised diameter length are gathered at three locations along the centerline (P1, P2, P3). Systolic and diastolic time instants were defined as the highest and lowest velocities, respectively. Care has to be taken here to emphasize the non-dimensionalization of the diameter. We wanted to project these velocity profiles onto the aforementioned reference to capture whether the velocity profiles are scale-dependent, or not.