There is another problem:
When I have smoothed the vessel and then want to create the hollow body, an opening (hole) is created at the tip of the aneurysm and the shape is greatly changed, that is, it flattens out. How can that be avoided?
You can represent vessel with finer resolution by cropping and oversampling the image before segmentation, using Crop volume module (use spacing scale of 0.5 or less).
Both the flattening and the whole are because there is no space in the segmentation’s binary labelmap for the growth of the aneurysm. The easiest is to leave some space around the vasculature before segmentation, when you crop the input volume. If you already segmented the image then you can adjust the segmentation’s extent by creating a ROI node that encloses the region you need and then use it as “Source geometry” in the “Segmentation geometry” window (that you can bring up by clicking the button next to the master volume selector in Segmentation module).