Segmentation - not active tools ?

I’m sorry for this question, I’m completly novice.

Operating system: Windows 10
Slicer version: 4.8.1
Expected behavior: Segmentation:

I expected, that after choosing the certain area of similar CT-density in one slice it can be extended to all the boundaries of the selected organ, within that slice, as well as in adjacent ones, embracing the whole organ of interest. Similarly to picking up of the color in Photoshop and Feeling with the color, but in 3 D. It is probably not like that.

With this intention I tried to use the tool “Level tracing”, hoping that it selects the level of density and somehow expand my segment over whole organ. It did not work, looks like inactive.

I used “paint tool” in Segmentation Editor to mark some area, hoping to extend boundaries, but how to do that.

I hope, there is no necessity to mark area of interest in all the slices separately…

E.g. how to select aorta to make 3D surface model ?

See detailed tutorials about how to segment efficiently in 3D:

I would recommend to use nightly version of Slicer, it works well and has many improvements since the last stable 4.8 version.