How to define vessel centerline

How can I make a good center line? when I put the point of origin and end in an angiotomography (panoramix) the screen closes automatically. how can i fix this problem? I need to know if with the center line I can make measurements from point A to point B by adding the fudicials. thanks for your help.

What do you do exactly? Place markups line? Place annotation ruler? In slice or 3D view?