How to obtain the complete center lines using VMTK?

I’m trying to obtain center lines from airway using VMTK. After dilating the pixels of airway, i got most of the center line, including from the small airway, looks like this:

my problem is, why the center lines are broken in the middle? and what would be the best way to get a complete center lines?

A similar problem seems to be happened here, the extract lines are not joined:

Please help or try to give some ideas how to achieve this. Thanks in advance.

Please send questions about VMTK toolkit to the new VMTK forum (see recent note from Luca Antiga).