VMTK centerline computation problem

Hi, I’m new to 3D slicer and medical image segmentation. Now I’m working at cerebral vessel segmentation. I have figured out a vessel segmentation of brain CT images as follow
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I want to use the VMTK extension to caculate the centerline of the vessels. However, I cannot select the segmented volume as vessel tree model. Actually there is nothing I can select as input of VMTK centerline computation module. Can you provide any advice to help me out or any guide of using the VMTK centerline computation module?

It is 3D Slicer 4.10 on Windows.
Thank you so much.