How to convert to .stl from .nii

Hi everyone,

I want to convert this .nii file directly into .stl file which can be used by Netgen, how can I do it without any segmentation in 3DSlicer? Because this file .nii was already a segmentation from itksnap.

The link of this file .nii:
https://drive.google.com/open?id=1IQ7_5ha_MisHcfpndhLi_KlV33S1k0pp

Thank you.

Xiaoyu.