Not able to select masks for registration in elastix

[Chih-Ying-Huang]

Operating system: Windows10
Slicer version: 5.6.2
Expected behavior: Registration between 2 abdominal CT image with masks
Actual behavior:
After import my binary mask as segmentations in slicer, I was not able to find a mask to select, as the following image shows:

[lassoan (Andras Lasso)]

Mask must be a label volume or segmentation. You can either convert the scalar volume to labelmap volume in Volumes module; or when you select the data for loading in the “Add data” window, choose “Segmentation” in Description column, or keep the description “Volume” and check the “LabelMap” option.

[Chih-Ying-Huang]

Thank you for your prompt reply, but I think I have imported my binary mask as segmentation:

But still, I am unable to select it from the masks in the elastix module.