afni.nimh.nih.gov

Yang,

There are several drawing modes in the ROI plugin. The default is "Filled Curve", which will automatically fill any shape you draw. The drawing mode "Open Curve" will allow you to draw a shape and not have it fill automatically. However, it does require freehand drawing. I don't think there's a drawing mode that will create perfect non-filled cirlces or ovals for you.

Good luck.

Peggy