Hi Zhaoqing,
There shouldn’t be any restrictions on the plane used for drawing ROIs, other than the fact that it needs to be aligned with the axes of the ROI image itself (this is only a problem if you’re drawing on a ROI image that has different dimensions, voxel sizes and/or orientation from the main image).
Do you mean that once you’ve drawn your ROI in the axial or sagittal plane, it appears as separate planes when viewed in the coronal plane? Or is it literally not possible to draw on the coronal plane as expected? Can you maybe post a screenshot to illustrate?
Cheers,
Donald.