Detect contours, modify ROI
Optionally, to calculate histogram click and 'Histogram icon' at 'List of images' widget.
To extract / detect contours - adjust level and window.
Open 'List of ROIs' widget - click at 'circle' icon in lower right corner.
Select ROI or create new one. To create new ROI right-click inside widget to open menu.
To draw click 'Plus on circle' icon at 2D view toolbar.
Another panel will appear.