You can draw a Region of Interest (ROI) to measure the area, average pixel value, and standard deviation of pixel values in an image. The graphic is added automatically as a bookmark. Open the Bookmark list to see its details.
1. From the Viewer, select an image.
2. In the Graphics tab of the toolbar, select Oval ROI 
from the ROI drop-down in the Measurements ribbon.
3. Click on the image where you want the ROI to start.
4. Click where you want the ROI to end.
The area of the ROI, the radius, and the average digital value and standard deviation of the digital values in the ROI are automatically measured.
To draw a circle, hold down the CTRL key while drawing.