I don’t know of a quick tool which does this like you have in the likes of image J or the software on the CT workstations (although there may be?)
The way I would do that (although there may be other ways) is to go to segment editor, then go to the paint effect.
You want a circular area of 200mm^2 and because the area of a circle is A = (pi * d^2) / 4 then the diameter of the circle needs to be approx 16mm (if I did my maths right).
Now click the small “%” button to toggle to absolute diameter mode for the paint brush size and enter the paint brush diameter as 16mm.
Then click once (do not click and drag) on the area you are interested in to paint a circle (as shown in yellow above). If you accidentally paint in the wrong spot just press the undo button.
Now go over to “Segment Statistics” module and enter your segmentation node (probably called “Segmentation”) and your volume as inputs. Then press apply and you will see the mean HU units of the area you have just painted on the resulting table.
