ROI of a ring area?

How do I define an ROI of a ring area, i.e, a big cirle and small cirle inside, and I want to select the area in between.

Do I have to define two ROIs first and name ROI3 = ROI1 -ROI2?

Answer 1

[Matt J]

For example,

[x,y]=deal(1:1000);
dsq=(x-400).^2 + (y(:)-300).^2;

ROI = (50^2<=dsq & dsq<=100^2 );

imshow(ROI)

[xi]

Thanks for your reply. However, what I'm looking after is an interactive ROI definition and managment tool. Users can create an ROI object by drawing ROI on top of an image using the 'freehand', 'polygon','circular', 'rectangle'... What's common is that, they all define a sets of edge points and close up to form an enclosed region, but what if I want to define a ring shape ROI object? Not necessarily two circles, can be any shape of a connected region with a hollow area in the middle.

[Matt J]

You will probably have to build that out of two separate ROIs.