How can I remove inside area from an ROI to analyse the outer area only?

I have recently started using MATLAB for image processing with MR images, and so far have found previously asked questions and examples to be a great help - but with this I am a little stuck! I have created an ROI on my dicom image using imfreehand() I have then dilated this ROI using the imdilate(I, strel('square',n)) function to create a new ROI. Ideally, I would now like to subtract the original ROI from the new ROI so that I am left with only the new area. Any help would be greatly appreciated!

Answer 1

[KSSV]

I = imread('cameraman.tif') ;
imshow(I)
h = imfreehand ;
pos = wait(h); 
%%Extract the ROI 
[ny,nx] = size(I) ;

[X,Y] = meshgrid(1:nx,1:ny) ;
idx = inpolygon(X(:),Y(:),pos(:,1),pos(:,2)) ;
Z = I ;
Z(idx) = NaN ;

imshow(Z) ;

[molly jane fitches]

This works great for drawing a new ROI - Any hints for an existing ROI?

[KSSV]

If you have positions for ROI, you can do the same eh...

[Image Analyst]

In what form is your existing ROI?

[molly jane fitches]

I thought so, thought it would be very easy to adjust but rather than returning an error it executes and displays the original image. I drew the ROI using imfreehand, then tried it as a double, and then tried using the column and row vectors from the boundary.