If you load an image into matlab, the data is stored in a matrix. You can then just extract a sub-matrix of the area you are interested in.

You can create a binary mask where it has 1 for a required pixel (pixels in circle area) and 0 for a non required pixel(pixels outside the circle).

Then you can just multiply the image with mask and get the required area.

% You can Mask the image using bsxfun() function maskedRgbImage = bsxfun(@times, rgbImage, cast(mask, 'like', rgbImage));

also have a look at this

https://in.mathworks.com/matlabcentral/answers/341305-how-to-impose-binary-mask-on-rgb-color-image

If you know center pixel coordinates (i.e. which pixel is you center for a circle) then you can just use a simple function to create structuring element in matlab.

strel('disk',r); where 'r' is radius of your circle.