Finding ROI in frequency domain.

I am looking for a method which can be used for finding the ROI on a matrix obtained by plotting power spectrum of an image. The power spectrum is shown below:

In this I am interested in the region between the two red circles(circles on the image were drawn manually). As it is in the frequency domain, the ROI can be calculated by selecting a frequency from a to b. But I am short of a method to do it. Anything in this regard would be helpful. Thank you.

The code used is:

 img= rgb2gray(imread ('finger3.jpg'));
 [M,N,~]=size(img);
  F = fft2(img);
  figure;imshow(mat2gray(log(1+abs(fftshift(F)))));
  psd = 10*log10(abs(fftshift(fft2(img))).^2 );
  figure(2); clf
  mesh(psd);
  figure;
  imshow(psd, []);
  axis on;
  colormap(jet(256));
  end

Answer 1

[Image Analyst]

Just mask with circles, like the FAQ shows: http://matlab.wikia.com/wiki/FAQ#How_do_I_create_a_circle.3F