There are probably other ways to do this, but this is what I rolled:
inpict = rgb2gray(imread('us2.bmp'));
% generate a mask
bb = inpict>5; % some threshold
bb = despeckle(bb,100); % remove small spots/holes
bb = bwpropfilt(~bb,'solidity',1); % find most solid circle
% apply the mask
inpict(bb) = 0;
If you need the mask to trim off some of the very inner content, you can dilate it after the bwpropfilt() call. This example uses despeckle() from MIMT, but you can do the same with bwareaopen()
bb = ~bwareaopen(~bwareaopen(bb,100),100);
