Need to remove the black background from image patches

I have patches of an image . I want to remove those patches which only contain the black background of the image . The image is medical CT scan image of the brain. As you can see in the attachment, the original image and the patches of that image, i want to remove those patches which are only black patches i.e patches of the image background.

FYI , please ignore label of the patch.

Answer 1

[Image Analyst]

Inside your loop where you're creating the subplots of the subimages, do this

for k = 1 : 121
      thisImage = ....whatever....
      if mean2(thisImage) < 20 % Or whatever value you want
          continue;
      end
      subplot(......
end

Answer 2

[Thorsten]

You can detect an all-black patch P if the maximum value is 0 (in the ideal case) or below a certain threshold, say 0.1 (if the image values range from 0 to 1)

threshold  = 0.1;
isblack = max(P(:)) < threshold;

And if it is black, you do not show the patch.