How can I use a slide bar to select a specific image that displays the best ROI within a movie?

I have a .mov stored in imstack and am running through all the images to analyze the change in pixel intensities over time, but I would like to employ a slide bar to select the image that produces the clearest ROI. Can anyone help me out?

Here is my code so far:

mask = roipoly;

mask = im2double(mask);

mask_size = 0;

curve = zeros(numel(t),1);

[r c] = size(imROI);

for n = 1:r

    for m = 1:c

        if mask(n,m) > 0

            mask(n,m) = 1;

            mask_size = mask_size + 1;

            curve = curve + squeeze(imstack(n,m,:));

        end

    end

end

curve = curve/mask_size;

end