You need to compute the histogram, then the CDF:
numBins = 100; % whatever....
[counts, edges] = histcounts(energyImage, numBins);
cdf = cumsum(counts);
Now figure out what energy you want to remain. Let's say it should be in the darker gray levels rather than the brighter ones. Let's say it's 3.5e7 for example. Pick whatever number is right for your situation. Then find the energy level of the CDF that has that energy
remainingEnergy = 3.5e7; % Whatever....
index = find(cdf > remainingEnergy, 1, 'first')
% Now find edge where this occurs
thresholdLevel = edges(index);
% Now threshold the image at that level:
binaryImage = energyImage < thresholdLevel;
It's probably the most efficient way.
