Help needed on quantifying the differences in contrast true and fake image

Hello there.
I'm working with medical imaging using GAN models, when it comes to compare true and fake images I'm using SSIM, however, contrast plays a big role in my project and SSIM can give me a high value (\~.95) when both images have the same shape but the contrast estimation is completely wrong in the fake image.

So, does anyone know a good metric to quantify the difference in contrast in true and fake images?

deeplearning fake image