How can i get the brisk's scale?

Hello, I'm trying to extract the scale from brisk's descriptors, but i don't know exactly how. I've already get the keypoints and the descriptors from my image. I saw in another posts that I can get the size from the descripor wich is proportional to the scale in this way: `size = scale * magnifier`) but i need the scale value. How can i get the scale value? Or the magnifier value? Can anyone help me? Thanks in advance,