I have no idea if it's a common technique or not, but a couple years ago I met some guys similarly using image analysis of spectrograms. They were trying to diagnose sleep apnea based on of heart rate data. They had a company developing a device and claimed to have patents on the technique, but I forget the name. I just remember that it seemed like a convoluted algorithm to me, and they agreed, but they claimed it worked better than any traditional approach they tried.