Disclosed herein is a method comprising: directing X-ray in a first wavelength range and X-ray in a second wavelength range are directed to a subject； introducing a contrast agent into the subject； capturing a first image with the X-ray in the first wavelength range and a second image with the X-ray in the second wavelength range； determining a differential image between the first image and second image； wherein strength of interaction between the contrast agent and the X-ray in the first wavelength range and the strength of interaction between the contrast agent and the X-ray in the second wavelength range are different.
