Dual Energy Imaging

In x-ray imaging, pixel values represent effective attenuation measurements of an object in the beam and can be expressed by the following equation: 

N = N0e-ut

Where N is the number of detected photons, N0 is the number of photons incident on the object, u is the attenuation coefficient and is material specific and a function of energy,  tis the object thickness. When imaging two distinct materials, the equation becomes: 

N = N0e-(ujtj+uata)

Only in limited situations with specific a priori knowledge can information about the thickness of both materials be discerned from a single measurement.