1. A method for determining the concentration of a substance in a body material that is composed of two different material components in an unknown ratio, comprising: recording two computed tomography pictures of the body material with the aid of a multi-energy computer tomograph in conjunction with two different spectral distributions of the x-radiation; reconstructing two image data records from the two computed tomography pictures, the image data records including x-ray attenuation values x; decomposing the x-ray attenuation values x for each voxel of interest of the two image data records into x-ray attenuation values of three material components of which a first and a second constitute the two different material components of the body material and a third constitutes the substance, wherein the decomposition is performed on the assumption that the x-ray attenuation value x M of the body material without the substance is composed of the x-ray attenuation values x M1 , x M2 of the first and second material component in accordance with the following equation: 
 x M =f*x M1 +(1- f )* x M2 , f being a volume fraction of the first material component in the body material; and determining the concentration of the substance for each voxel of interest on the basis of the decomposition.