1. A method for recognizing and marking blood vessels of a lung with the aid of a CT examination using at least two different x-ray energy spectra, the method comprising: scanning a patient, at least in a region of the lung, with two different x-ray energy spectra, with the patient having only a single contrast agent administered; and reconstructing an at least two dimensional tomographic image for each x-ray energy spectrum which reproduces local spectrum-specific absorption properties of the scanned region, wherein a surrounding area is defined for a multiplicity of voxels for which an average ratio is calculated with the aid of local absorption values of the at least two x-ray energy spectra, the average ratio representing a measure for a ratio of a proportion of the single contrast agent to a proportion of soft tissue in the surrounding area of the respectively considered voxel, and upon the average ratio dropping below a threshold value, the respectively considered voxel is considered to have reduced blood circulation and is marked in the reconstructing at least two dimensional tomographic image.