1. A method for recognizing and marking contrast agents in blood vessels of the 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 contrast agents in the blood stream; reconstructing an at least two dimensional tomographic display for each x-ray energy spectrum which reproduces the local spectrum-specific absorption properties of the scanned region, wherein a neighborhood is defined for a multiplicity of voxels for which an average ratio is calculated with the aid of the local absorption values of the at least two x-ray energy spectra, the neighborhood specifying a measure for the ratio of a proportion of contrast agent to a proportion of soft tissue in the neighborhood of the respectively considered voxel, and upon the ratio dropping below a threshold value, the respectively considered voxel is considered to have reduced circulation and is marked in the reconstructing at least two dimensional tomographic display.