An x-ray system is disclosed including a selector for finding an optimum combination between the contrast medium and the energy spectrum of an x-radiation for a scan to optimize the noise-to-contrast ratio. A method for creating X-ray images is also provided. The x-ray images are created with the aid of contrast media by taking into account an optimal combination between the contrast medium and the energy spectrum of an X-radiation used for a scan. A method for the use of a lanthanide-containing complex to produce a contrast medium for optimizing the combination between the contrast medium and the radiation to obtain a maximum contrast-to-noise ratio in an X-ray image is also provided.