1. A method for noise reduction of CT image data, comprising: scanning an object under examination; creating at least two CT image datasets, each dataset created on the basis of a different x-ray energy spectrum; splitting the at least two CT image datasets into at least two split-up image datasets respectively, with a lowest local frequency band and at least one high local frequency band; determining noise in the at least one of at least two CT image datasets for each x-ray energy spectrum; calculating at least one new image dataset using an unchanged split-up one of the two CT image datasets with the lowest local frequency band and one of the at least two CT image datasets created from a noise-minimized weighted combination of split-up image datasets which originate from the scans with different x-ray energy spectrums.