1. An image system for retaining a contrast when merging image data of at least two images of a patient, comprising: apparatus for receiving and processing images; an image processing unit operating within the apparatus that: mixes image data of a second image of the patient over with image data of a first image of the patient using an alpha-channel-based mixing technique, determines a compensating pixel value term for compensating a reduction of the contrast occurring during the mix-over, and brightens pixel values of the first image prior to being mixed-over with a foreground region of the second image by applying the compensating pixel value term for retaining the contrast according to the formulas: 
 I FV + ( n x ,n y ):= I FV ( n x ,n y )+α·Δ I F ∀( n x ,n y )ε FV ; and 
 I FH + ( n x ,n y ):= I FH ( n x ,n y )+α·Δ I F ∀( n x ,n y )ε FH; wherein α is a blending factor 0<α<1; I FV (n x , n y ) means an intensity of a pixel at coordinates (n x , n y ) in a foreground region FV of the first image; I FH (n x , n y ) means an intensity of a pixel at coordinates (n x , n y ) in a background region FH of the first image; ΔI F is an offset value; and “intensity” means a grey-scale value or a color luminance value.