the image comes out of a long program in which I am not able to change the relation of the numbers that represent the pixels ...
It is somewhat hard to understand what it is you cannot do here with Mathematica. You just need to define a relation between pixel value and color. An even more simple approach might be this:
img = Import["https://cdn.drg.de/media/image/21865/100p/Abb-3-CT-Abdomen-copy-Universitaetsklinikum-Tuebingen.jpg"];
Colorize[img, ImageSize -> ImageDimensions[img], ColorFunction -> #] & /@ ColorData["Gradients"][[;; ;; 3]]
Does that help? Regards -- Henrik