In a method, a water equivalent diameter (abbreviated to WED) of a slice plane of an examination volume is received via an interface. In an embodiment, a noise level based on the water equivalent diameter is determined via a computer unit, the noise level being an upper threshold value for the noise of a CT image dataset. Furthermore a reference dose parameter is determined based on the noise level and the water equivalent diameter via the computer unit, the reference dose parameter corresponding to a first x-ray dose, which would be absorbed in the slice plane during a recording of a first CT image dataset of the examination volume upon the noise of the first CT image dataset corresponding to the noise level. Finally, the reference dose parameter is indirectly proportional to a power of the noise level.
