In a method, a topogram of an examination volume is received via an interface. A water equivalent diameter (abbreviated to WED) of a slice plane of the examination volume is then determined, via a computer unit, based on the topogram. Thereafter, via the computer unit, a noise level is determined based on the WED, the noise level being an upper threshold value for noise of a CT image dataset. Finally, via the computer unit, a reference dose parameter is determined, based on the noise level determined and the WED, the reference dose parameter corresponding to a first x ray dose, absorbable in the slice plane during a recording of a first CT image dataset of the examination volume upon noise of the first CT image dataset corresponding to the noise level determined.
