A method for determining a modulation function for modulating intensity of X-rays provided by an X-ray source of a CT-system for CT-imaging a slice of a region a person's body, the method comprising: acquiring X-ray attenuation data for a first substantially anterior-posterior or lateral view of the slice; determining a first maximum X-ray attenuation from the attenuation data; determining a length of a projection of the slice along an axis in the plane of the slice that is substantially orthogonal to the view angle using the attenuation data; determining a second maximum X-ray attenuation for a second view orthogonal to the first view based on the determined length; and using the first and second attenuation maxima to determine the modulation function.