1. A method for establishing a topogram, based on a spiral acquisition of an examination region using a computed tomography system, the method comprising: reconstructing a spatial three-dimensional image of the examination region using the spiral acquisition; establishing a first topogram of the examination region by a parallel projection of the spatial three-dimensional image along a first projection direction; computing an envelope of the examination region by segmenting the examination region in the spatial three-dimensional image; and determining scattering parameters based on the computed envelope of the segmented examination region.