For cone beam tomography, the ideal beam geometry (fan beam) is purposefully altered to increase energy efficiency and reduce measurement time. This means that the ideal beam geometry is found only in the center plane. It is defined as the plane that is perpendicular to the rotary axis and is at the focus spot of the X-rays.   In all the other layers of the object that lie parallel to the center plane, the beam geometry deviates more severely  from  the  ideal  geometry as the distance to the center plane increases.