One of three stages of CBCT reconstruction, which consists of two distinct steps, HU correction and ring correction, and that applies to 3D images. The HU mapping provides a linear mapping of the arbitrary units of the raw CT numbers, which are the standard intensity scale used in computed tomography (air = -1000 HU and water = 0 HU). Ring correction is performed with a similar approach to that used by Sijbers and Postnov (Phys. Med. Biol. 49:14 (2004) but with an enhancement to account for partial or nonuniform rings. See also CBCT reconstruction.