One of three steps in the reconstruction of a CBCT dataset, which produces corrected 2D projections from raw 2D projections. In addition to normalization and logarithmic conversion, preprocessing consists of two distinct steps, spectrum correction and scatter correction:

• Spectrum correction compensates for the beam-hardening effect by accounting for the output spectrum of the X-ray tube and the material through which each ray passes (Titanium filter, bowtie filter, and so on) as well as the spectrum sensitivity of the detector. It also performs a (guessed) correction of the beam hardening caused by the object itself. • Scatter correction estimates the amount of X-ray scatter in each projection based on the object visible in the projections, the machine configuration, and precomputed simulations. This estimated scatter contribution is subtracted from the measured X-ray transmission in the projections. See also CBCT reconstruction.