2.1. BP

The BP algorithm is referred to as "shift-and-add" (SAA) [9], whereby projection images taken at different angles are electronically shifted and added to generate an image plane focused at a certain depth below the surface. The projection shift is adjusted such that the visibility of the features in the selected plane is enhanced, whereas those in other planes are blurred. By using a digital detector, the image planes at all depths can be retrospectively reconstructed from one set of projections. The SAA algorithm is valid only if the motion of the X-ray focal spot is parallel to the detector.
