After filtering, the existing projection images are reverse-projected (commonly known as back projection).  The back projection of the 2D X-ray images can be imagined as though the procedure for generating the projection images using the measuring machine is mathematically reversed by the software.   Back projection is performed for all radiographic images created, in the direction of the location of the focal spot, taking into consideration the geometric positions of the relevant machine components and the current rotary position of the work piece.  A virtual measurement object volume is thus created by super imposing.  Each of the projection values from the various rotary positions are added to all the voxels along the beam path.  This is done along the same lines as the original measurement.  The image reconstruction is complete when all measured projection images have been back-projected.

See in image above calculating volume data by back projection of filtered radiographic images: a) Object b) X-ray beam path in one section plane c) Principle of step wise back projection and superimposing d) Result of reconstruction with different numbers of back projections for a real work piece