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For two arbitrary positions (primed and unprimed) of the unit consisting of a point source (spots) and a detector (line) rotating around a centre of rotation, one exemplary measured “ray” is displayed. It enters the volume to be reconstructed at entry point A (or A′) and exits it in point B (or B′). The distance 
the ray traverses through a particular voxel 
provides a simple measurement to compute the contribution of the “ray” to the grey value in voxel 
