Fig. 2
From: GPU based parallel acceleration for fast C-arm cone-beam CT reconstruction
FDK reconstruction coordinate system definition. The definition of the volume voxel coordinate system axis X, Y, Z with origin O and the flat panel plane coordinate system U,V with origin O1. Assume voxel x1 and x2 are symmetric with the plane s formed by axis Z and axis V, and the distance to plane s is d, their projections on the flat panel plane px1 and px2 are symmetric with axis V, with the same distance pd to axis V. Therefore we only need to calculate one half of the volume voxels. The calculation for the other half can be accomplished by a mirror action