FIG. 9.

Schematic illustration of why a unique d_xy-wave symmetry was observed in the NPS of ROI B: when a small bowtie filter was used during the scan of a large object, the amount of x-ray attenuation experienced by x-rays intersecting with ROI B strongly depends on the projection angle θ. When θ = 135^∘ or 225^∘, the combined attenuation from the bowtie filter and the object reaches its maximum; therefore, when the corresponding noisy detector data are backprojected during the FBP reconstruction, diagonal noise streaks form and dominate the 2D NPS of the final CT image.