Local and global 3D noise power spectrum in cone-beam CT system with FDK reconstruction

Purpose: The authors examine the nonstationary noise behavior of a cone-beam CT system with FDK reconstruction. Methods: To investigate the nonstationary noise behavior, an analytical expression for the NPS of local volumes and an entire volume was derived and quantitatively compared to the NPS estimated from experimental air and water images. Results: The NPS of local volumes at different locations along the z -axis showed radial symmetry in the f_x - f_y plane and different missing cone regions in the f_z direction depending on the tilt angle of rays through the local volumes. For local volumes away from the z -axis, the NPS of air and water images showed sharp transitions in the f_x - f_y and f _y - f_z planes and lack of radial symmetry in the f _x - f_y plane. These effects are mainly caused by varying magnification and different noise levels from view to view. In the NPS of the entire volume, the f_x - f_y plane showed radial symmetry because the nonstationary noise behaviors of local volumes were averaged out. The nonstationary sharp transitions were manifested as a high-frequency roll-off. Conclusions: The results from noise power analysis for local volumes and an entire volume demonstrate the spatially varying noise behavior in the reconstructed cone-beam CT images.