[A Simple Method for Measuring the Anisotropic Two-dimensional Noise Power Spectrum in Helical CT Scan]

Recently, a new method has been devised to measure the anisotropic 2-dimensional noise power spectrum (2D-NPS) in computed tomography. The 2D-NPS varied with the X-ray tube angle θ in the helical scan; it was defined as 2D-NPS_θ. However, the method requires many scans to obtain the 2D-NPS_θ for each θ with less statistical variability and is laborious. In the present study, the 2D-NPS_θ was assumed to be an identical anisotropic 2D-NPS that rotated around the origin of the spatial frequency domain in conjunction with the rotation of the X-ray tube. We defined the identical 2D-NPS as 2D-NPS_rot and proposed its measurement method with fewer scans.

The 2D-NPS_θ (θ from 0° to 180° in an increment of 30°) were obtained from noise images acquired by a hundred scans of a water phantom. In the proposed method, the 2D-NPS_θ were obtained from noise images by 2 scans, rotated backward around the origin by θ to generate the identical 2D-NPS, and averaged to generate the 2D-NPS_rot.

The 2D-NPS_rot, when it was rotated by θ, agreed well with the corresponding 2D-NPS_θ. Absolute values of the mean and standard deviation of percentage errors of the 2D-NPS_rot with the corresponding 2D-NPS_θ at each θ were less than 0.70% and 6.12%, respectively.

The proposed method was suggested to be valid for simple measurement of anisotropic 2D-NPS.