Method for measuring noise power spectrum depending on X-ray tube angle in spiral orbit of helical CT scan

The noise power spectrum (NPS) in computed tomography (CT) images potentially varies with the X-ray tube angle in a spiral orbit of the helical scan. The purpose of this study was to propose a method for measuring the NPS for each angle of the X-ray tube. Images of the water phantom were acquired using a helical scan. As a conventional method, we measured the two-dimensional (2D) NPS from each image and averaged them; the obtained 2D-NPS was referred to as NPS_conventional. In the proposed method, we made the X-ray tube angle θ (0°≤θ<360°) to correspond to the image according to each slice position of the images that located within the travel distance of the CT scan table per 360° rotation of the X-ray tube. We obtained the 2D-NPS from each image and assigned the θ (0°, 30°, 60°, 90°, 120°, 150°, 180°); the obtained 2D-NPS was referred to as NPS_Sθ. The NPS_Sθ was compared to the NPS_conventional. Also, we investigated the dependency of the NPS_Sθ on the θ. The NPS_conventional was found to be isotropic, and in contrast, the NPS_Sθ was anisotropic. The NPS_Sθ showed a continuously rotational change while increasing the θ. There was an excellent correlation (R²>0.999) between the rotation angle of NPS_θ and the θ. The proposed method was demonstrated to be effective for evaluating anisotropic noise characteristics depending on the X-ray tube angle. (author)