Method for measuring noise-power spectrum independent of the effect of extracting the region of interest from a noise image

This study aimed to evaluate the impact of region of interest (ROI) size on noise-power spectrum (NPS) measurement in computed tomography (CT) images and to propose a novel method for measuring NPS independent of ROI size. The NPS was measured using the conventional method with an ROI of size P × P pixels in a uniform region in the CT image; the NPS is referred to as NPS R = P . NPSs R =256, 128, 64, 32, 16, and 8 were obtained and compared to assess their dependency on ROI size. In the proposed method, the true NPS was numerically modeled as an NPS model , with adjustable parameters, and a noise image with the property of the NPS model was generated. From the generated noise image, the NPS was measured using the conventional method with a P × P pixel ROI size; the obtained NPS was referred to as NPS' R = P . The adjustable parameters of the NPS model were optimized such that NPS' R = P was most similar to NPS R = P . When NPS' R = P was almost equivalent to NPS R = P , the NPS model was considered the true NPS. NPSs R =256, 128, 64, 32, 16, and 8 obtained using the conventional method were dependent on the ROI size. Conversely, the NPSs (optimized NPSs model ) measured using the proposed method were not dependent on the ROI size, even when a much smaller ROI ( P = 16 or 8) was used. The proposed method for NPS measurement was confirmed to be precise, independent of the ROI size, and useful for measuring local NPSs using a small ROI.