Development of the noise theoretical formula for analyzing noise components on CT images

This study derived a theoretical formula for analyzing noise components in computed tomography (CT) images and evaluated the relationship between CT doses and each noise component on CT images. We devised a theoretical formula for estimating the noise components using CT numbers and the relative noise standard deviation (SD) of the CT images. The CT quality control phantom’s CT images were obtained at different CT doses using two CT scanners. We measured the mean CT numbers and relative noise SD at each rod position on the CT images and estimated each noise component on the CT images using a theoretical formula. The relative noise SD on CT images was represented as a function of the CT doses corresponding to the number of photons penetrating the phantom. The second-order, first-order, and constant terms in the theoretical formula indicate the structural, quantum, and electronic noise components, respectively. The content ratio of quantum noise exceeded 70% at CT doses routinely used in clinical settings and was higher than that of the other noise components. The theoretical formula developed in this study can be useful for optimizing CT doses and developing image reconstruction methods to reduce noise on CT images.