Quantitative evaluation of the effect of changes in effective energy on the image quality in X-ray computed tomography.

Abstract

The X-ray effective energy differs for each computed tomography (CT) scanner even at the same tube voltage because of differences in the bow-tie filter and additional filter. Even when scanning with the same tube voltage and dose setting, these differences in effective energy result in different image noise levels. Although this qualitative change is known, the related quantitative changes have not been clarified. In this study, using two CT scanners with the same geometric specifications and detector configurations, we quantitatively assessed the reduction in image noise accompanying the increase in effective energy. We also clarified the fluctuations in CT number. For both CT scanners, the effective energy, the standard deviation (SD) of the noise image when using two water phantoms with diameters of 240mm and 320mm, and CT numbers of the sensitometry module were measured. Further, the dose required to obtain the same image noise level in each CT scanner was calculated. The effective energy difference was 5.5keV to 10.7keV, and the difference tended to be larger when the scan field of view was larger. The SD differences were 24% and 14% for the 320-mm and 240-mm phantoms, respectively. For converting to the dose required to obtain the same SD, the dose can be reduced by 42% and 24% for the 320-mm and 240-mm phantoms, respectively. The CT number difference of both CT scanners was small. Therefore, higher effective energy contributes to the reduction of image noise.