Visual impression of photographic density of lung, thorax, and mediastinum in chest CT images: verification of optical illusion by visual subjective evaluation of medical images

Abstract

Even if the brain in head CT images is shown physically at the same photographic density, optical illusion (assimilation, contrast, etc.) occurs and practical density can be observed psychologically differently. Therefore, in this study, in order to clarify the visual characteristics of lightness perception in chest CT images, I attempted to compare it with the visual impression of the photographic density of lung, thorax, and mediastinum, using visual subjective evaluation. The results were as follows. (1) In the case of the preferential window setting of lung, both thorax and mediastinum, which surround the lung, are recognized as a wide white frame with contrast, and the photographic density of the lung psychologically seems blackish as a result of the "picture frame effect". (2) In the case of the preferential window setting of thorax and mediastinum, the visual impression of the photographic density varies among observers. (3) In the case of the virtual double window setting of lung, thorax, and mediastinum, under the influence of both high-density and low-density areas in the radiological anatomy of thorax and mediastinum, the photographic density of lung psychologically appears whitish as a result of the "grouping theories" of lightness computation. Further, under the influence of original gray lung, the photographic densities of thorax and mediastinum psychologically appear to be whitish.