Evaluation of subjective assessment of the low-contrast visibility in constancy control of computed tomography

The purpose of the present work was to investigate the reliability of subjective assessments of the low-contrast visibility in constancy control of computed tomography (CT). Axial CT images of a low-contrast phantom were acquired on an 8-slice multi-detector CT scanner at nine tube current settings ranging from 75 to 440 mA. Five medical physicists assessed the visibility of the low-contrast details in two sessions. In the first session, containing 54 images, the visibility was rated on an absolute scale by determining the number of visible details in each contrast group in each image. In the second session, 180 image pairs were presented to the observers with the task of determining if the two images had been acquired under identical conditions or not. In the absolute session, both the intra- and inter-observer variabilities were high. In the relative session, the variability was smaller, but an exposure difference of 50 % was needed for all observers to correctly identify a change in all cases. In conclusion, the present study indicates that subjective assessments of the low-contrast visibility in constancy control of CT are not reliable.