Temporal subtraction of chest CT in lung cancer screening

Purpose: Detection of change between CT images is crucial in lung cancer screening. We analysed whether subtraction images can detect change in nodular size between successive low-dose CT images. Methods and Materials: Given two successive CT scans lung segmentation and non-rigid registration between the two scans is performed and a subtraction image is obtained by subtracting the deformed prior scan from the current scan. Subtraction images of a total of 111 participants of a lung cancer screening trial that were referred to a pulmonologist on the basis of the last CTwere studied. An experienced radiologist annotated all relevant nodular changes by inspecting the subtraction images. The two original images were available side-by-side for confirmation. In addition, the quality of the subtraction images, was scored on a 1-5 scale. Results: The quality of the subtraction images was rated high: only six subtraction images (4\%) had a rating lower than 4, meaning that the images were significantly degraded by artefacts. In the referred cases 58 new nodules > 5 mm were found. Furthermore 45 nodules with significant growth were noted. A significant nodule was missed in 8 cases, of which 2 were stable and hence not visible on the subtraction image, 1 was missed because of its pleural location and 5 showed a mean increase in the diameter of 21\%. Conclusion: Temporal subtraction of CT images is a promising tool for the visual detection of change, especially significant growth of nodules, between successive CT scans.