Relative impact of detector noise and anatomical structure on lung nodule detection

A four-alternative forced-choice experiment was conducted to investigate the relative impact of detector noise and anatomical structure on detection of subtle lung nodules. Sets of four independent backgrounds from each of three regions (heart, ribs, and lung field between the ribs) were derived from a very low-noise chest-phantom capture. Simulated nodules of varying contrast and fixed diameter (10 mm) were digitally added to the centers of selected background images. Subsequently, signal-dependent noise was introduced to simulate amorphous selenium radiographic detector performance at typical 80, 200, 400, 800, or higher speed class exposures. Series of four nodule contrasts each were empirically selected to yield comparable ranges of delectability index (d') for each background type and exposure level. Thirty-six observers with imaging, expertise performed the nodule detection task, for which the signal and location were known exactly. Equally detectable nodule contrasts for each background type and exposure level were computed and their squares plotted against detector noise variance. The intercepts and slopes of the linear regressions increased in the order of lung, heart, and ribs, correlating with apparent anatomical structural complexity. The regression results imply that the effect of anatomical structure dominated that of capture device noise at clinically relevant exposures and beyond.