Size discrimination in computed tomographic images. Effects of feature contrast and display window.

Studies show that features on computed tomographic (CT) images in clinical formats become less detectable when the images are produced with wider CT display windows. We studied the effects of feature contrast and the display window on observer performance in higher-order tasks that involved discriminating small size differences between features on CT images. The features to be discriminated were pairs of disks (9.0 or 9.5 mm in diameter) superimposed on CT images of water phantoms. Sets of image stimuli for two different types of size-discrimination tasks were generated with various CT contrasts specified for the superimposed features and were produced on film transparencies with display windows ranging from 90 to 2880 Hounsfield units (HU) in width. Observers' performance improved with increasing CT contrast in both size discrimination tasks. Unlike performance in feature-detection tasks, however, size discrimination was unaffected by changing the CT display window over a factor of 16 (from 90 to 1440 HU). Performance fell only at the widest display window (2880 HU), for which CT noise was essentially invisible. These results suggest that the effect of changing the CT display window may depend on the spatial frequency content of image information required for a given task.