Problems with contrast-detail curves for CT performance evaluation.

Contrast-detail curves have been used frequently to describe the low contrast performance characteristics of computed tomography (CT) scanners. However, such curves can produce misleading conclusions if the effects of all variables influencing CT images are not considered. As shown in this experimental study, improperly designed contrast-detail curves disguise differences in CT performance when the same object is imaged with different x-ray spectra. These problems arise because contrast is defined as the difference in system-dependent CT numbers rather than the actual difference in the object. An alternate approach to CT performance evaluation using "difference-detail" curves is offered.