This experiment studied the detectability of lesions superimposed at specified locations on CT images of water phantoms. Each set of images contained 120 pedestals (a constant CT number added to circular areas of the water-phantom images), 60 of which also had a circular lesion profile of a given size and contrast superimposed on their centers. Lesion detectability (estimated from observers' ROC curves) was measured for 24 separate sets of images, representing four different levels of contrast for each of six lesion sizes (3.0 to 16.0 mm in diameter). Direct calculations of each lesion's "signal-to-noise ratio" (SNR), from the physical CT values :in each set of images, predicted its detectability for observers, and there was no systematic change in the linear relation between detectability and lesion SNR as a function of lesion size. The levels of contrast required to produce either a given level of lesion SNR or a given level of detectability were approximately linear functions of the lesion's diameter on a log-log plot, and parallel to the contrast-detail curve that we measured independently with the same image format. The slope of these curves (about -1.0) is consistent with those measured by other investigators, and is inconsistent with the signal detection predictions that assume an ideal ramp function for the CT noise power spectrum. These results suggest that the ideal noise power spectrum may not be realized in actual CT scanners, and that the actual deviations at low spatial frequencies affect the detectability of large lesions.

Date Published: 11 June 1985
PDF: 5 pages
Proc. SPIE 0535, Application of Optical Instrumentation in Medicine XIII, (11 June 1985); doi: 10.1117/12.947234

Published in SPIE Proceedings Vol. 0535:
Application of Optical Instrumentation in Medicine XIII
Samuel J. Dwyer III; Roger H. Schneider, Editor(s)