Detection of small focal lesions in CT images: effects of reconstruction filters and visual display windows.

The detectability of small, high-contrast lesions was measured on CT images, simulations of those obtained by the EMI Mark I scanner. Images were reconstructed using five reconstruction filters (kernels), which varied the image sharpness and noise level. Different sets of images were produced using various CT display windows, six different window sizes and four different display level settings. The measured lesion detectability for observers increased from 1.6 to 2.4 as the reconstruction kernel became smoother, and it decreased only slightly at the largest display window (1000 CT numbers wide). These effects were predicted by changes in the signal-to-noise ratio, as calculated for the lesion-matched filter applied to each set of physical CT images. This filter computes the cross-correlation of the CT image and the lesion profile at the specified possible locations for the lesion.