Investigation of optimal display size for detecting ground-glass opacity on high resolution computed tomography using a new digital contrast-detail phantom.

The purpose of this study was to clarify the relationship between display sizes of high resolution computed tomography (HRCT) images for detecting ground-glass opacity (GGO) and observer performance using a digital contrast-detail (d-CD) phantom. A structure of the d-CD phantom was determined on the basis of the actual images of GGOs and background noises of 22 patients who were diagnosed as GGO by chest HRCT. The d-CD phantom has a 512×512 matrix in size and has total of 100 holes: the diameter of these holes increases stepwise from 2 to 20 pixels with 2 pixels interval in a vertical direction and the CT value varies stepwise from 2 to 200 HU in a horizontal direction. The observer performance study was carried out for three different display sizes (30 cm×30 cm as an enlarged size, 13 cm×13 cm as an original size, and 7 cm×7 cm as a reduced size) using a 2-megapixels LCD monitor, and it was analyzed using Friedman and Wilcoxon statistical tests. As a result, the observer performance for the original display and the reduced display sizes was superior to that for the enlarged size (P=0.006 and 0.037 for the original display and the reduced display sizes, respectively), whereas there was no significant difference between the original display and reduced display sizes (P=0.77). The d-CD phantom enables a short-term evaluation of observer performance and is useful in analyzing relationship between display size and observer performance.