THRESHOLD SELECTION FOR CT ANGIOGRAPHY SHADED SURFACE DISPLAY OF THE RENAL-ARTERIES

The purpose of this study was to define objective criteria to calculat e a tissue segmentation threshold for shaded surface display (SSD) ren dering of the renal arteries with computed tomography angiography. Con trast-enhanced spiral CT scans were obtained through the renal arterie s of nine patients. Six sets of SSD images were rendered for each pati ent with lower threshold values ranging from 80 to 130 Hounsfield unit s (HU) by increments of 10 HU. Visceral organ enhancement was measured in the aorta, liver, spleen, pancreas, and kidney. The segmentation t hreshold for each patient was determined by evaluation of the SSD imag es alone as well as by comparison with conventional arteriograms. The ideal threshold, as shown by comparison with conventional arteriograph y, was better correlated with a threshold value selected by qualitativ e evaluation of SSD images alone (r(s) = .42), than with measured enha ncement in visceral organs (r(s) = -.289 to .009). The degree of steno sis was overestimated in a single renal artery (1 of 18) because of an inappropriate threshold selected by evaluation of the SSD images alon e. In comparison with a segmentation threshold calculated from measure d enhancement of visceral organs, a segmentation threshold selected by qualitative evaluation of the resulting SSD images is more likely to approximate the ideal threshold. Given the subjective nature of such t hreshold selection, further evaluation is warranted to determine wheth er threshold selection may result in inaccurate grading of stenosis. ( C) 1995 W.B. Saunders Company