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Colonic Polyps: Effect of Attenuation of Tagged Fluid and Viewing Window on Conspicuity and Measurement—In Vitro Experiment with Porcine Colonic Specimen¹

¹ From the Department of Imaging, University College Hospital, University of London, 250 Euston Rd, London NW1 2BU, England (S.A.T., S.H.); and Department of Intestinal Imaging, St Mark's Hospital, London, England (A.S., D.B., L.G., J.S.). Received June 13, 2005; revision requested August 9; revision received August 22; accepted September 15; final version accepted October 17. Address correspondence to S.A.T. (e-mail: csytaylor{at}yahoo.co.uk).

Purpose: To investigate effect of attenuation of tagged fluid and viewing window on polyp conspicuity and measurement with porcine colonic specimen.

Materials and Methods: Eleven (3–10-mm-diameter) polyps were created in porcine colon and the specimen submerged in saline. Four–detector row CT was performed after gas distension and after filling with six barium sulfate suspensions (attenuation, 100–1000 HU). Two readers independently measured maximal two-dimensional polyp diameter on each data set with the following four viewing windows and window levels and window widths, respectively: colon (–150 HU, 1500 HU), lung (–500 HU, 1500 HU), bone (500 HU, 2500 HU), and abdomen (40 HU, 400 HU). In consensus, polyp conspicuity (compared with air data set) was assigned a grade of 1–4 for each viewing window (grade 1, not seen or barely visible; grade 4, optimally seen). For statistical analysis, conspicuity grades were collapsed to a two-point scale. Data were analyzed with Mann-Whitney, Kruskal-Wallis, and ² tests.

Results: Accuracy of polyp measurement was independent of viewing window for attenuation of tagged fluid of 100–300 HU but differed significantly for 500–1000 HU (P < .001); that for colonic and bone viewing windows was superior (median size difference, 1.0 mm; interquartile range, 0.5–1.5). Conspicuity differed significantly according to viewing window at all attenuation values (P < .001). For 100–300 HU with abdominal viewing window, 83% (24 of 29) of observations were assigned grade 3 or 4 (best). For 500–1000 HU with bone viewing window, 94% (30 of 32) of observations were assigned grade 3 or 4 (superior). Overall conspicuity was best with bone viewing windows at 700 HU.

Conclusion: Polyp conspicuity and measurement in tagged data sets were optimized at 700 HU with bone viewing windows. At less than 300 HU, conspicuity improved with abdominal viewing windows.

© RSNA, 2006

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