DETECTION OF COLONIC POLYPS IN A PHANTOM MODEL - IMPLICATIONS FOR VIRTUAL COLONOSCOPY DATA-ACQUISITION

Purpose: Virtual colonoscopy is a new method of colon examination in w hich computer-aided 3D visualization of spiral CT simulates fiberoptic colonoscopy. We used a colon phantom containing various-sized spheres to determine the influence of CT acquisition parameters on lesion det ectability and sizing. Method: Spherical plastic beads with diameters of 2.5, 4, 6, 8, and 10 mm were randomly attached to the inner wall of segments of plastic tubing. Groups of three sealed tubes were scanned at 3/1, 3/2, 5/1, and 5/2 collimation (mm)/pitch settings in orientat ions perpendicular and parallel to the scanner gantry. For each acquis ition, image sets were reconstructed at intervals from 0.5 to 5.0 mm. Two blinded reviewers assessed transverse cross-sections of the phanto ms for bead detection, using source CT images for images for acquisiti ons obtained with the tubes oriented perpendicular to the gantry and u sing orthogonal reformatted images for scans oriented parallel to the gantry. Results: Detection of beads of greater than or equal to 4 mm w as 100% for both tube orientations and for all collimator/pitch settin gs and reconstruction intervals. For the 2.5 mm beads, detection decre ased to 78-94% for 5 mm collimation/pitch 2 scans when the phantom sec tions were oriented parallel to the gantry (p = 0.01). Apparent elonga tion of beads in the slice direction occurred as the collimation and p itch increased. The majority of the elongation (similar to 75%) was at tributable to changing the collimator from 3 to 5 mm, with the remaind er of the elongation due to doubling the pitch from I to 2. Conclusion : CT scanning at 5 mm collimation and up to pitch 2 is adequate for de tection of high contrast lesions as small as 4 mm in this model. Howev er, lesion size and geometry are less accurately depicted than at narr ower collimation and lower pitch settings.