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Multi–Detector Row CT Colonography: Effect of Collimation, Pitch, and Orientation on Polyp Detection in a Human Colectomy Specimen¹

¹ From the Departments of Intestinal Imaging (S.A.T., S.H., C.I.B., N.F.), Pathology (I.C.T.), Surgery (K.K.), and Endoscopy (B.P.S.) and the Cancer Research UK Colorectal Cancer Unit (S.H., B.P.S., W.A.), St Mark’s Hospital, Northwick Park, Watford Rd, Harrow, Middlesex HA1 3UJ, England; and General Electric Medical Systems, Slough, England (P.R.M.). Received May 10, 2002; revision requested June 14; final revision received February 5, 2003; accepted March 3. Address correspondence to S.H. (e-mail: s.halligan@ic.ac.uk).

View larger version (161K): [in a new window]   Figure 1. Colectomy specimen prior to submersion in 20 L of normal saline solution containing 60 mL diatrizoate meglumine. The Foley catheter (arrow) used for insufflation has been inserted into the ileocecal valve.

View larger version (167K): [in a new window]   Figure 2a. Correlation between the pathologic specimen and the corresponding CT image data. (a) Short section of the sigmoid segment from the colectomy specimen shows a cluster of large polyps (arrows). (b) Endoluminal CT colonographic image of the same sigmoid segment, rendered as an opened and flattened projection (ie, virtual dissection). The same cluster of large polyps is well demonstrated (arrows), allowing polyp-to-polyp correlation with the pathologic specimen.

View larger version (145K): [in a new window]   Figure 2b. Correlation between the pathologic specimen and the corresponding CT image data. (a) Short section of the sigmoid segment from the colectomy specimen shows a cluster of large polyps (arrows). (b) Endoluminal CT colonographic image of the same sigmoid segment, rendered as an opened and flattened projection (ie, virtual dissection). The same cluster of large polyps is well demonstrated (arrows), allowing polyp-to-polyp correlation with the pathologic specimen.

View larger version (32K): [in a new window]   Figure 3. Bar graph shows the detection rate for polyps of 2-4 mm (n = 21) across the 12 CT data sets. All parameters had a significant effect on detection of polyps in this size category.

View larger version (37K): [in a new window]   Figure 4. Bar graph shows the detection rate for polyps of 5-9 mm (n = 11) across the 12 CT data sets.

View larger version (35K): [in a new window]   Figure 5. Bar graph shows the detection rate for polyps of 10-15 mm (n = 6) across the 12 CT data sets. Allowing for perceptual error, there was no difference in the rate of detection across the parameters tested.

View larger version (123K): [in a new window]   Figure 6a. Increased polyp detection with improving CT parameters. (a) Endoluminal colonographic image rendered from a CT data set acquired with a collimation of 2.5 mm, pitch of 6, and tube current of 50 mA. A 4-mm sessile polyp is barely visible (arrow). (b) Endoluminal colonographic image of the same section of colon rendered from a CT data set acquired with a collimation of 1.25 mm, pitch of 3, and tube current of 150 mA. The 4-mm polyp is now well depicted (curved arrow), and an additional 2-mm polyp also is visible (open arrow).

View larger version (127K): [in a new window]   Figure 6b. Increased polyp detection with improving CT parameters. (a) Endoluminal colonographic image rendered from a CT data set acquired with a collimation of 2.5 mm, pitch of 6, and tube current of 50 mA. A 4-mm sessile polyp is barely visible (arrow). (b) Endoluminal colonographic image of the same section of colon rendered from a CT data set acquired with a collimation of 1.25 mm, pitch of 3, and tube current of 150 mA. The 4-mm polyp is now well depicted (curved arrow), and an additional 2-mm polyp also is visible (open arrow).

View larger version (135K): [in a new window]   Figure 7. Endoluminal CT colonographic image of the transverse colonic segment. A 6-mm polyp (arrow) is situated behind a haustral fold. This polyp was missed by the observer during the prospective interpretation but was well depicted in all 12 CT data sets interpreted retrospectively.

View larger version (9K): [in a new window]   Figure 8. Graph shows the effect of collimation on polyp detection according to polyp size. The odds ratio (), which is given with 95% CI, is the odds of detection at a collimation of 1.25 mm in comparison with the baseline odds of detection at a collimation of 2.5 mm. The odds of polyp detection were significantly greater with a 1.25-mm collimation for both polyp groups, although the effect was much larger for polyps of less than 5 mm.

View larger version (10K): [in a new window]   Figure 9. Graph shows the effect of pitch on polyp detection according to polyp size. The odds ratio (), which is given with 95% CI, refers to the odds of detection at a pitch of 3 in comparison with the baseline odds of detection at a pitch of 6. The odds of polyp detection were significantly greater with a pitch of 3 for polyps of less than 5 mm, but no significant effect was observed for detection of polyps of 5 mm or greater.

View larger version (18K): [in a new window]   Figure 10. Graph shows the effect of tube current on polyp detection according to polyp size. The odds ratio (), which is given with 95% CI, refers to the odds of detection with a tube current of 100 or 150 mA in comparison with the baseline odds of detection with 50 mA. The odds of detecting polyps of less than 5 mm were significantly greater with a tube current of 150 mA, but tube current had no significant effect for detection of polyps of 5 mm or greater.

View larger version (35K): [in a new window]   Figure 11. Bar graph shows the conspicuity of polyps of 2-5 mm (n = 26) across the 12 CT data sets. White bars = polyp faintly depicted and possibly present (grade 1), gray bars = polyp fairly well depicted and probably present (grade 2), black bars = polyp very well depicted and definitely present (grade 3). In general, additional polyps detected with improving parameters tended to be grade 1 in conspicuity, whereas previously detected polyps increased in conspicuity grade.

View larger version (38K): [in a new window]   Figure 12. Bar graph shows the conspicuity of polyps of 6-15 mm (n = 12) across the 12 CT data sets. Gray bars = polyp fairly well depicted and probably present (grade 2), black bars = polyp very well depicted and definitely present (grade 3). Practically all polyps of 5 mm or more were rated as very well depicted (grade 3), regardless of the technical parameters used.

View larger version (127K): [in a new window]   Figure 13a. Increased polyp conspicuity with improving CT parameters. (a) Endoluminal colonographic image rendered from a CT data set acquired with a collimation of 2.5 mm, pitch of 6, and tube current of 50 mA. A 3-mm polyp (arrow) on a haustral fold was assigned grade 1 conspicuity (ie, rated as faintly depicted and possibly present). (b) Corresponding endoluminal colonographic image rendered from a CT data set acquired with a collimation of 1.25 mm, pitch of 3, and tube current of 150 mA. On this image, the 3-mm polyp (arrow) was very well depicted and therefore was assigned grade 3 conspicuity.

View larger version (131K): [in a new window]   Figure 13b. Increased polyp conspicuity with improving CT parameters. (a) Endoluminal colonographic image rendered from a CT data set acquired with a collimation of 2.5 mm, pitch of 6, and tube current of 50 mA. A 3-mm polyp (arrow) on a haustral fold was assigned grade 1 conspicuity (ie, rated as faintly depicted and possibly present). (b) Corresponding endoluminal colonographic image rendered from a CT data set acquired with a collimation of 1.25 mm, pitch of 3, and tube current of 150 mA. On this image, the 3-mm polyp (arrow) was very well depicted and therefore was assigned grade 3 conspicuity.

View larger version (14K): [in a new window]   Figure 14. Bar graph shows the average detection rate for polyps of 2-4 mm across the 12 data sets by specimen orientation. The average numbers of polyps detected were five of 11 in the longitudinal orientation, one-fourth of six in the transverse orientation, and two of four in the oblique orientation. These results confirm that performance was poor in the transverse orientation (ie, with the colon parallel to the CT gantry).