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Gastrointestinal Imaging |
1 From the Departments of Radiology (M.M.M., J.B.K., K.R., V.R.) and Gastroenterology (R.J.F., J.B.M.), Beth Israel Deaconess Medical Center, Harvard Medical School, 330 Brookline Ave, Boston MA 02215. Received November 10, 1999; revision requested December 21; revision received February 14, 2000; accepted February 22. Address correspondence to M.M.M. (e-mail: mmorrin@caregroup.harvard.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Two hundred patients underwent CT colonography in both supine and prone positions. A five-point scale was used to assess the effect of contrast enhancement on overall reader confidence and bowel wall conspicuity. Eighty-one patients underwent CT colonography with complete colonoscopic or surgical correlation; diagnostic accuracy was compared in 48 patients who received contrast material and 33 who did not.
RESULTS: Bowel preparation was ideal in 38 (19%) of 200 patients. Enhanced prone CT images had significantly better scores for reader confidence (4.9 ± 0.1 vs 4.6 ± 0.1, P < .005) and bowel wall conspicuity (4.6 ± 0.2 vs 4.2 ± 0.2, P < .005) compared with those of nonenhanced prone images despite no significant difference in bowel distention (3.8 ± 0.2 vs 3.9 ± 0.1, P = .8). Enhancement significantly improved the ability to depict medium (6–9-mm) polyps (75% vs 58%, P < .05). Three large (10–19-mm) polyps were detected only with contrast enhancement; two remained submerged despite dual positioning.
CONCLUSION: The use of intravenously administered contrast material significantly improved reader confidence in the assessment of bowel wall conspicuity and the ability of CT colonography to depict medium polyps in suboptimally prepared colons.
Index terms: Colon, CT, 75.12111, 75.12112, • Colon neoplasms, CT, 75.12111, 75.12112, 75.311 • Colon neoplasms, diagnosis, 75.12111, 75.12112, 75.311 • Computed tomography (CT), contrast media
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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However, before CT colonography can be considered a potential screening alternative to colonoscopy, its ability to depict medium and large polyps must be improved. Currently, the diagnostic accuracy of CT colonography is limited by factors such as poorly prepared colons, which make it difficult to distinguish between stool and mucosal lesions. Also, fluid-filled colons hide submerged polyps, and flat or sessile polyps smaller than 1 cm often can be overlooked on CT images. Distinguishing the colonic wall from luminal contents in patients with poorly prepared colons remains a major problem in the interpretation of CT colonograms. While efforts to tag stool and subtract it from reformatted images are still only in the developmental stages (8), Chen and colleagues (9) have recently demonstrated that scanning the patient in both the supine and prone position improves evaluation of the colon and enhances sensitivity for polyp detection.
In an effort to further improve the diagnostic accuracy of CT colonography, we assessed the potential benefits of combining intravenously administered contrast material with dual positioning. The specific aims of this study were to determine whether the use of contrast material administered during CT colonography improves overall reader confidence in the assessment of the colon, large-bowel wall conspicuity, and diagnostic accuracy in the detection of colorectal polyps and cancers.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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CT Colonographic Technique
All patients underwent standard bowel preparation 24 hours prior to colonoscopy or bowel resection by using either a standard barium enema preparation (Fleet Prep Kit 1; Fleet Pharmaceutical, Lynchburg, Va; n = 165) or a balanced polyethylene glycol solution (GoLYTELY; Braintree Laboratories, Braintree, Mass; n = 35). Patients referred after incomplete colonoscopy underwent CT colonography within 2 hours, as previously described (10). Our first 80 patients received intravenously administered glucagon hydrochloride (Eli Lilly, Indianapolis, Ind; 1 mg) before CT colonography as was our initial practice. We subsequently abandoned this practice because glucagon did not substantially improve colonic distention. As a result, we reserved the use of glucagon for those patients who had severe bowel spasm that limited distention.
Patients were placed in the right lateral decubitus position on the CT table, a 12-F balloon-tipped rectal tube was inserted, and room air was gently insufflated into the colon to patient tolerance. A standard CT scout image was obtained in the supine position to assess the degree of colonic distention. All patients underwent scanning in the supine position and then turned to the prone position for further scanning in which a single breath-hold acquisition was used to examine the entire colon. Prone CT images were obtained in patients who received an intravenous administration of 150 mL ioversol (Optiray 320; Mallinckrodt, St Louis, Mo) at a rate of 3.5 mL/sec after a delay of 45 seconds. Concerns regarding excessive radiation and contrast material exposure precluded the aquisition of contrast material–enhanced images in both the supine and prone positions.
CT was performed by using a Hi-Speed helical CT scanner (GE Medical Systems, Milwaukee, Wis; n = 125), a LightSpeed QX/i scanner (GE Medical Systems; n = 55), or a Somatom Plus 4 helical CT scanner (Siemens Medical Systems, Iselin, NJ; n = 20). Helical CT images were acquired by using the following parameters: 3-mm collimation with a table speed of 6 mm/sec (pitch of 2), 120 mA, 120 kVp, and 512 x 512 matrix. Transverse images were reconstructed at 1.5-mm section overlap. Images were obtained with the multidetector LightSpeed scanner (GE Medical Systems) by using a 2.5–5.0-mm section thickness with a 11.25–15.00 mm/sec table speed per rotation, a high-speed mode with 1.25–2.50-mm image spacing, 200 mA, 120 kVp, and a mean scanning time of 22–30 seconds.
CT data were transferred to a workstation (Advantage Windows; GE Medical Systems) equipped with navigator software, which permitted the radiologist (M.M.M., J.B.K.) to obtain both a multiplanar reformation of the air-distended colon and an endoluminal perspective through the entire distended colonic lumen. Magnified transverse images were viewed in a rapid cine sequence, and three-dimensional surface-shaded endoluminal images were generated in areas of bowel that could not be confidently evaluated by using the magnified transverse sequences alone. Images were viewed by using both soft-tissue windows (level, 70 HU; width, 500 HU) and lung windows (insensitive to contrast enhancement; level, 750 HU; width, 500 HU), as previously described (10).
Data Analysis
Magnified transverse images were evaluated independently by two abdominal radiologists (M.M.M., J.B.K.) who had interpreted CT colonograms for over 18 months. Differences in assessments were resolved by consensus. The colon was divided into 10 anatomic segments: rectum; distal, middle, and proximal sigmoid colon; distal and proximal descending colon; distal and proximal transverse colon; ascending colon; and cecum. Nonenhanced supine images were compared with nonenhanced prone images in the 85 patients who did not receive contrast material, and nonenhanced supine images compared with enhanced prone images in the 115 patients who received contrast material.
Overall bowel preparation in terms of fluid and stool content in the lumen was assessed by using the following five-point scale: 1 was full, 2 was 25%–50% filled, 3 was less than 25%, 4 was less than 25% in noncontiguous bowel segments, and 5 was none. Fluid and stool scores less than or equal to 4 represented adequate preparation and enabled full visualization of the colon, provided that both fluid and stool were displaced with dual positioning.
The degree of colonic distention in all 10 segments was also scored by using the following five-point scale: 1 was collapsed, 2 was poorly visualized, 3 was entire segment visualized but underdistended, 4 was entire segment visualized and well distended, and 5 was overdistended. A distention score of greater than or equal to 3 represented adequate distention. Overall reader confidence in the diagnosis of any colonic abnormality and both overall and segmental bowel wall conspicuity were also assessed by using the following five-point scale for confidence and conspicuity: 1 was none, 2 was poor, 3 was fair, 4 was good, and 5 was excellent.
Among the 81 patients who had complete colonoscopic and surgical correlation, we compared the sensitivity and specificity of CT colonography among those who received contrast material (n = 48) with those who did not (n = 33). The detection of colonic masses (>20 mm), large polyps (10–19 mm), medium polyps (5–9 mm), small polyps (<5 mm), as well as normal colons, was assessed. Results were calculated on a per polyp and per patient basis by using colonoscopic or surgical findings as the standard. Transverse CT colonograms obtained in patients who received contrast material were also assessed by using window settings that were sensitive or insensitive to contrast enhancement as described previously.
Statistical Analysis
A statistician at our institution performed the statistical analysis. As scores for bowel preparation, distention, reader confidence, and bowel wall conspicuity were skewed, paired data were analyzed by using the Wilcoxon signed rank test. Enhanced prone images were compared with nonenhanced prone images by using the Mann-Whitney U test. Differences in sensitivity for the detection of polyps with or without enhancement were assessed by using the McNemar test. Significance was inferred at a confidence level of 5%.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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.001) and overall bowel wall conspicuity (4.2 ± 0.2 vs 3.8 ± 0.1; P = .001). Improvement in bowel wall conspicuity due to the change in position from supine to prone was seen in all bowel segments with the exception of the transverse colon. When prone images were compared with supine images obtained in patients who received contrast material, the combination of prone positioning and contrast enhancement also significantly improved both overall reader confidence in the assessment of the colon (4.9 ± 0.1 vs 3.4 ± 0.2; P = .001) and overall bowel wall conspicuity (4.6 ± 0.2 vs 3.8 ± 0.1; P = .001). As expected, bowel distention was significantly better in the prone position compared with the supine position for both the group that did not receive contrast material (3.9 ± 0.1 vs 3.4 ± 0.1; P = .001) and the group that did (3.8 ± 0.2 vs 3.5 ± 0.1; P = .001). However, when prone images were compared among patients who received contrast material and those who did not, the enhanced images had significantly better scores in terms of reader confidence in the assessment of the colon (4.9 ± 0.1 vs 4.6 ± 0.1; P < .005) and overall bowel wall conspicuity (4.6 ± 0.2 vs 4.2 ± 0.2; P < .005) despite no significant difference in bowel distention (3.8 ± 0.2 vs 3.9 ± 0.1; P = .8) (Fig 2). These results are summarized in Table 1.
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The findings at colonoscopy and/or surgery and the diagnostic accuracy of CT colonography are summarized in Table 2. The overall sensitivity and specificity of CT colonography in revealing large masses and polyps (>10 mm) were 100% (16 of 16) and 90% (18 of 20), respectively; for medium and small polyps, sensitivity was 65% (13 of 20) and 33% (nine of 27), respectively. The addition of contrast enhancement significantly improved the sensitivity in the detection of medium polyps (75% [six of eight] vs 58% [seven of 12]; P < .05, McNemar test). All masses were correctly diagnosed in both the group that received contrast material and the group that did not. There was no significant difference in the detection of either small polyps (31% [five of 16] vs 36% [four of 11]; P > .05, McNemar test) or large polyps (88% [seven of eight] vs 91% [10 of 12]; P > .05, McNemar test).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Unlike conventional colonoscopy, where adherent stool or intraluminal pools of fluid can easily be irrigated or aspirated to reveal the underlying mucosal surface, CT colonography is dependent on proper bowel preparation. Unfortunately, as our study findings demonstrate, dual positioning cannot always overcome the problems of poor bowel preparation. For example, in 26 (13%) of 200 patients, the degree of dependent fluid or stool noted on supine images was such that it did not displace sufficiently when the patients were in the prone position to permit clearer visualization of the entire circumference of the bowel wall.
While suboptimal bowel preparation accounted for the majority of false-negative lesions depicted at CT colonography, inadequate distention and collapsed colonic segments, particularly in the ascending colon, accounted for two of three false-positive masses and two of three false-positive medium polyps. In retrospect, five of these six cases would still have been misinterpreted as polyps despite the intravenous administration of glucagon and contrast material and prone positioning; this observation demonstrates the difficulty in the interpretation of poorly distended colonic segments.
The use of intravenously administered contrast material to enhance the bowel wall during CT imaging is a relatively recent development in gastrointestinal imaging. To our knowledge, Amin et al (13) were the first to describe the use of intravenously administered contrast material during dynamic helical CT of an air-insufflated colon in the detection and staging of four cases of colorectal cancer. While the same group (14) recently published their results in 38 patients with colonic tumors, to our knowledge there are no studies in the current literature that report the value of intravenously administered contrast material in the improvement of colorectal polyp detection at CT colonography. On the basis of the observation that polyps enhance (15) while fecal material does not, techniques involving the use of intravenously administered gadopentetate dimeglumine, which enhances the mucosal-luminal interface, have been used with magnetic resonance colonography (16).
We must acknowledge the possibility that the improved diagnostic accuracy in the group that underwent enhanced CT may reflect a learning curve bias; the majority of enhanced CT colonographic examinations were performed later in the study when the radiologists may have improved on their ability to interpret the images. The fact that intravenously administered contrast material improves diagnostic accuracy suggests that similar results may be obtainable with use of an orally administered contrast agent, which opacifies the fluid feces, in the bowel preparation. Several researchers (8,17) are developing a method for preparation-free virtual colonoscopy, which, if feasible, could eliminate the safety concerns and costs pertinent to the use of intravenously administered contrast material.
It is interesting to note that in our study, the addition of contrast material significantly increased our ability to detect medium and large polyps, but it had no effect on our ability to detect small polyps. Despite the fact that three large polyps that otherwise would not have been detected were correctly identified in the enhanced CT group, the similar accuracy rates in the detection of large polyps between the enhanced and nonenhanced groups is rather disappointing. However, closer analysis reveals that the results probably reflect the limited number of polyps in each group. Furthermore, only one large polyp was missed in both the enhanced and nonenhanced groups. In both situations, the missed polyps were large flat lesions that, despite enhancement, were depicted as adherent to the colonic wall and were therefore interpreted as normal bowel wall.
While larger randomized series are needed to further define the role of contrast material in CT colonography, our results demonstrate that the intravenous administration of contrast material significantly improves reader confidence in the assessment of the colon, improves bowel wall conspicuity, and enhances the detection of medium and large polyps in suboptimally prepared colons, some of which would have been missed despite prone imaging. Our results also demonstrate that imaging of polyps 5 mm or smaller remains beyond the diagnostic limits of enhanced CT colonography and that sessile or flat polyps larger than 10 mm can still be difficult to distinguish from the bowel wall despite contrast enhancement.
Another potential advantage of the use of intravenously administered contrast material at CT colonography is the ability to provide images of the bowel wall, extracolonic tissues, and liver at one setting. While this benefit may have limited relevance in a screening population, it may substantially aid our ability to use CT colonography to stage colorectal cancers by helping us to assess the depth of involvement of a possible mucosa-based lesion; to identify pericolic spread, including lymph nodes; and to detect colorectal liver metastases (14).
In a separate study (18), we found that the use of intravenously administered contrast material during CT colonography proved useful in the assessment of patients suspected of having extensive colorectal cancer; it permitted identification of invasion of pericolic fat planes, adjacent organs, and metastatic spread to distant sites such as the liver. Of note, all 13 patients in whom cancer was correctly staged at CT colonography received contrast material, while none of the three patients in whom the degree of transmural spread of colorectal cancer was incorrectly staged received contrast material. Furthermore, incidental liver lesions such as cysts or hemangiomas are commonly noted at CT colonography. Unfortunately, this finding often results in further radiologic work-up including formal contrast-enhanced CT. The use of intravenously administered contrast material at the initial CT examination may help us to avoid additional work-up in some cases.
The main limitations of the intravenous administration contrast material are the additional costs (estimated to be $60–$70 per patient with nonionic contrast material), as well as the increased risk of reactions (reported to occur in approximately 3%–4% of patients) although the incidence of notable reactions is probably closer to 0.5% (19,20). Fortunately, none of our patients experienced contrast material–related reactions, and it is worth noting that the incidence of sedation-related adverse events at colonoscopy is substantial (21).
In conclusion, our findings demonstrate that dual positioning increases overall reader confidence in the assessment of the colon, bowel wall conspicuity, and diagnostic accuracy of CT colonography. This finding reflects both the ability of dual positioning to displace fluid and stool in a large proportion of patients with poor bowel preparation and improved bowel distention in the prone position. In addition, we demonstrated that intravenous administration of contrast material significantly improves reader confidence and bowel wall conspicuity. This enhancement of the mucosal-luminal interface is of particular clinical importance in patients with suboptimally prepared colons; a substantial proportion of our patients comprised these patients.
We demonstrated that the use of intravenously administered contrast material significantly improves the evaluation of the colon and the diagnostic accuracy for the detection of medium and, most likely, large polyps. One potential approach in the use of intravenously administered contrast material at CT colonography would be to administer contrast material during prone positioning of patients who have suboptimally prepared colons, as depicted during initial scanning in the supine position. According to our study findings, this practice would eliminate the use of contrast material in more than 85% of patients. However, large prospective clinical studies are required to validate this approach.
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FOOTNOTES |
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This paper was Awarded the 1999 RSNA Trainee Research Award: Gastrointestinal Section
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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