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Gastrointestinal Imaging |
1 From the Department of Radiology (114), Veterans Affairs Medical Center, 4150 Clement St, San Francisco, CA 94121 (J.Y., N.N.K., R.K.H., P.R.G.K., S.D.W.); and Departments of Radiology (J.Y., N.N.K., R.K.H., P.R.G.K., S.D.W.) and Gastroenterology (G.A.A.), University of California School of Medicine, San Francisco. Received March 28, 2001; revision requested May 21; final revision received June 25, 2002; accepted July 16. Address correspondence to J.Y. (e-mail: judy.yee@radiology.ucsf.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: CT colonography and colonoscopy were performed in 182 patients. Distention and preparation of eight colonic segments were rated separately on a scale of 1–4 (1, segment completely distended or no residual material; 4, segment collapsed or large amounts of residual material). The distention, preparation, and polyp detection data were compared with regard to each position alone and then in combination. CT findings were correlated with colonoscopic findings.
RESULTS: The percentage of colonic segments with grade 1 distention and preparation was 93.7% (1,364 of 1,456) and 66.6% (969 of 1,456), respectively, with combined scanning; 86.4% (1,258 of 1,456) and 52.1% (759 of 1,456), respectively, with supine scanning alone; and 85.6% (1,246 of 1,456) and 57.1% (831 of 1,456), respectively, with prone scanning alone. The sensitivity for detection of colorectal polyps 10 mm or larger, 5.0–9.9 mm, and smaller than 5 mm and polyps of all sizes was 92.7%, 79.8%, 60.3%, and 69.9%, respectively, with combined scanning. Sensitivity was 58.5%, 47.2%, 36.3%, and 42.1%, respectively, with supine scanning and 51.2%, 41.6%, 30.2%, and 36.3%, respectively, with prone scanning. The improved sensitivities for use of combined versus individual scanning positions were highly significant (P < .001) for polyps in all size categories.
CONCLUSION: Colonic distention and preparation at CT colonography were significantly improved by using supine and prone scanning in combination, and results correlated directly with improved sensitivity of polyp detection.
© RSNA, 2003
Index terms: Colon, CT, 75.12115 • Colon neoplasms, 75.311 • Colonoscopy, 75.128 • Computed tomography (CT), image processing, 75.12117 • Computed tomography (CT), three-dimensional, 75.12115, 75.12117
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The accuracy of CT colonography depends on many factors, including colonic distention, bowel preparation, data acquisition parameters, postprocessing techniques, image display, and image analysis. Each factor contributes to the quality of the examination and to the diagnostic reliability of the test. While many researchers are evaluating optimal scanning protocol, postprocessing techniques, and image analysis tools (8–13), there has been less investigation of optimization of colonic distention and preparation (14–17).
Typically, patients are scanned in both supine and prone positions, since theoretically, regions of the colon that are obscured because of poor colonic distention or residual material (fluid or stool) are depicted by using complementary positions. Suboptimal colonic distention and residual material limit evaluation of the colonic surface at CT colonography and can increase both false-positive and false-negative findings.
To our knowledge, results of only two studies (14,15) have been published in which evaluation of supine and prone scanning at CT colonography was performed. In a retrospective unblinded study, Chen et al (14) evaluated 23 patients and found that combined supine and prone CT scanning improved colonic distention and increased the sensitivity for colorectal polyp detection. However, colonic preparation was not evaluated. In a more recent study, Fletcher et al (15) found that combined scanning had better sensitivity for polyp detection compared with scanning in the supine position alone for polyps with diameters of 5.0–9.9 mm and 10 mm or larger. However, the performance of prone scanning alone and in combination with supine scanning was not evaluated. Quantification of distention and preparation for individual colonic segments was not studied.
The purpose of our study was to compare colonic distention, adequacy of colonic preparation, and colorectal polyp detection as assessed for supine and prone scanning separately and in combination during CT colonography.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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CT colonography was performed first, followed by fiberoptic colonoscopy on the same day. Room air was used to distend the colon and was administered by means of a rectal tube until the patient refused additional insufflation. CT of the abdomen was performed to rapidly assess the degree of colonic distention. If adequate colonic distention had not been achieved, air insufflation was administered again according to patient tolerance. Helical CT data were acquired in the supine and then in the prone positions by using a HiSpeed CT/i scanner (GE Medical Systems, Milwaukee, Wis) with an 800-msec gantry rotation period. During a single breath hold, CT images of the colon were obtained by using 3-mm collimation, 120 kVp, 150 mA, and a pitch of 1.5–2.0. The transverse CT images were reconstructed by using a standard algorithm with 50% overlap.
CT data sets were interpreted by two radiologists (J.Y., R.K.H.) with a dedicated computer workstation (UltraSparc I 170E; Sun Microsystems, Mountain View, Calif). Transverse images were enlarged and evaluated with a viewer program that allowed rapid scrolling through the data set. Reformatted and surface-rendered endoluminal images were produced by using commercially available software (Navigator; GE Medical Systems). The software features a four-quadrant display, which includes the transverse, coronal, and sagittal reformation planes and the endoluminal image. When the endoluminal images are viewed dynamically, the transverse CT images and coronal and sagittal reformatted CT images automatically track to the region of interest as the observer manually maneuvers through images of the colon.
Each radiologist independently interpreted the images obtained in the supine position first, the prone position second, and then both supine and prone images together. Complete two-dimensional (transverse with reformation) and three-dimensional endoluminal interrogation was performed. When there was a discrepancy between the two radiologists, a consensus reading was performed. The readers were blinded to patient history and both colonoscopic and pathologic findings. The initial evaluation consisted of separately grading colonic distention and preparation on the enlarged transverse CT images obtained in the supine position and then in the prone position. The colon was divided into eight segments: cecum, ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, sigmoid colon, and rectum. A total of 1,456 segments (eight segments per patient) were evaluated for each position.
The degree of distention and the quality of preparation were graded by using a numeric scale of 1–4. A segment with a distention score of 1 allowed easy navigation through it by way of endoluminal images (>75% of estimated maximal distention). A score of 2 was assigned when the lumen was 51%–75% distended, and a score of 3 was assigned when the lumen was 25%–50% distended. A score of 4 indicated that the segment was completely collapsed, such that the opposing colonic walls were in contact or the lumen was so small that endoluminal navigation was impossible (<25% of maximal distention). Preparation was scored on a similar scale. Preparation was assessed according to the amount of residual fluid and stool in the segment. Segments with no residual fluid or stool were graded as 1 (0% of the lumen filled with material). Those with small amounts of material were graded as 2 (<25% filled). Segments with moderate amounts of residual material were graded as 3 (25%–50% filled), and those with large amounts were graded as 4 (>50% filled). For long or tortuous segments, the grade was assigned according to the area within the segment with the worst distention or preparation. Any subsegmental area with poor distention and/or preparation can cause a lesion to be overlooked. The "combined" score was obtained by taking into account how much of the colonic luminal surface could be evaluated in a segment on images obtained in both supine and prone positions. An improved combined score occurs when areas within a segment that could not be evaluated well because of residual material or inadequate distention in one position may be evaluated well in the opposing position because of shifting of residual material and/or improved distention.
The total number of segments that had grades 1–4 of distention or preparation at CT colonography were tabulated for the supine and prone positions separately and in combination. At each colonic location, the percentage of segments with each grade of preparation or distention was also calculated. We used strict criteria for what was considered to be optimally distended or prepared—only segments with grade 1 distention or preparation were considered optimal.
Evaluation for colonic polyps was performed by using enlarged transverse images, followed by complete navigation through three-dimensional endoluminal images of the colon from rectum to cecum and back. The maximum diameter and segmental location of all lesions were recorded, as well as whether the particular lesion was observed in the supine or prone position or in both. Interpretation time for each of the two radiologists with grading of segmental distention, preparation, and polyp detection by using both two- and three-dimensional images ranged between 15 and 45 minutes. Polyp size and location recorded during fiberoptic colonoscopy were used as the standard for comparison.
The sensitivities for detection of colorectal polyps 10 mm or larger, 5.0–9.9 mm, and smaller than 5.0 mm and polyps of all sizes were calculated by evaluating images obtained in the supine and prone positions separately and in combination. Colorectal polyps detected during CT colonography were further separated according to location, and the sensitivities at each location were calculated. Sensitivity and specificity were also tabulated per patient for polyps 10 mm or larger, 5.0–9.9 mm, or smaller than 5 mm, and for polyps of all sizes. Positive and negative predictive values were calculated for findings obtained in the supine and prone positions separately and in combination.
A retrospective analysis was performed for false-positive and false-negative findings of polyps in the supine and prone positions separately and in combination. Causes of false-positive findings of polyps were categorized as poor preparation, thickened or bulbous fold, perceptual error, motion artifact, or a combination of any of these. Each false-negative finding for which fiberoptic colonoscopy demonstrated one or more polyps 5.0 mm or larger was retrospectively reviewed. In addition, each individual false-negative finding of a polyp measuring 10 mm or larger was reviewed. The causes for missed polyps were categorized as poor preparation, poor distention, perceptual error, motion artifact, or a combination of any of these.
The statistical significance of differences between polyp detection sensitivities, distention, and preparation for supine and prone positions separately and in combination was determined by using the McNemar test for comparisons according to individual location or polyp size. The Wilcoxon signed-rank test was used for overall colonic comparisons. For determining differences in sensitivity between different colonic segments within a particular scanning position, a Fisher exact test was used to establish statistical significance. For all tests, a finding was considered significant if P < .05. Because multiple observations (segments or polyps) from the same person may not be independent, we repeated the McNemar and Fisher exact tests by using generalized estimating equation methods (18) whenever possible. Because these did not produce any qualitative differences in interpretation, and because the generalized estimating equation of the McNemar test could not be performed for many tests because of zero counts in some cells, these results are not presented. P values from the generalized estimating equation models, when derivable, were slightly larger than those derived with the McNemar test, while they were slightly smaller than those derived with the Fisher exact test.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Sensitivity according to polyp location showed different trends in the prone position. For prone scanning, the hepatic flexure (45.5%, five of 11 polyps, P = .022), sigmoid colon (52.8%, 47 of 89 polyps, P < .001), and rectum (50.0%, 23 of 46 polyps, P < .001) were all areas of relatively higher sensitivity for polyp detection than was the cecum (11.1%, four of 36 polyps). For the prone position, the rectum (50.0%, 23 of 46 polyps) had a significantly higher sensitivity than that of the ascending (25.0%, 10 of 40 polyps, P = .026) or transverse (26.3%, 15 of 57 polyps, P = .015) colon. Also, for the prone position, sensitivity in the sigmoid colon (52.8%, 47 of 89 polyps) was higher than that in the ascending (25%, 10 of 40 polyps, P = .004) and descending (26.9%, seven of 26 polyps, P = .026) colon. The combination of supine and prone scanning showed that sensitivities in the transverse colon (73.7%, 42 of 57 polyps, P = .046), sigmoid colon (73.0%, 65 of 89 polyps, P = .034), and rectum (76.1%, 35 of 46 polyps, P = .037) were all significantly higher than the sensitivity in the descending colon (50.0%, 13 of 26 polyps). Each of these differences was accompanied by corresponding significant differences in the number of segments with grade 1 preparation and/or distention, except for the transverse colon and rectum scanned in the supine position, which were statistically similar in the number of segments with grade 1 preparation and distention.
Sensitivity for detection of patients with polyps was also significantly improved by scanning with both supine and prone positions. Combined scanning had an overall sensitivity of 90.4% (103 of 114 patients) for detection of patients with polyps of all sizes compared with the sensitivities of 64% (73 of 114 patients, P < .001) and 59.7% (68 of 114 patients, P < .001) for supine and prone scanning alone, respectively. The specificity for detection of patients with polyps was 82.4% (56 of 68 patients) for combined scanning and 85.3% (58 of 68 patients, P = .157) for supine scanning. The specificity for detecting patients with polyps with prone scanning was 97.1% (66 of 68 patients), which was significantly higher than that with combined and supine scanning. CT colonography had high positive predictive values: 89.6% (103 of 115 patients) for combined, 88.0% (73 of 84 patients) for supine, and 97.1% (68 of 70 patients) for prone scanning. Negative predictive values were higher for combined (83.6%, 56 of 67 patients) than for supine (58.6%, 58 of 99 patients) or prone (58.9%, 66 of 112 patients) scanning alone.
A detailed retrospective analysis was performed to determine the causes of false-positive and false-negative findings. At review, false-positive findings were believed to result from poor preparation, perceptual error, thick or bulbous haustral folds, motion artifact, or any combination of these (Figs 5, 6). The most common cause of false-positive findings of polyps 10 mm or larger at combined scanning was poor preparation (Table 3). The major cause of false-positive findings of patients with polyps 5 mm or larger at combined scanning was also poor preparation (Table 4). There were only three false-negative findings of polyps measuring 10 mm or larger when using combined scanning: Two resulted from poor distention, and one resulted from motion artifact (Table 5) (Fig 7). There were also only three patients with false-negative findings of polyps 5 mm or larger; two resulted from a combination of poor preparation and distention, and one resulted from a combination of poor preparation, poor distention, and motion artifact (Table 6).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Combined scanning also yielded an overall increase in sensitivity for detection of patients with polyps of any size. In our study, however, there was no improvement in the specificity for combined scanning over supine or prone scanning alone. This is to be expected, given the fact that by increasing the sensitivity by using combined scanning, we are automatically decreasing the specificity, since the number of false-positive findings of polyps seen with combined scanning is the sum of those seen during supine and prone scanning alone.
In a study by Fletcher et al (15), who compared supine scanning alone versus combined supine and prone scanning in 180 patients, an increase of 11.6% in the sensitivity for colorectal polyp detection was found for lesions 10 mm or larger. Additionally, similar to our results, they did not find any improvement in specificity with combined scanning. There are a few notable differences between the two studies. The sensitivity for polyps of 5.0–9.9 mm was 79.8% in our study, compared with their reported sensitivity of 47.2%. In addition, the sensitivity for polyps 10 mm or larger was 92.7% in our study, compared with 75.2% in their study.
Higher polyp detection sensitivities may be related to different technical parameters used in each study, such as our use of 3-mm collimation compared with 5-mm collimation in their study. There were also differences in postprocessing techniques. Fletcher et al (15) used proprietary software with volume rendering, compared with our use of a commercially available software package with surface rendering. Studies are currently being performed to examine the differences in sensitivity with use of different software and rendering methods. Image review was performed by two radiologists in our study, compared with their use of one radiologist to interpret the images obtained with supine scanning and another to read the images obtained with combined scanning. Their method may lead to more missed polyps because only one reader interpreted the images obtained with prone scanning.
One possible criticism of our interpretation method is that use of one radiologist to review the images obtained with supine and prone scanning separately and in combination might lead to a study bias. For example, after having seen a polyp on the image obtained with supine scanning, the reader may look more carefully for the same polyp on the image obtained with prone scanning, or vice versa. This type of bias, however, would only lead to an increase in polyp sensitivity for supine or prone positions alone and would tend to diminish differences seen between images obtained with one or two scanning positions. The finding of substantial improvement with combined scanning despite this interpretation method weighs more heavily in favor of our conclusions.
Although Fletcher et al (15) did not find any difference in polyp detection sensitivities between individual colonic segments, our results indicate that there are distinct areas in the colon in which polyp sensitivity is better than in others. These differences typically correlated with intersegmental differences in distention and/or preparation. Combined scanning resulted in an increased number of segments with grade 1 distention both overall and for all individual segments when compared with supine or prone scanning alone, with the exception of supine scanning of the cecum, ascending colon, and hepatic flexure. This is related to the fact that the percentages of these segments with grade 1 distention scores for supine scanning were already high (96.2%, 96.2%, and 95.6%, respectively), and therefore, the percentages for combined scanning (97.3%, 96.7%, and 96.7%, respectively) were not significantly improved. Combined scanning led to a significant increase in the number of segments with grade 1 preparation, both overall and for each individual location.
In analyzing supine versus prone scanning, we found that supine scanning had significantly higher sensitivity for polyp detection in the cecum and transverse colon. This was correlated with the finding that a significantly larger number of transverse colon segments had grade 1 preparation and distention with supine scanning than with prone scanning, although a similar pattern was not identified for the cecum. Significantly higher sensitivity was found for the sigmoid colon with prone scanning. This was supported by a significantly larger number of sigmoid colonic segments that had grade 1 distention at prone scanning.
Sensitivity differences were not always accompanied by differences in preparation scores. Residual material typically shifts when the patient is repositioned, but the same amount of material may still remain in a particular segment. Thus, many segments continued to be considered poorly prepared because of residual material, although complementary positions enabled visualization of portions of the colonic wall that were previously obscured. As expected, colonic segments that were located posteriorly, including the sigmoid and descending colon, showed significantly better distention with prone scanning, while the transverse colon, which is anteriorly located, showed significantly better distention with supine scanning. The percentage of segments with grade 1 distention was lower in the sigmoid colon than in other segments with both supine and prone scanning, suggesting that additional factors such as passage of bowel gas distally and the known relatively higher incidence of diverticulosis and muscular hypertrophy in the sigmoid colon may play a role in this particular colonic segment. However, these factors were not specifically evaluated in the present study.
Residual material in the colon (fluid and/or stool) was found to be a more common problem than was poor distention, as demonstrated by the high percentage of segments found to be poorly prepared. Even with combined scanning, most colonic segments continued to have a substantial percentage with poor preparation, including the cecum (60.4%), ascending colon (57.1%), transverse colon (46.2%), descending colon (30.8%), and rectum (29.1%). This can be explained in part by the strict definition of optimal preparation in our study as those segments with no residual material (grade 1), because even a small amount of fluid or stool can obscure polyps at CT colonography.
Data acquisition parameters and the presence of artifacts such as those due to motion greatly influence polyp detection sensitivity and specificity. The use of multi–detector row CT scanners should significantly reduce the presence of motion artifacts caused by shorter scan times. At retrospective analysis, most false-negative findings of polyps were due to poor preparation and/or distention. This underscores the importance of combined scanning and highlights the need for investigation of various colonic distention techniques and bowel cleansing agents.
The most common cause of false-positive findings of polyps was poor preparation. Another cause was the presence of thick folds. Optimization of colonic cleansing and methods that allow differentiation of residual stool from a polyp would improve specificity (17). Investigators are currently evaluating the use of fecal-tagging agents and electronic subtraction CT colonography to help distinguish stool from polyps (19–21). An additional area of research is the use of intravenous contrast material to improve the sensitivity and specificity of CT colonography (22). The bowel preparation materials used in our study consisted of magnesium citrate and polyethylene glycol, which are favored by gastroenterologists for fiberoptic colonoscopy. However, this regimen may not be optimum for CT colonography. Use of polyethylene glycol tends to leave more residual fluid in the colon, compared with use of saline cathartics such as sodium phosphate.
In conclusion, CT colonography protocols must include use of two positions, typically supine and prone. CT scanning in two complementary positions improves both colonic segmental distention and preparation, which correlates with significantly increased sensitivities for polyp detection.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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REFERENCES |
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- = supine scanning, -
- = prone scanning, -
- = combined scanning. P values: * = combined versus supine scanning, P < .05; 
= combined versus prone scanning, P < .05; 
= supine versus prone scanning, P < .05.
