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Gastrointestinal Imaging |
1 From the Departments of Diagnostic Radiology, East 2 (M.R.C., C.D.J., J.G.F., L.A.W., K.E.C.), Information Services (J.E.R.), Gastroenterology and Internal Medicine (D.A.A.), and Biostatistics (W.S.H.), Mayo Clinic, 200 First St SW, Rochester, MN 55905; and Medefield, Sydney, Australia (K.T.). Received July 6, 2000; revision requested August 19; revision received November 6; accepted December 12. Supported by National Institutes of Health grant R01-CA75333. Address correspondence to C.D.J. K.T. is an employee of Medefield Pty, the manufacturer of the stool marker material.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Fifty-six patients with suspected or known polyps were assigned to five groups. Two to seven doses of 225 mL of dilute contrast material were orally administered during 24 or 48 hours. Transverse CT images were assessed for effectiveness of stool labeling. Colonoscopy was performed in all patients and was the standard. Two radiologists blinded to prior imaging and colonoscopic results assessed polyp detection.
RESULTS: For each group, average stool labeling scores and ranges were as follows: 24 hour two dose, 16% and 8%–21%; 24 hour five dose, 53% and 27%–66%; 48 hour four dose, 38% and 22%–48%; 48 hour six dose, 68% and 54%–77%; and 48 hour seven dose, 88% and 75%–98%. Sensitivity for the two radiologists for the identification of patients with polyps 1 cm or larger for each group was as follows: 24 hour two dose, 50% and 67%; 24 hour five dose, 100% and 100%; 48 hour four dose, 58% and 75%; 48 hour six dose, 56% and 67%; and 48 hour seven dose, 100% and 80%.
CONCLUSION: Ingestion of contrast material adequately labels stool for lesion identification; a 48-hour lead time and multiple doses of contrast material are required. Sensitivity for polyp detection in patients with adequate stool labeling approaches the sensitivity for polyp detection in prepared colons.
Index terms: Colon, CT, 75.12115, 75.12143, 75.1283 • Colon neoplasms, 75.30 • Computed tomography (CT), contrast media, 75.12115, 75.12143, 75.1283
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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Elimination of the onerous bowel preparation has the potential to dramatically enhance patient compliance for colorectal cancer screening (11). CT colonography represents a structural approach to colorectal cancer screening that may obviate cathartic preparation. In the near term, identification of stool on the CT colonographic images will likely require ingestion of a contrast agent to label the stool. Sheppard and co-workers (12) recently reported the use of an in vitro animal model for CT colonography with an admixture of barium and stool. This approach yielded 94% sensitivity and 80% specificity for simulated polyps larger than 3 mm. Weishaupt and co-workers (13), using magnetic resonance (MR) imaging, examined two patients who had ingested meals containing lactulose, simethicone, and gadolinium tetraazacyclododecanetetraacetic acid prior to the examination. Although these patients did not have polyps, the signal from this labeled stool was homogeneous and indistinguishable from that obtained with gadolinium tetraazacyclododecanetetraacetic acid enema required in MR colonographic examinations. These findings suggest that it is possible to effectively label stool and detect polyps in the unprepared colon. To our knowledge, to date, practical and effective stool labeling regimens have not been developed, and no sensitivity data exists for polyp detection in these patients.
In the conventionally prepared colon, detection of colorectal polyps 1 cm or larger at CT colonography has ranged from 75% to 91%, with specificities ranging from 86% to 96% (14–18). With this technique, volumetric helical CT data of the abdomen and pelvis are acquired after insufflation of the colon, and, by using specialized imaging software, both two- and three-dimensional endoluminal images are reviewed for interpretation. Potentially, stool labeled with dilute barium sulfate will be of high enough attenuation to be clearly differentiated from soft tissue and polyps; it is not known whether virtual preparation will affect the performance characteristics of CT colonography. The presence of contrast material–labeled stool will affect the sensitivity and specificity of CT colonography for the detection of colorectal polyps and cancers.
The purpose of this study was to evaluate the feasibility of CT colonography of the unprepared colon, with two goals: (a) to develop an effective method of labeling stool with contrast material in the unprepared colon and (b) to assess the sensitivity for polyp detection in the unprepared colon containing barium contrast–labeled stool. To our knowledge, this study represents the first human study of CT colonography with an unprepared colon.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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Patients were referred to the study on the basis of a prior barium enema examination (37 patients), a flexible sigmoidoscopic examination (17 patients), or because of a prior history of colon polyps (two patients). In the 54 patients who underwent a prior examination (barium enema, sigmoidoscopy), the number of days between the prior study and the CT colonographic examination ranged from 3 to 62 days (mean, 13 days ± 13). Selection criteria included age greater than 18 years, the ability to give written consent, and a known or highly suspected polyp 1 cm or larger. Exclusion criteria included the presence of a colostomy, inflammatory bowel disease, acute diverticulitis, pregnancy, polypectomy in the past 6 weeks, or severe claustrophobia. Informed consent was obtained from all patients included in this study, and the protocol was approved by the institutional review board.
The CT examinations were performed with a multi–detector row CT scanner (Lightspeed Qxi; GE Medical Systems, Milwaukee, Wis) with a section thickness of 5 mm, table speed of 15 mm/sec, a reconstruction interval of 3 mm, field of view to fit (range, 35–49 cm2), 50 mA, 120 kVp, and usually one 20-second breath hold (two breath holds were uncommonly used for tall patients), with the acquisition of approximately 150 transverse images per examination. These technical parameters were chosen to match those used in previous efforts (19) with a single-section spiral CT scanner; the technique values for the multi–detector row CT used in this study (120 kVp, 50 mA) were chosen so that the image noise would match that found with the single-section technique (120 kVp, 70 mA).
Glucagon (Eli Lilly, Indianapolis, Ind; E-Z-Em, Westbury, NY; 1 mg) was administered subcutaneously to nondiabetic patients 10 minutes prior to the examination; insufflation of the colon was performed by using approximately 2 L of carbon dioxide (to patient tolerance without pain). Adequacy of colonic distention was determined by reviewing the anteroposterior scout image. Images of patients in both prone and supine positions were acquired. Colonoscopy was performed in all patients after subsequent full preparation, which served as the standard for polyp detection. The number of days between the CT colonographic examination and the colonoscopic examination ranged from 0 to 48 days (mean, 4 days ± 9).
Image Analysis
Image processing was performed with a workstation (Ultra2; Sun Microsystems, Mountain View, Calif) and customized computer software developed at our institution, as described in earlier reports (20–22), and that was specifically developed for this effort. The data were examined by using the transverse CT images with concurrent display of both two- and three-dimensional reformatted images. For this study, each pixel in each transverse section with a value more than the threshold of 150 HU was electronically labeled (purple). This threshold was arbitrarily chosen as a value that allowed identification of barium contrast-labeled stool with only negligible labeling of soft tissues.
To quantify the effectiveness of labeling the stool, a scoring system was established. Each transverse image was electronically labeled as described earlier, and the colon was then examined and scored in six segments: cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum. Each segment of the colon was given a visual subjective score of 0%, 25%, 50%, 75%, or 100% for the effectiveness of stool labeling, with a score of 0% assigned for stool labeling equal to background (unlabeled stool) and a score of 100% assigned for complete homogeneous labeling of stool. Examples of scoring for each quartile are shown in Figure 3. Two radiologists (C.D.J., J.G.F.) separately reviewed the transverse images of each patient by using the electronic threshold tool (with 150 HU as the threshold value), and the transverse section location of each identified polyp and polyp size were recorded without the radiologists having knowledge of prior imaging or colonoscopic results. These radiologists were aware of the entrance criteria for this study.
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values were calculated for the two reviewers. The result of the colonoscopic examination was used as the standard for the assessment of the presence of polyps. |
RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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Polyp Detection
Thirty-seven of the 56 patients had at least one colorectal polyp 1 cm or larger (Table). The sensitivities for polyp detection in the various groups for these polyps for the two reviewers blinded to the suspected location of polyps or cancers and to the results of subsequent colonoscopy are listed in the Table.
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Because of the study design, few patients had a negative colonoscopic finding. Contained in the Table I is the specificity for the two reviewers for each of the study groups. Because none of the patients in the 24-hour two-dose group had a negative colonoscopic finding, no specificity could be calculated. For the other four groups, a total of 19 patients did not have a polyp 1 cm or larger. The corresponding specificities ranged from 40% to 83% for these four groups for the two reviewers. All false-positive findings of polyps 1 cm or larger in this study were the result of misidentification of unlabeled stool as a polyp. As can be seen in the Table, the total number of false-positive findings of polyps 1 cm or larger ranged from 0 to 2 in each group.
The measure of agreement between the two reviewers for the sensitivity for polyp detection yielded a value of 0.54 and a percentage agreement of 77% (44 of 57 polyps). In the assessment of patients, the value for sensitivity was 0.41 (76% agreement, 28 of 37 patients) and for specificity, the value was 0.36 (74% agreement, 14 of 19 patients).
Figures 6 and 7 show magnified transverse CT images of polyps adjacent to labeled stool. In these images, all tissue with attenuation values more than 150 HU, including stool containing barium sulfate and bone, is electronically labeled purple. Polyps can be readily distinguished from adjacent stool when the stool is well labeled with barium sulfate.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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CT colonography can potentially affect two of these compliance barriers: preparation and examination inconvenience and/or discomfort. CT colonographic examination is considered more comfortable than the other colorectal screening tests because data acquisition lasts only a few seconds, and interpretation is performed with the data collected rather than with further patient examination. To date, CT colonography requires thorough purgation of the colon for accurate examination. By removing the inconvenience and discomfort of bowel preparation, a major compliance obstacle is eliminated. Such an advance could have a major effect on population screening rates.
In this study, we addressed two important issues necessary for the development of a useful full structural colorectal examination in an unprepared colon: stool labeling and polyp detection. Similar to the need for thorough cleansing in the prepared colon is the requirement for optimal stool labeling in the unprepared colon. This study demonstrates that, without altering diet or colonic transit time, 48-hour preparation is required when dilute barium sulfate is used as an oral marker. The effectiveness of stool labeling is related to the amount of barium administered during the 48-hour period. Overall, stool tagging was improved from an average of 38% with a 48-hour four-dose schedule (10.6 g barium sulfate) compared with 68% with a 48-hour six-dose schedule (16 g barium sulfate).
Adequate stool labeling is important to allow adequate polyp detection. In our study of the unprepared colon, two reviewers correctly identified 80% and 100% of patients with polyps 1 cm or larger in the 48-hour seven-dose group, which had the highest average stool labeling score throughout the colon. These sensitivities compare favorably with reported (14–18) sensitivities of 85%–100% in the prepared colon. Identification of polyps 1 cm or larger in the group that received the least amount of oral contrast material in this study, 24-hour two-dose group, was suboptimal, with polyp detection sensitivities of 18% and 55%. In groups with intermediate labeling, the 24-hour five-dose, 48-hour four-dose, and 48-hour six-dose groups, polyp detection sensitivity ranged from 50% to 67% for the two reviewers. The total number of false-positive findings of polyps 1 cm or larger in this study is low, with no significant difference in the number of false-positive findings of polyps 1 cm or larger between the poorly labeled and the well-labeled groups.
The lack of a difference in the number of false-positive findings in polyps 1 cm or larger between the groups may be because the number of patients in this study was insufficient for us to discern the difference but also because stool has CT characteristics that are different from those of a polyp. While a polyp usually has a smooth margin and is homogeneous, stool often has an irregular margin and contains air and heterogeneous internal density. Despite the entrance criteria of prior imaging or sigmoidoscopy, with evidence of a polyp 1 cm or larger, 19 patients did not have a polyp 1 cm or larger when examined with colonoscopy. Although the radiologists who participated in this study had a high expectation that each patient would have at least one polyp 1 cm or larger, the specificity for the two reviewers, who had no knowledge of the colonoscopic results, ranged from 67% to 84% among the 48-hour groups.
Further optimization of stool labeling is required to effectively label all stool throughout the colon. It is possible that further improvement in stool tagging could be achieved with higher doses of barium administered during the same period. A formal assessment of patient preferences and compliance with use of the oral marker was not performed. However, anecdotally, the vast majority of patients did not complain about the liquid barium. Those who had undergone preparation for a colon examination in the past commented about the ease of this examination. Further evaluation is necessary to determine the expected effect of obviation of colonic preparation on patient compliance with colon cancer screening. It will also be important to determine whether the ingestion of multiple doses of dilute oral contrast material during 2 days results in complications, such as constipation, diarrhea, or allergic reactions. Evaluation of a larger group of patients in a screening population with optimal stool labeling is necessary to determine the sensitivity and specificity in the target population.
Effective stool labeling with barium sulfate allows identification and electronic labeling of stool. However, evaluation of the colon is potentially difficult when a large amount of stool is present in the colon. Although not investigated in this study, electronic removal of well-labeled stool is potentially feasible. This electronic purgation could potentially reduce interpretation time and improve polyp detection. This area of development is worthy of future investigation but will require considerable technical expertise, since the colon wall and lesions could be easily removed in the stool subtraction process. It is possible that a combination of stool labeling with barium sulfate and a mild laxative would provide higher sensitivity and specificity for polyp detection, particularly in the sigmoid colon and rectum.
In conclusion, stool labeling with a dilute barium suspension is feasible but requires oral administration of contrast material during 48 hours, with multiple intervening administrations. Detection rates for polyps 1 cm or larger in patients with well-labeled stool approach the published detection rates in patients with cleansed colons. Further investigation of this promising technique is warranted.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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