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Gastrointestinal Imaging |
1 From the Department of Radiology, Division of Abdominal Imaging (S.L., M.M.), and Department of Medicine (E.J.B.), NYU Medical Center, Tisch Hospital, 560 First Ave, Suite HW 207, New York, NY 10016. Received June 12, 2003; revision requested August 26; revision received September 17; accepted October 21. Address correspondence to M.M. (e-mail: michael.macari@med.nyu.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: From December 2001 to March 2003, 113 patients underwent computed tomographic (CT) colonography prior to colonoscopy. For all confirmed polyps that were 5 mm and larger, images obtained with CT colonography were retrospectively analyzed by one author to determine if the polyp was present on both data sets or on only one data set. Retrospective evaluation of these polyps for ventral or dorsal location within the colonic lumen was performed for data sets obtained with patients in the prone and the supine position. The data sets were further reviewed by another author to determine the cause of positional change, when present.
RESULTS: Twenty-six patients had a total of 49 histologically proved colorectal polyps that were 5 mm and larger. Eight of 49 colorectal polyps were depicted only on images obtained with the patient in the supine or prone position. Of the remaining 41 polyps that were depicted on images obtained with the patient in the supine and the prone position, 11 moved from a dorsal to a ventral location or vice versa relative to the colonic surface when the patient changed position. Five of these polyps were pedunculated on a stalk. Six were sessile; two were located in the sigmoid colon, two in the transverse colon, one in the ascending colon, and one in the cecum. In these cases, polyp mobility was related to positional changes of the colon in the mesentery, as opposed to true mobility of the polyp.
CONCLUSION: In this series, 27% of polyps moved from a ventral location to a dorsal location relative to the colonic surface when the patient was turned from the supine to the prone position; thus, polyps appeared to be mobile. Thus, a mobile filling defect cannot be assumed to be residual fecal material at CT colonography.
© RSNA, 2004
Index terms: Colon, CT, 75.1282 • Colon neoplasms, 75.311 • Colonoscopy, 75.1282
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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There are three observations that are made with two- and three-dimensional imaging that help distinguish residual fecal material from polyps. The first observation is the presence of internal gas or areas of high attenuation. This finding indicates a lesion is residual fecal material, since colorectal polyps are homogeneous in attenuation (1,2). The second observation is morphology. Polyps and small cancers have rounded or lobulated smooth borders. Residual fecal material may have a similar morphology; however, if a lesion shows geometric or irregular angulated borders, it is residual fecal material (3). The third observation is mobility of a lesion. Stool tends to move to the dependent surface of the colonic mucosa when a patient is turned from the supine to the prone position (2–4). Polyps reportedly maintain their ventral or dorsal position with respect to the bowel surface, regardless of patient position.
Lesion mobility, as described with analysis of data sets obtained with patients in the prone and the supine position, has been helpful in distinguishing fecal material from colorectal polyps. In one study, the addition of imaging patients in the prone position decreased the incidence of false-positive diagnoses caused by the presence of stool by 83% when compared with images obtained with patients in the supine position alone (5). The reason for this is that stool changed position when patients were turned from the supine to the prone position. We have noted, however, that there are several limitations to the use of mobility as a criterion for differentiation of polyps and fecal material. Some colorectal polyps are detected with only one view because of inadequate distention or the presence of residual fecal material in certain colonic segments (6). Occasionally, fecal material can be adherent to the colon wall and does not move. Finally, lesions that appear to be mobile may be polyps.
The purpose of this study was to determine the frequency with which polyps change positions with respect to the bowel surface and to determine the cause of this movement.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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On the day prior to initiation of the study, bowel preparation with either oral hydration with two 45-mL doses of phosphosoda (Prep 1; Fleet Pharmaceuticals, Lynchburg, Va) or 4 L of polyethylene glycol electrolyte solution (Golytely; Braintree Laboratories, Braintree, Mass) was administered. Immediately before CT colonography, the patient was asked to evacuate any residual fecal material from the rectum.
Procedures
CT colonography was performed with a Plus 4 Volume Zoom multi–detector row CT system (Siemens Medical Systems, Forcheim, Germany). No bowel relaxant was used in this study. A flexible rubber catheter was inserted into the rectum, and the colon was insufflated with room air to patient tolerance (minimum of 40 puffs). The catheter was left in the rectum, and a scout image was obtained to verify adequate bowel distention. If bowel distention was adequate, the CT examination was performed. If bowel distention was not adequate, additional air was insufflated into the rectum. Two examinations, the first with the patient in the supine position and the second with the patient in the prone position, were performed. Several additional puffs of air were administered before the second examination. CT parameters included 4 x 1-mm detector configuration, 120 kV, 0.5-second gantry rotation, and 50 mAs (effective). The beam pitch varied from 1.5 to 1.8 to enable anatomic coverage of the entire colon in a single breath hold. CT images were reconstructed as 1.25-mm-thick sections, with a 1-mm reconstruction interval. CT colonography examinations were interpreted by using a Vitrea 2 workstation (Vital Images, Plymouth, Minn). Colonoscopy was performed by a board-certified gastroenterologist with 7 years of experience in colonoscopy (E.J.B.) or a senior gastroenterology fellow who was supervised by the gastroenterologist.
Interpretations and Comparisons
For both CT colonography and colonoscopy, the colon was divided into the following six segments: the cecum; the ascending, transverse, descending, and sigmoid colon; and the rectum. In addition, the size (measured in millimeters) and morphology (flat, sessile, or pedunculated) of all detected lesions were recorded for both sets of images. All lesions detected with CT colonography and endoscopy were imaged. Lesions detected with CT colonography were saved from the workstation as PowerPoint files (Microsoft, Redmond, Wash), and polyps detected with colonoscopy were photographed endoscopically. On a weekly basis, the images obtained with the two examinations were compared. Colonoscopy was used as the reference standard for determination of the presence of colorectal polyps. In cases where a polyp was detected with both examinations in the same segment and had similar morphology but a slight (within 4 mm) discrepancy in size, the lesion was considered a match. In these cases, the radiologist and gastroenterologist reviewed both endoscopic and CT colonographic data to determine the most appropriate size of the polyp.
For all confirmed polyps that were 5 mm and larger, CT colonographic images were retrospectively analyzed at the workstation by a senior medical student (S.L.) with 6 months of experience in CT colonography data interpretation. Data were evaluated to determine if the polyp was present on both data sets or if it was present on only one. In cases where the polyp was present on only one data set, the reason for this discrepancy was evaluated by a radiologist (M.M.) with 5 years of experience in CT colonography interpretation. In cases where the polyp was present on both data sets, the data were evaluated (S.L.) to determine if the location of the polyp relative to the colonic surface changed when the patient was turned from the supine to the prone position. Positional changes were defined as movement of the polyp from the dorsal to the ventral wall of the colonic lumen (with gravity) or from the ventral to the dorsal wall of the colonic lumen (against gravity) when the patient was moved from the supine to the prone position and vice versa. The findings were then reviewed (M.M.) to determine the cause of positional change. When a positional change was noted, the polyp was evaluated to determine if it was pedunculated or sessile. In addition, the segment of the colon in which the polyp was present was evaluated. Pedunculated polyps were presumed to show mobility because of the stalk and changes with gravity. Mobile sessile lesions were assumed to be in segments of the colon that were on a long mesentery.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Thirty (73%) of the 41 polyps detected on both data sets did not change position relative to the colonic surface. Eleven (27%) of the 41 polyps detected on data sets obtained with patients in the supine or prone position changed location relative to the colonic lumen when the patient was turned from the supine to the prone position. Nine polyps changed with gravity—moving from the dorsal to the ventral surface of the colon lumen—when the patient was turned from the supine to the prone position, and two polyps changed against gravity—moving from the ventral to the dorsal surface of the colon lumen. Of the 11 polyps that changed position, five were pedunculated on a stalk. The mean size of these polyps was 17.2 mm (range, 11–27 mm). Three of these lesions were located in the sigmoid colon, and two were located in the descending colon. (Figs 1, 2). In all five cases, movement of the patient from the supine to the prone position resulted in repositioning of the head of the polyp to the dependent surface of the colon.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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It has been observed that residual fecal material is located on the dependent surface of the bowel wall when the patient is turned from the supine to the prone position; thus, residual fecal material is mobile (1). Moreover, colorectal polyps are attached to the colonic wall and are not expected to change position when the patient is turned. In this study, we evaluated the reliability of this criterion to aid in the differentiation of polyps and fecal material. Of the 41 polyps larger than 5 mm that were present on data sets obtained with patients in both the supine and the prone position, 11 (27%) changed position relative to the colonic lumen when the patient was turned. Five (45%) of these polyps were pedunculated; the remaining six (65%) were sessile.
There are two causes for these positional changes. With regard to pedunculated polyps, the head of the polyp is inherently mobile, since it is usually on a long thin stalk. When the patient is turned, the bulbous head of the polyp moves with gravity and changes its relative position. Identification of the morphology of these polyps (head and stalk) usually allows one to recognize these mobile lesions as true polyps and not as mobile fecal material. A previous report in which CT colonography was compared with colonoscopy, however, showed that a large pedunculated colorectal polyp was misinterpreted as residual fecal material secondary to apparent mobility (8). Occasionally, the morphology of large lesions can be difficult to assess. Recognition of polyp morphology is facilitated by the use of coronal and sagittal multiplanar reformatted images, endoluminal views, and transverse images. Moreover, recognition of the morphology of colorectal polyps may be aided by using thin-section CT techniques (3).
With regard to sessile polyps, apparent mobility can be attributed to rotation of the colon on its mesentery within the peritoneal cavity when the patient is turned. This is true not only for sessile lesions but also for pedunculated lesions on a stalk. It is possible that some of the pedunculated lesions in this study may have appeared mobile as a result of colon mobility and not as a result of gravitational changes because of the stalk. The colon is not a fixed structure, and positional abnormalities are common (9–14). The rectum, descending colon, and ascending colon are located in the extraperitoneal space. The sigmoid colon, transverse colon, and cecum are located in the peritoneal cavity and may be on a long mesentery, allowing these segments of colon to rotate on the mesentery. In addition, variation in the position of the ascending colon is not uncommon, and portions of the ascending colon are frequently mobile (13).
In this study, five of the six mobile sessile polyps were located in a colonic segment that is within the peritoneal cavity. Two of these sessile lesions were found in the sigmoid colon. The ability of the sigmoid colon to twist within the peritoneal cavity is further demonstrated by the fact that it is the segment of the colon in which volvulus most frequently occurs (12). Two additional mobile sessile lesions were found in the transverse colon. A long mesentery also allows the transverse colon to rotate within the peritoneal cavity. The cecum, which is the second most common site of volvulus, is completely enveloped by peritoneum and is freely mobile in the abdomen of most individuals (12). This is recognized in a study that showed that only 35% of appendices are located within 5 cm of the McBurney point (13).
In the current study, no sessile lesion changed position when located in the descending colon or rectum. These segments of the colon tend to be fixed in the extraperitoneal space; therefore, polyps in these segments would not be expected to be mobile. However, we have demonstrated a potential pitfall of imaging patients in the supine and the prone position in the characterization of lesions detected with CT colonography on the basis of mobility alone. Despite this potential limitation, the importance of imaging patients in both the supine and the prone position can not be overemphasized. These positions allow redistribution of residual fecal material and gas so that segments that are filled with fecal material or not well distended at one examination will likely be dry and distended at the other. This facilitates complete visualization of the luminal surface of the colon. Moreover, when a lesion is depicted with CT colonography, all the imaging characteristics that are present—including segmental location, morphology, and attenuation—usually allow for the differentiation of fecal material from polyps.
There are some limitations to this study. First, this was a retrospective evaluation, and a relatively small number of polyps were visualized on both data sets. Of the 41 polyps that were seen on both data sets, however, 11 (27%) demonstrated mobility. This is a relatively large percentage and underscores the importance of complete evaluation and characterization of any lesion detected with CT colonography. Second, careful evaluation of the pedunculated lesions in this series did allow correct prospective characterization by an experienced reader. Moreover, of the six sessile lesions that moved, two remained on the nondependent surface during both examinations, which is a clue to the correct diagnosis. These lesions, although mobile, are very unlikely to represent fecal material. While they could represent adherent fecal material, this observation should strongly suggest the diagnosis of mobile polyp. The remaining four sessile lesions moved to the dependent surface when the patient changed position, which suggests that they are fecal material.
In conclusion, radiologists who interpret CT colonography data sets should be aware of the limitations of mobility as a sole criterion to differentiate stool and polyps. Pedunculated polyps and sessile polyps in segments of the colon that are mobile may mimic stool by moving to the dependent surface when patients are repositioned. Whenever a filling defect is seen in the colon, all its characteristics—including mobility, attenuation, and morphology—need to be assessed before a determination of fecal material or polyp is rendered.
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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 L Bogoni, P Cathier, M Dundar, A Jerebko, S Lakare, J Liang, S Periaswamy, M E Baker, and M Macari Computer-aided detection (CAD) for CT colonography: a tool to address a growing need Br. J. Radiol., January 1, 2005; 78(suppl_1): S57 - S62. [Abstract] [Full Text] [PDF]
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