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1
Department of Radiology, Division of Abdominal Imaging, New York University
Medical Center, 560 First Ave., Ste. HW 206, New York, NY 10016.
2
Department of Medicine, Division of Gastroenterology, New York University
Medical Center, New York, NY 10016.
Received October 26, 1999;
accepted after revision December 8, 1999.
Address correspondence to M. Macari.
Abstract
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SUBJECTS AND METHODS. Forty-two patients undergoing colonoscopy screening were examined with CT colonography before endoscopy. Data were examined following one of two methods. In method 1, axial 2D data sets were examined in a cine mode. If findings were suggestive of abnormality, focal areas were examined with 3D CT colonography. In method 2, data sets were examined exactly as in method 1, and subsequent to that review, data sets were examined with simultaneous 3D "fly-through" CT colonography (surface-rendered images) and multiplanar reformatted images.
The time required to examine CT colonography using each technique was recorded and abnormal findings were documented. Results of methods 1 and 2 were compared with findings on colonoscopy.
RESULTS. Colonoscopy detected 16 polyps in 13 patients (polyp size, 2-10 mm). Ten polyps measured 5 mm or less, five measured between 6 and 9 mm, and one measured 10 mm or more. Using method 1, two of 10 polyps measuring less than 5 mm, three of five polyps measuring between 6 and 9 mm, and one polyp measuring 10 mm were detected. We noted no false-positive polyps. Average evaluation time was 16 min. With method 2, the same polyps were seen as with method 1. No additional polyps were detected, and the average evaluation time was 40 min.
CONCLUSION. Axial 2D CT colonography can be performed quickly and is comparable with complete 2D and 3D CT colonography in detecting colorectal polyps.
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Currently, researchers are evaluating CT colonography as a method for colorectal cancer screening [8,9,10]. This minimally invasive technique requires only 5-10 min (exclusive of time associated with bowel preparation) and no sedation or analgesia. Preliminary clinical evaluation of CT colonography shows promise in detecting polyps and cancer of the colon and rectum [9,10,11,12,13]. These findings favorably compare with studies that have compared double-contrast barium enema with colonoscopy in detecting lesions of similar size [9].
Offsetting the initial promise of CT colonography for wide-scale screening are technical issues related to data display and the mechanics of interpretation [9, 10]. Few formal studies have addressed the efficient use of two-dimensional (2D) or three-dimensional (3D) image data [12, 14]. We examined the clinical use of time-efficient CT colonography (axial 2D cine review with supplemental 3D imaging for problem solving) and devised a strategy that examines a large number of patients while preserving the inherent sensitivity and specificity.
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All patients underwent a standard bowel preparation prescribed by the individual colonoscopist. Polyethylene-glycol solution (GoLytely; Braintree Laboratories, Braintree, MA) or 45 ml of phospha soda (24-hr Fleet 1 preparation; Fleet Pharmaceuticals, Lynchburg, VA) was used. Patients underwent CT colonography 1 hr before conventional colonoscopy screening.
Immediately before CT colonography, patients were asked to evacuate any residual fluid or fecal material from the rectum. Then, patients were placed on the CT table in the supine position, and 1 mg of glucagon was administered IV to decrease bowel peristalsis and spasm and to facilitate hypotonia. A flexible rubber catheter was inserted into the rectum and the colon was insufflated with room air to patient tolerance (minimum, 40 puffs). The catheter was left in the rectum. A single low-dose scout CT image was obtained to verify adequate bowel distention. If adequate bowel distention was present, the CT examination was performed. If bowel distention was inadequate, additional air was insufflated into the rectum (10 puffs).
CT was performed on a helical HiSpeed Advantage or CTI scanner (General Electric Medical Systems, Milwaukee, WI) at 120 KvP, 150 mAs, using a 5-mm collimation, pitch of 2.0, and 2.5-mm reconstruction interval. Two acquisitions, the first in the supine and the second in the prone position, were performed. Both acquisitions were performed in a single breath-hold (approximately 25-30 sec). CT images were transferred to a remote GE Advantage workstation, equipped with commercial Navigator software (General Electric Medical Systems) capable of performing 2D and 3D data rendering.
Two abdominal radiologists independently examined CT data sets at the workstation. Both radiologists had training in the interpretation of CT colonography. CT colonographic images were examined without knowledge of colonoscopy findings. The CT data sets were examined in one of two ways:
Method 1
Observer 1 examined the axial supine and prone 2D data sets in a cine mode
at the workstation. Speed of review (cine frame rate) was determined by the
reviewer. Three-dimensional threshold-rendered CT colonography was used only
as a problem solver if an abnormality was detected or suspected during axial
2D review. For patients examined with method 1, only the focal area was imaged
with 3D CT colonography.
Method 2
In method 2, the same axial supine and prone data sets were examined by a
different interpreter (observer 2) in a cine mode exactly as that in method 1.
The data was then completely examined using GE Navigator software. GE
Navigator software enables the simultaneous review of 3D CT colonographic
(surface-rendered), coronal and sagittal multiplanar reformatted, and axial
images. Using GE Navigator software, the data sets were examined in both an
antegrade and retrograde fashion. Method 2 was designed as a more thorough
evaluation of the colon because the entire colon was examined with 2D and 3D
techniques.
For purposes of data recording and comparison of CT colonography with conventional colonoscopy, the colon was divided into six segments: the cecum, ascending colon, transverse colon, descending colon, sigmoid, and rectum. The time to complete the colonic evaluation included the time to examine all segments, using both supine and prone techniques, with a sufficient level of confidence that disease (polyp) was present or absent. Each interpreter examined the data in this fashion because positive results (polyp) required follow-up colonoscopy. If no polyp was present, then no further evaluation was necessary. We recorded the time it took for each interpreter to complete the evaluation at the workstation using methods 1 and 2 (excluding time to transfer images to the workstation).
Findings for each patient were documented on separate worksheets. Colonographic data were analyzed for the presence or absence of colonic polyps. The size and location of polyps detected on CT colonography were documented on the basis of anatomic segment location. The CT colonographic data were then compared with findings on conventional colonoscopic images obtained within 24 hr of CT. True-positive findings were noted when CT colonography and conventional colonoscopy depicted lesions in the same anatomic segment. True-negative findings were noted when CT colonography and conventional colonoscopy revealed no abnormalities in the same region. False-positive findings were noted when CT colonography revealed abnormalities that were not present on conventional colonoscopy. False-negative findings were noted when lesions were detected on conventional colonoscopy but not on CT colonography.
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Using method 1, examinations lasted an average of 16 min (range, 10-23 min) and 3D rendering was used in 62% of patients (26/42) to further examine suspected abnormalities depicted during 2D cine review (Figs. 1A,1B,1C,1D,2A,2B,2C,2D,3A,3B). Using method 2, examinations lasted an average of 40 min (range, 28-62 min).
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Colonoscopy detected 16 polyps in 13 patients. The polyps measured between 1 and 5 mm (n = 10), 6 and 9 mm (n = 5), or larger than 10 mm (n = 1). Of 16 polyps, six were prospectively visible on CT colonography. These polyps measured 10, 8, 8, 7, 4, and 4 mm and were identified with methods 1 and 2. All polyps were identified during initial cine review. No additional polyps were identified with 3D threshold-rendered views using method 2.
Ten polyps were overlooked on CT colonography; these polyps measured 2 mm (n = 2), 3 mm (n = 5), 4 mm (n = 1), and 6 mm (n = 2). These polyps were not visible with either method 1 or method 2. One patient had a 6-mm polyp that was overlooked on CT colonography because of the incomplete distention of the colon. The remaining polyps were overlooked on CT colonography because of small size (Fig. 4A,4B).
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We noted no false-positive findings using method 1. However, using method 2, we noted three false-positive findings in one patient. False-positive findings were recorded for 3-4 mm lesions depicted only during the endoluminal evaluation. When correlating the CT colonography and colonoscopy findings for this patient, we determined that the false-positive findings were probably related to residual stool.
Of 16 polyps detected on colonoscopy, two were in the cecum, seven in the ascending colon, two in the transverse colon, and five in the sigmoid colon. On CT colonography, zero of two polyps were visible in the cecum, three of seven in the ascending colon, one of two in the transverse colon, and two of five in the sigmoid colon.
For method 1, the overall sensitivity for polyp detection was 38%, specificity was 100%, positive predictive value was 100%, and negative predictive value was 73%. For method 2, the sensitivity for polyp detection was 38%, specificity was 96%, positive predictive value was 86%, and negative predictive value was 72%. Differences between methods 1 and 2 are attributable to one patient for whom false-positive findings were revealed with method 2 but not with method 1.
Although the overall sensitivity for polyp detection was low, most false-negative CT findings accounted for polyps measuring less than 5 mm. For polyps measuring 6 mm or more, the sensitivity of CT colonography was 67% (4/6). For polyps measuring 7 mm or more, the sensitivity of CT colonography was 100% (4/4).
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Complete fiberoptic colonoscopy allows the most thorough evaluation of the colon with the added benefit of biopsy for suggestive regions. Researchers have noted limitations to the widespread use of fiberoptic colonoscopy, including expense, time, sedation requirements, potential risk of perforation, and failure to complete the examination in up to 10% of patients [17]. Sigmoidoscopy fails to reveal the entire colon; therefore, complete colonic screening is impossible [3], and false-positive findings for fecal occult blood tests of colonic cancer can lead to further testing and expense. One study revealed that more than 50% of occult heme-positive stool findings are from sources in the upper gastrointestinal tract [18]. Presently, despite the availability of colonoscopy and other screening techniques, the morbidity and mortality rates associated with colorectal carcinoma remain high.
Currently, researchers are evaluating CT colonography as an alternative method for polyp detection. Preliminary data from CT colonography testing reveal promise in detecting polyps and cancer of the colon and rectum [9,10,11,12,13]. A major challenge facing radiologists is the development and implementation of user-friendly display programs that insure a complete colonic evaluation, a minimal false-positive rate, and a high negative predictive value (so that the results of a normal examination are sufficient to exclude disease). If the examination is to be disseminated to the radiologic community, the complete evaluation must be performed in a time-efficient manner.
Several basic principles of CT colonography have been established. It is generally accepted that a "clean" colon is essential to avoid misinterpretations. Scanning in the supine and prone positions is necessary to ensure adequate bowel distention [19]. Moreover, by obtaining data sets in both positions, small amounts of residual fecal material may be distinguished from polyps. If a filling defect changes its position during the examination, the finding does not represent a polyp Fig. 1A,1B,1C,1D).
Despite agreement on the benefits of CT colonography, dispute continues on how best to examine CT colonography data [11, 12, 14]. Some advocate evaluating a combination of complete axial 2D and 3D data sets [12]. Others suggest that primary reliance on axial 2D data images, with supplemental 3D images for problem solving, has similar sensitivity to the technically more thorough 2D and 3D evaluation [14]. Our data supports the latter strategy.
In our study, 16 polyps were identified during colonoscopy. Both methods performed similarly in polyp detection and revealed one polyp measuring 1 cm or more (1/1). Neither method was superior for polyps measuring less than 1 cm. The sensitivity of both methods was 60% (3/5) for polyps measuring between 6 and 9 mm and 20% (2/10) for polyps measuring 5 mm or less. However, for polyps 6 mm or larger, the sensitivity of CT colonography was 67% (4/6). Two 6-mm polyps were overlooked, one of which was obscured by incomplete distention despite supine and prone scanning. When the colon is incompletely distended, adequate evaluation is impossible and colonic examinations are incomplete. In one patient, lesions were detected on 3D CT colonography that were not detected on conventional colonoscopy. These lesions were small (<5 mm), they probably represented residual fecal material, and were considered false-positive findings on conventional colonoscopy.
No polyps were detected on 3D endoluminal images that were not detected on initial 2D images. Three-dimensional CT helped to discriminate polyps from prominent haustral folds when differentiation could not be determined on 2D images alone (Fig. 3A,3B). In this study, using method 1, 3D CT colonography was used to supplement initial axial review in 61% of patients. This rate of supplemental 3D review is similar to that of a previous report in which CT colonography was compared with a method similar to the method 1 in our study [14]. In our study, the primary difference between methods 1 and 2 was time. Using axial cine review and supplemental 3D images (when necessary), CT colonography can be performed in a time-efficient manner. The prudent use of 3D imaging is necessary to confirm the morphology of suggestive areas depicted during axial cine review. However, it is likely that continued experience with 2D CT colonography will lead to better discrimination of polyps and folds. The use of supplemental 3D CT colonography will thus be reduced and the time necessary for evaluation will be decreased even more.
Our study had some limitations. It is clear from our study, and others, that many small polyps (<7 mm) are overlooked on CT colonography (2D, 3D, or a combination of techniques) [10, 12, 14]. A recent report on 46 patients undergoing CT colonography and colonoscopy revealed a low sensitivity of CT colonography in detecting not only small polyps, but also larger flat lesions [20]. The results of this study suggest that CT colonography may not be an accurate screening method for colorectal polyps. However, as pointed out in an editorial on this series, it is too early to pass judgement on CT colonography based on one report [21]. Our results suggest that continued evaluation of time-efficient CT colonography is warranted. All imaging techniques that examine the colon overlook small polyps. The exact sensitivity of double-contrast barium enema in detecting polyps in this size range is dependent on the skill of the radiologist. One study revealed a sensitivity of 81% for double-contrast barium enema when compared with colonoscopy in diagnosing polyps measuring at least 10 mm [22]. The sensitivity for smaller polyps was less.
Colonoscopy may also overlook small polyps. In a study by Rex et al. [23], researchers describe disparate detection rates for lesions even when colonoscopy is performed "back-to-back." Using colonoscopy, these researchers reported the rate of overlooking polyps less than 5 mm to be 27%.
Given the limitations of colonic imaging, some researchers have attempted to define a "clinically significant" polyp [2]. Most small polyps are hyperplastic polyps, have no malignant potential, and are clinically insignificant. In fact, even small adenomatous polyps may be clinically insignificant. For adenomatous polyps, particularly polyps measuring less than 1 cm, it takes approximately 10 years to transform into invasive cancer [2, 3].
Another limitation of our study was the small number of total polyps and large polyps included. Our patients were asymptomatic undergoing first-time colonic screening with colonoscopy. Therefore, it was likely that we would find few significant polyps and encounter a large number of examinations with normal findings. The number and size distribution of polyps encountered in our study are similar to previous reports of polyp frequency in asymptomatic patients [24]. The above characterizes a "screening population." Patients with a strong family history or patients with symptoms such as bleeding, change in bowel habits, or pain should be referred directly to fiberoptic colonoscopy.
A final limitation of our study was the relatively small number of patients included. Currently, multiinstitutional trials are underway that may allow sufficiently powerful statistical analysis to indicate the optimal way for radiologists to learn how to best examine the colon with CT.
Future directions include the quick transfer of 2D images into focused 3D images and more robust surface-rendered algorithms to improve detail of detected abnormalities. Multislice scanning, with its ability to approach isotropic voxel display and preserve resolution in any plane, should also improve CT findings for smaller polyps.
In conclusion, 2D axial CT colonography, with supplemental 3D endoluminal CT colonography, provides an adequate interpretation of the CT colonography examination. Moreover, 2D axial CT colonography can be performed quickly, enabling the technique's use for screening purposes.
Acknowledgments
We thank the CT technologists of New York University, including Jennifer
McNew, Tanya Yeargin, Carol Gomez-Garcia, Fred Indiviglia, Elba Cardona,
Warren Hendicks, Debbie Banini, and Emilio Vega.
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M. E. Zalis, J. Perumpillichira, C. D. Frate, and P. F. Hahn CT Colonography: Digital Subtraction Bowel Cleansing with Mucosal Reconstruction— Initial Observations Radiology, March 1, 2003; 226(3): 911 - 917. [Abstract] [Full Text] [PDF]
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E. G. McFarland, T. K. Pilgram, J. A. Brink, R. A. McDermott, C. V. Santillan, P. W. Brady, J. P. Heiken, D. M. Balfe, L. B. Weinstock, E. P. Thyssen, et al. CT Colonography: Multiobserver Diagnostic Performance Radiology, November 1, 2002; 225(2): 380 - 390. [Abstract] [Full Text] [PDF]
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J. Yee CT Screening for Colorectal Cancer RadioGraphics, November 1, 2002; 22(6): 1525 - 1531. [Full Text] [PDF]
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M. Macari, E. J. Bini, X. Xue, A. Milano, S. S. Katz, D. Resnick, H. Chandarana, G. Krinsky, K. Klingenbeck, C. H. Marshall, et al. Colorectal Neoplasms: Prospective Comparison of Thin-Section Low-Dose Multi-Detector Row CT Colonography and Conventional Colonoscopy for Detection Radiology, August 1, 2002; 224(2): 383 - 392. [Abstract] [Full Text] [PDF]
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R. E. van Gelder, H. W. Venema, I. W. O. Serlie, C. Y. Nio, R. M. Determann, C. A. Tipker, F. M. Vos, A. S. Glas, J. F. W. Bartelsman, P. M. M. Bossuyt, et al. CT Colonography at Different Radiation Dose Levels: Feasibility of Dose Reduction Radiology, July 1, 2002; 224(1): 25 - 33. [Abstract] [Full Text]
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P. W. Hung, D. S. Paik, S. Napel, J. Yee, R. B. Jeffrey Jr, A. Steinauer-Gebauer, J. Min, A. Jathavedam, and C. F. Beaulieu Quantification of Distention in CT Colonography: Development and Validation of Three Computer Algorithms Radiology, February 1, 2002; 222(2): 543 - 554. [Abstract] [Full Text] [PDF]
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J. T. Ferrucci Colon Cancer Screening with Virtual Colonoscopy: Promise, Polyps, Politics Am. J. Roentgenol., November 1, 2001; 177(5): 975 - 988. [Full Text] [PDF]
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J. Yee, G. A. Akerkar, R. K. Hung, A. M. Steinauer-Gebauer, S. D. Wall, and K. R. McQuaid Colorectal Neoplasia: Performance Characteristics of CT Colonography for Detection in 300 Patients Radiology, June 1, 2001; 219(3): 685 - 692. [Abstract] [Full Text] [PDF]
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M. Macari and A. J. Megibow Pitfalls of Using Three-Dimensional CT Colonography with Two-Dimensional Imaging Correlation Am. J. Roentgenol., January 1, 2001; 176(1): 137 - 143. [Full Text]
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