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Clinical Observations |
1 Department of Radiology and Research Institute of Radiology, University of
Ulsan College of Medicine, Asan Medical Center, 388-1 Poongnap-Dong,
Songpa-Gu, Seoul, South Korea, 138-040.
2 Department of Internal Medicine, University of Ulsan College of Medicine, Asan
Medical Center, Seoul, South Korea.
Received December 13, 2004;
accepted after revision May 29, 2005.
Address correspondence to H. K. Ha
(hkha{at}amc.seoul.kr).
Abstract
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CONCLUSION. Less than 50% of flat lesions in our population could be visualized by blinded and unblinded review. Before they could be visualized, flat lesions were 2 mm or greater in height and 7 mm or greater in diameter. Lesions with a height of 1 mm or less were not seen on CT colonography. Contrast enhancement, location on a haustral fold, and abnormal 2D and 3D morphology contributed to lesion conspicuity.
Keywords: colon • colonography • colonoscopy • CT
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Patient Population
Two hundred and thirteen consecutive patients underwent colonoscopy at our
institution during the period between January 2004 and July 2004 with the
following indications: patients referred for newly diagnosed colon cancer
(n = 180) or suspected of having colon cancer (n = 20),
routine screening (n = 5), and others (n = 8). Of those
patients, 32 flat colonic polyps were found in 21 patients. Six of the 21
patients did not undergo CT colonography, and five underwent CT colonography
after polypectomy. The remaining 10 patients, between 49 and 74 years old
(mean ± SD, 59.2 ± 8.4 years), with a total of 18 flat polyps,
comprised the study cohort. In those 10 patients, the time difference between
CT colonography and colonoscopy (mean ± SD) was 4.4 ± 7.7
days.
CT Colonography
Study technique—Bowel preparation was performed with a
low-residue diet followed by an oral magnesium carbonate preparation (Magcorol
Soln, Taejoon Pharm) and 10 mg of bisacodyl (Dulcolax, Boehringer Ingelheim
Korea) the evening before CT colonography. CT colonography was performed after
intramuscular injection of 20 mg of scopolamine N-butylbromide
(Buscopan, Boehringer Ingelheim Korea) if no contraindication was present. The
colon was gently insufflated with room air according to the patient's
tolerance in a left lateral decubitus position using a flexible rubber tube
placed in the rectum. The patient was first scanned in the prone position and
then in the supine position. Before scanning, an anteroposterior CT scout
image was obtained to ensure adequate bowel distention. Further air
insufflation was performed when collapsed bowel segments were identified.
During the scanning, the rectal tube was removed. CT was performed from the
top of the diaphragm down to the anus during a breath-hold with a 16-MDCT
scanner (Somatom Sensation 16, Siemens Medical Solutions) with the following
parameter settings: beam collimation, 16 x 0.75 mm; slice thickness, 1
mm; reconstruction interval, 0.7 mm; beam pitch, 1; gantry rotation time, 0.5
sec; table speed, 24 mm/sec; field of view to fit; 120 kV; and 150 mAs.
Scanning in the supine position was performed after IV injection of 150 mL of
iopromide (Ultravist 370, Schering) at a rate of 2.7 mL/sec through a 20-gauge
angiographic catheter inserted in the antecubital vein. Contrast enhancement
was aimed at detecting possible liver metastasis because most of the patients
either had colon cancer or were suspected of having colon cancer; the scanning
was performed 75 sec after initiation of contrast injection.
Image analysis—One board-certified gastrointestinal radiologist who had experienced more than 100 CT colonography cases with colonoscopic correlation, interpreted the CT colonography images at a dedicated workstation using a software package with volume-rendering capabilities (V-works 5.0, CyberMed). The observer was blinded to the colonoscopic findings and was requested to report flat lesions. A 2D search was done to select colonic lesions using transverse images and also, when the transverse plane was not perpendicular to the bowel segments to be looked at, oblique images that were perpendicular to the long axes of the bowel segments as primary 2D views. The rationale for including oblique images as primary views was that we assumed the flat lesions would be even more difficult to detect when they were parallel to the review plane versus when they were perpendicular to the review plane because some flat lesions would only slightly project into the bowel lumen. To be classified as a flat lesion, the lesion height had to be less than half the lesion diameter [2, 6]. When a suspected lesion was identified, the morphology was examined on coronal and sagittal reformations and 3D endoluminal views. An irregular, angular, or barlike shape was considered to suggest a false lesion. The observer examined the internal attenuation of the suspected lesion by modifying the window width and level to depict small gas bubbles, a finding that is consistent with stool. The images were reviewed from both the prone and the supine data sets. Review of the CT colonography images was performed during one session using a standard colon window setting (width, 1,500 H; level, -200 H) and subsequently with an intermediate soft-tissue window setting (width, 400 H; level, 20 H). Contrast enhancement of a suspected lesion could be noted on the soft-tissue window setting of the supine images, which was used to distinguish colonic lesions from feces. The location of a lesion was specified as being one of the eight segments: the cecum (C), ascending colon (A), hepatic flexure (HF), transverse colon (T), splenic flexure (SF), descending colon (D), sigmoid colon (S), or rectum (R). Locations other than the cecum were further recorded as proximal, mid, or distal within each segment. The height from the luminal surface and width of the polyp were measured on 2D images using an electronic ruler after appropriate magnification.
CT Colonographic and Colonoscopic Comparison and Data Analysis
Colonoscopy was considered the reference standard in this study. Four
board-certified gastroenterologists, each of whom had performed more than
1,000 examinations, performed the colonoscopy using a standard
videocolonoscope (CF series, Olympus Optical). Lesion location was described
the same way as on CT colonography. Lesion morphology and size were described.
Flat lesions were defined as mucosal elevations with a height less than half
the lesion diameter [2,
6]. Lesion size, including the
height of the flat lesion, was determined by comparing the lesion with open
endoscopic biopsy forceps pushed against the lesion or by direct measure of
the resected specimen with a ruler when the lesion was retrieved in toto. For
a given flat lesion to be considered a true-positive match between CT
colonography and colonoscopy, the lesion had to be in the same colonic segment
or in the adjacent portion of the adjacent segments, and the two recorded
widths and heights had to be the same within a 50% margin of error. Flat
lesions that were missed on the blinded review were reassessed by two
radiologists in retrospect with the knowledge of the colonoscopic findings,
and judgments were made regarding the causes of the false-negative results.
Lesion conspicuity was qualitatively compared between the colon window 2D
view, soft-tissue window 2D view, and the 3D endoluminal view. For lesions
that could not be found on unblinded review but which were located in
distended and clean colonic segments, lesion height, width, location (haustral
fold vs haustra), and histopathology were compared with those of visualized
lesions. Fisher's exact test was used, and a p value less than 0.05
was considered statistically significant. For all the flat lesions that were
found on CT colonography but were not present at colonoscopy, all the
available information, such as surgical and pathologic reports or follow-up
colonoscopy results, was reviewed to verify if those were really
false-positive findings. The lesions heights, except for those of
false-positive lesions, referred to in the article were from colonoscopic
measurement.
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47.8%) were detected
on the blinded review (Figs.
1A,
1B,
1C,
1D,
2A,
2B,
2C and
2D). On the unblinded review,
of the 14 lesions that were missed at the blinded review, a 7 x 2-mm
(width x height) hyperplastic polyp was found (Fig.
3A,
3B,
3C and
3D); six lesions could not be
visualized because of excessive luminal fluid, poor bowel preparation, and/or
poor bowel distention; and the remaining seven lesions could not be found
despite optimal preparation and distention of the relevant bowel segments.
Except for a 15 x 1-mm adenocarcinoma in situ that was not visualized
because of excessive luminal fluid and poor bowel distention, the other two
cancerous lesions were detected on blinded review (Fig.
1A,
1B,
1C and
1D). All the flat lesions that
were detected were found on the colon window setting, and no additional flat
lesion was found when the intermediate soft-tissue window setting was used.
However, three lesions could be confidently distinguished from feces by noting
their contrast enhancement when the soft-tissue window was used (Fig.
2A,
2B,
2C and
2D). Two lesions located on the
haustral fold showed higher conspicuity on the 3D endoluminal view than on the
colon window setting 2D view (Fig.
1A,
1B,
1C and
1D and
Table 1). For the lesions that
could not be found on unblinded review but which were located in distended and
clean colonic segments, lesion height was a significant factor that determined
lesion visibility; five of five lesions 2 mm or higher versus zero of seven
lesions 1 mm or less in height could be visualized (p = 0.001)
(Table 1). Lesion width
(p =1) and histopathology (p = 0.576) were not associated
significantly with lesion visibility (Table
1). In terms of location, three of three lesions located on the
haustral fold and two of nine lesions located on the haustra were visualized
(p = 0.046); however, all the three lesions on the haustral fold were
2 mm in height, whereas the seven haustral lesions not visualized were 1 mm or
less in height (Table 1).
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Seven false-positive flat lesions were found in six patients, that is, in the transverse colon (n = 1), sigmoid colon (n = 5), and the rectum (n = 1). Lesion heights were 2 mm (n =5) and 3 mm (n = 2), and the widths were from 6 to 10 mm (median, 8 mm). Of those, four false-positives in the sigmoid colon and one in the rectum were confirmed to be false-positive by pathologic inspection of the relevant colonic segments after surgery; however, in two false-positives further confirmation was unavailable.
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The difficulty of diagnosing flat lesions on CT colonography is also thought to be related to several other factors. Plaquelike morphology may be easily mistaken for feces. As shown in the three flat lesions of our study that were confidently distinguished from feces with the help of contrast enhancement, IV contrast may improve interpreter confidence and lesion detection [11]. IV contrast may be used as a problem-solving tool when a potential flat lesion is found and there is doubt; however, the use of the fecal tagging with an oral contrast agent [12] may be a more feasible way to distinguish flat lesions from feces. Because lesions were detected in this study using 2D search and conspicuity was better with 3D endoluminal images in two lesions on the haustral fold, sensitivity of CT colonography may be improved by using a search method of 3D endoluminal fly-throughs. Our study is limited in that 3D fly-throughs were not used. The roles of fecal tagging and 3D fly-throughs for detecting flat polyps may be worth further study. Increasing awareness of the flat lesions and familiarity with their morphologic features would also be helpful in correctly diagnosing these lesions.
Regarding the negative impact of missed flat lesions on screening CT colonography, a recent report [10] suggested that flat lesions were not a significant drawback for screening in an asymptomatic low-risk population. According to some investigators [13], hyperplastic polyps accounted for most flat lesions in a low-risk screening population. It is also not yet known how many flat lesions in a screening population would be 1 mm or less in height. The scale of the negative impact of missed flat lesions on screening CT colonography may need further study.
In two false-positive lesions, confirmation was lacking. These lesions could be false-negative findings of colonoscopy. With regard to the detection of flat polyps, colonoscopy, as performed with standard technique, is relatively insensitive. Special techniques such as high-magnification chromoscopy and dye spraying are necessary to maximize the detection of flat polyps on colonoscopy [6].
Our study has limitations. First, only disease-positive flat-lesion cases were included, and the reviewer was requested to report flat lesions rather than adopting a conventional interpretation of CT colonography. Therefore, although the reviewer was blinded to any relevant information, there could be a bias in favor of a suspicion that the patients harbored flat polyps. Second, as most of our study patients had advanced colon cancer, complete bowel preparation was difficult and many flat lesions were inevitably unable to be detected because of poor bowel preparation and/or excessive luminal fluid.
In conclusion, less than 50% of flat lesions in our population could be visualized by blinded and unblinded review. Visualized flat lesions were 2 mm or greater in height and 7 mm or greater in diameter. Lesions with a height of 1 mm or less were not seen on CT colonography.
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