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1 Institute of Diagnostic Radiology, University Hospital Duesseldorf,
MNR-Klinik, Moorenstrasse 5, Duesseldorf 40225, Germany.
2 Department of Gastroenterology, Hepatology, and Infectious Diseases,
University Hospital Duesseldorf, Duesseldorf, Germany.
3 Institute of Informatics, Heinrich-Heine-University, Duesseldorf,
Germany.
Received February 10, 2004;
accepted after revision April 15, 2004.
Address correspondence to M. Cohnen
(cohnen{at}med.uni-duesseldorf.de).
Abstract
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SUBJECTS AND METHODS. After standard bowel cleansing, 137 patients (77 men, 60 women; mean [± SD] age, 57.1 ± 11.3 years) underwent high-resolution video colonoscopy within 2 hr after ultra-low-dose MDCT colonography had been performed. Ultra-low-dose MDCT colonography was performed with patients in the supine position only using 10 mAs (effective weighted CT dose index, 0.94 mGy). After mathematic noise reduction by nonlinear gaussian filter chains, using dedicated software (ECCET), images were analyzed by two blinded observers in simultaneously displayed interactive 2D and 3D modes. Findings of ultra-low-dose MDCT colonography were compared with the results obtained with high-resolution video colonoscopy.
RESULTS. Calculated effective doses were 0.7 and 1.2 mSv for men and women, respectively. Ultra-low-dose MDCT colonography detected 84 (62%) of 135 lesions: 11 (78.6%) of 14 large polyps (> 10 mm), 12 (85.7%) of 14 medium polyps (9.9-5 mm), and 61 (57%) of 107 small polyps (< 5 mm). On a patient-by-patient basis, an overall sensitivity of 70.3% with a specificity of 80.8% was calculated. False-positive findings were seen mostly for small lesions (eight medium and 29 small lesions). Two of the three false-negative lesions were retrospectively detected in contrast-enhanced cleansing fluid; one was a flat lipoma not detectable on ultra-low-dose MDCT colonography.
CONCLUSION. Despite an effective dose of approximately 1 mSv, MDCT colonography using an ultra-low-dose technique performs as well as MDCT colonography with a standard dose, according to published data. After mathematic noise reduction, 82% of polyps larger than 5 mm can be detected.
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Therefore, this study was performed to assess the sensitivity and specificity of MDCT colonography in the ultra-low-dose technique in comparison to the gold standard, high-resolution video colonoscopy.
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Ultra-low-dose MDCT colonography was performed using a Somatom Plus 4 Volume Zoom scanner (Siemens Medical Solutions) after a standard oral colonic preparation starting on the day before the examination using bisacodyl (Prepacol, Guerbet) combined with a liquid diet of 4-5 L of a polyethylene glycolelectrolyte solution (Endofalk, Falk Pharma).
After careful digital rectal examination, insufflation of room air into the colon was achieved by the patients themselves using an air bulb to the subjective maximum tolerance. During this procedure, the patient was advised to turn from a left-sided to a prone position and further to a right-sided position to facilitate homogeneous bowel distention. Colonic distention was assessed on the digital topogram. When necessary, further air insufflation was performed. No IV medication was given. Examinations were performed in the supine position only.
Patients were instructed to slowly exhale if they could not maintain the entire breath-hold. Breath-hold time ranged between 20 and 30 sec. Total radiation exposure was calculated with a commercially available software package (CTExpo version 1.3, G. Stamm and H.D. Nagel) [10]. This software package calculates effective doses according to International Commission on Radiological Protection 60 [11] using the respective tissue-weighting factors. Estimated effective doses both for men and women were reported using conversion factors for the abdomen.
Scanning parameters were as follows: detector collimation was 4 x 1 mm; reconstructed slice width was 1.25 mm with a reconstruction increment of 0.7 mm. Scanning was performed with an 8-mm table feed accounting for a pitch of 8 (or 2 in classical terms). The rotation time was 0.5 sec with 120 kVp. The tube current was set at the lowest milliampere setting possible: 10 effective mAs corresponding to 40 electric mA resulted in an effective weighted CT dose index of 1.14 mGy as indicated by the implemented CT software.
After transfer to a remote double PC-based workstation, the reconstructed images underwent a mathematic noise-reduction algorithm using edge-preserving nonlinear gaussian filter chains [12] (Figs. 1A, 1B). Dedicated software (ECCET, www.eccet.de), which was developed by our Department of Informatics, segments colonic lumen automatically using a threshold of -750 H and generates a 3D volume. ECCET allows a simultaneous visualization of 2D multiplanar reconstructions and high-resolution interactive real-time 3D views and has the ability to switch directly between 2D and 3D mode.
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Ultra-low-dose MDCT colonographs were interpreted by an experienced radiologist as well as one gastroenterologist. Any observed lesion was classified as tumor, polyp, or residual stool by consensus, and size and segmental location were recorded. Lesion size was determined using electronic caliper measurements in 3D images, and location of lesions was attributed to eight colonic segments (cecum, ascending colon, right flexure, transverse colon, left flexure, descending and sigmoid colon, and rectum). All classified lesions were compared with the findings of high-resolution video colonoscopy regarding location (same or adjacent segments) and size (estimated size, ± 2 mm). Concerning size, three groups were differentiated: large polyps 10 mm or more, medium polyps 9.9-5 mm, and small polyps less than 5 mm in diameter.
Colonic dilatation was regarded as optimal when all or all but one segment were well expanded. Moderate dilatation was defined as occurring when dilatation was suboptimal in two or more segments; a collapse of one or more segments was regarded as insufficient. Residual stool or fluid was noted as well as noise-related artifacts.
High-resolution video colonoscopy—considered the gold standard—was performed by an experienced endoscopic gastroenterologist using the standard technique directly after ultra-low-dose MDCT colonography. Video colonography was performed using a high-resolution endoscope, processor, and light source (Olympus Evis Exera 160 Line, PCF-Q160AL Endoscope) with the patient under conscious sedation with midazolam and propofol. All tumors or polyps were photodocumented, and their size was assessed relative to an opened biopsy forceps. Positioning in the colon material for a histolopathologic examination was possible in 98% of lesions because either resection by biopsy or removal by snare polypectomy was done. Both the endoscopist and the ultra-low-dose MDCT colonography observers were blinded to each other's results.
Cross-table analysis, including Spearman's rank correlation test as well as statistical evaluation using the Student's t test and Wilcoxon's signed rank test at a significance level of 0.05, was performed using commercially available software (SPSS version 11.0 [Statistical Package for the Social Sciences] for Windows [Microsoft]).
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Colonic distention was regarded as optimal or moderate in 97.2% of patients. Residual stool was found in 15 patients examined with ultra-low-dose MDCT colonography. Residual fluid was seen to a moderate degree in one third of patients, although a large amount of cleansing fluid was noted in 15.1% of colonic segments, obscuring more than one third of the colonic surface, mostly in the ascending colon and the descending colon. Noise-related artifacts were mostly observed in the pelvic region: 20.2% of patients with ultra-low-dose MDCT colonography showed endoluminal "snow" artifacts. When the threshold of -750 H was increased to -650 H, these artifacts were greatly diminished and nonvisible without loss of diagnostic confidence.
Calculation of the effective dose is based on the effective weighted CT dose index. For an entire MDCT scan of the abdomen with an approximately 400-mm scanned length, an effective dose of approximately 0.7 mSv for men and 1 mSv for women, respectively, has to be expected.
Table 1 gives detailed results concerning tumors and polyps. Overall, 84 (62%) of 135 lesions were seen on ultra-low-dose MDCT colonography. Detailed analysis reveals that in ultra-low-dose MDCT colonography, 11 (78.6%) of 14 large lesions were seen, although 12 of 14 medium polyps and 61 of 107 small polyps were detected as compared with high-resolution video colonoscopy, accounting for a sensitivity of 85.7% for medium-sized and 57% for small polyps, respectively (Table 1 and Figs. 2A, 2B). Spearman's correlation coefficient for ultra-low-dose MDCT colonography versus high-resolution video colonoscopy was 0.87 for large lesions, 0.62 for medium lesions, and 0.5 for small lesions, with a significance level of p < 0.001 (Figs. 3A, 3B).
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Overall, the negative predictive value of ultra-low-dose MDCT colonography was 75.6% and the positive predictive value was 76.3%. Fourteen patients with false-positive findings on ultra-low-dose MDCT colonography would have undergone high-resolution video colonoscopy unnecessarily. However, 19 patients with false-negative findings would not have undergone colonoscopy, which includes five patients with false-negative findings for lesions 5 mm or larger.
For lesions of 5 mm or larger, sensitivity of ultra-low-dose MDCT colonography per polyp was 82.1%. On a per-patient basis, sensitivity, specificity, and positive and negative predictive values were 86.3%, 93%, 70.4%, and 97.2%, respectively.
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Our study confirms first-published results showing a high sensitivity and specificity of ultra-low-dose MDCT colonography for large and medium polyps [7, 9, 16]. The sensitivity for polyps 5 mm or larger was 82.1%. On a per-patient basis, sensitivity and specificity were 86.3% and 97.2%. These data show that ultra-low-dose MDCT colonography delineates polyps with a sensitivity and specificity comparable to those of the standard-dose technique as previously published [1, 3, 12, 17-20]. A large meta-analysis recently showed that, on average, CT colonography has a sensitivity of 81% (range, 79-82%) for the detection of polyps 1 cm or more in size (per-polyp basis), combined with a high specificity of 95% [20]. These data decreased to a sensitivity of 62% for polyps 6-9 mm and 43% for polyps smaller than 5 mm. On average, pooled data show a sensitivity and specificity of 88% and 81%, respectively, on CT colonography in patients with a polyp 1 cm or more on a per-patient basis [21].
Positive and negative predictive values of approximately 75% in our study indicate that—if ultra-low-dose MDCT colonography had been used as the only examination—four patients (10.2%) would have undergone conventional colonoscopy unnecessarily and 19 others (13.9%) would not have been sent for high-resolution video colonoscopy, although a polyp was present. These figures include three patients with polyps 5 mm or larger who would have been missed. For medium polyps 5 mm and larger, the negative predictive value of ultra-low-dose MDCT colonography was 97.2%, which would be acceptable even in a screening situation.
The false-negative rate, especially for the large polyps, needs further explanation: Two pedunculated polyps 12 and 14 mm in diameter removed by colonoscopy could not be detected either on 2D or on 3D virtual endoscopic views, even on meticulous retrospective analysis. The most likely reason was the residual fluid found in the colonic segments in which these polyps were found on high-resolution video colonoscopy. One other polyp turned out to consist of a thin lipoma of 15 mm in diameter with a height of not more than 5 mm in the cecal pole that could not be seen on ultra-low-dose MDCT colonography despite adequate distention without residual fluid. Most likely, it could not be differentiated from the abdominal fat surrounding the bowel. The two medium polyps not seen prospectively on ultra-low-dose MDCT colonography were located in the sigmoid colon, which was only moderately distended in one patient and partially collapsed in the other.
As has been reported, observer-independent factors are important: inadequate bowel preparation with residual stool or cleansing fluid and improper colonic distention are critical factors [3]. The IV application of bowel relaxants does not improve colonic distention on MDCT colonography [22, 23]. However, dual positioning of the patient was found to improve bowel-wall conspicuity and the detection rate of endoluminal lesions [22, 24, 25]. The aim of this study was to assess the ultra-low-dose technique with the least possible effective dose. Therefore, double scanning was not intended. However, because possible sequelae of radiation exposure must be weighed against the drawback of missing polyps of a relevant size, dual positioning should be used in a screening situation in cases of moderate or large amounts of residual fluid. In addition to dual positioning, a different approach would be to use oral contrast material to allow electronic subtraction of residual cleansing fluid [26, 27]. Future work will be directed to this approach.
The first large trial to examine virtual CT colonography as a screening technique in direct comparison with conventional colonoscopy yielded excellent results, with sensitivity and specificity exceeding 90% for polyps 6 mm or more [28]. However, if virtual CT colonography should become part of a screening program to reduce mortality rates from colorectal cancer, a reduction in effective dose compared with standard CT examinations is unavoidable. If patients are scanned in both positions, a mean effective dose of 10 mSv or more for the scanner type used in this study is to be expected. The low-dose technique leads to an increase in image noise because the amount of photons reaching the detector is diminished [29]. This increase may deteriorate lesion conspicuity in terms of detectability on low-contrast imaging. The imaging of high-contrast structures allows a higher noise level with a satisfactory image quality and diagnostic reliability [30]. Thus, it seemed feasible to decrease the dose in virtual CT colonography even to an effective dose of approximately 1 mSv because endoluminal lesions show an inherently high contrast to the surrounding colonic air or gas. Because subjectively perceived noise is inversely related to window width, image analysis was done with a wide window center and setting [31]. After application of the mathematic filter algorithm, no relevant noise-related artifacts were noted on 3D renderings of the colonic surface [12].
In addition to false-negative results, virtual colonoscopy is known to produce false-positive results, especially for smaller lesions [1, 3, 13]. This was true for our study as well. The number of false-positive findings was invertedly correlated to size, with a total of 29 false-positive small lesions. These were most likely due to fecal residue. However, one of eight medium false-positive lesions clearly identified as air-free pedunculated lesions on ultra-low-dose MDCT colonography was not found on high-resolution video colonoscopy. Because conventional colonoscopy may have miss rates of more than 20%, some "false-positive" findings on MDCT colonography may represent false-negative findings on high-resolution video colonoscopy [32, 33]. To solve this problem, a repeat colonoscopy (back-to-back) should be performed immediately in the same patient, especially after discordant findings are noted between ultra-low-dose MDCT colonography and high-resolution video colonoscopy.
Limitations of our study include that the examination was performed in only one position, the use of a cleansing agent without contrast medium, and the lack of repeat colonoscopy to exclude false-negative findings in high-resolution video colonoscopy. Furthermore, the data shown cannot be transferred to a screening situation because we did not clinically examine not apparently healthy persons. One hundred thirty-five lesions were found in 54 (39.4%) of 137 patients, resulting in a higher than expected incidence for a screening situation [34, 35]. Furthermore, we did not observe any malignant lesions in the study group undergoing ultra-low-dose MDCT colonography, so that the feasibility in this regard cannot be evaluated. Last, extracolonic disease cannot reliably be assessed so that incidental findings may be missed [36-38]. In our study population, however, one abdominal aortic aneurysm and a right-sided ureteric stone were found and altered the subsequent patient treatment. In cases of suspected or known cancer, a standard-dose abdominal CT examination with IV contrast medium is mandatory to exclude lymphadenopathy or organ metastases.
In conclusion, ultra-low-dose MDCT colonography seems to be feasible as an examination to detect lesions in the lumen of the colon with a high negative predictive value for lesions of 5 mm or more. Performance may be improved by fecal and fluid tagging, potentially minimizing false-negative and false-positive findings. Therefore, MDCT colonography in a low-dose technique may play a significant role in screening for colorectal cancer because radiation exposure is minimized.
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