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Gastrointestinal Imaging |
1 From the Department of Radiological Sciences, University of Rome–La Sapienza, Policlinico Umberto I, Viale Regina Elena 324, Rome, Italy 00161 (R.I., A.L., C.C., F.M., S.T., F.P., R.P.); and Department of Diagnostic Radiology, Yale University School of Medicine, New Haven, Conn (J.A.B.). From the 2002 RSNA scientific assembly. Received October 30, 2002; revision requested January 9, 2003; final revision received April 8; accepted April 30. Address correspondence to R.I. (e-mail: riannaccone@tiscali.it).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: One hundred fifty-eight patients underwent multi–detector row helical CT colonography (beam collimation, 4 x 2.5 mm; table feed, 17.5 mm/sec; voltage, 140 kV; and effective dose, 10 mAs) followed by conventional colonoscopy. Conventional colonoscopy served as the reference standard. Two radiologists interpreted CT colonographic images to assess the presence of polyps or carcinomas. Sensitivity was calculated on both a per-polyp and a per-patient basis. In the latter, specificity and positive and negative predictive values were also calculated. Weighted CT dose index was calculated on the basis of measurements obtained in a standard body phantom. Effective dose was estimated by using commercially available software.
RESULTS: CT colonography correctly depicted all 22 carcinomas (sensitivity, 100%) and 52 of 74 polyps (sensitivity, 70.3%). Sensitivity for detection was 100% in all 13 polyps 10 mm or larger in diameter, 83.3% in 20 of 24 polyps 6–9 mm, and 51.3% in 19 of 37 lesions 5 mm or smaller. With regard to the per-patient analysis, CT colonography had a sensitivity of 96.0%, a specificity of 96.6%, a positive predictive value of 94.1%, and a negative predictive value of 97.7%. The total weighted CT dose index for combined prone and supine acquisitions was 2.74 mGy. The simulated effective doses for complete CT colonography were 1.8 mSv in men and 2.4 mSv in women.
CONCLUSION: Lower-dose multi–detector row helical CT colonography ensures substantial dose reduction while maintaining excellent sensitivity for detection of colorectal carcinomas and polyps larger than 6 mm in diameter.
© RSNA, 2003
Index terms: Cancer screening • Colon, CT, 75.12115 • Colon neoplasms, 75.311, 75.321 • Computed tomography (CT), multi–detector row, 75.12115
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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The major potential drawback of reducing the radiation dose from a CT examination is that the increased noise level may generate images of low diagnostic quality. It has been demonstrated, however, that the high contrast that exists between the colonic wall and the air insufflated to distend the colon can be used to reduce the radiation dose of CT colonography with both single– (28) and multi–detector row (26,27,29) helical CT. With regard to multi–detector row helical CT in particular, a recent study by van Gelder et al (26) investigated the effect of different simulated effective tube currents on the performance of CT colonography. These authors concluded that an effective tube current of 30 mAs allows accurate polyp detection with a radiation dose of 3.6 mSv. In another recent article, Macari et al (27) demonstrated the efficacy of a thin-section (1.0-mm) low-dose (50-mAs) scanning protocol with a radiation exposure of 5.0 mSv for men and 7.8 mSv for women.
To our knowledge, no other study has investigated the possibility that CT colonography could be performed with even less radiation exposure to patients. Further reductions in radiation dose could be important in proposing CT colonography as an initial screening method for colorectal cancer.
Thus, we undertook this study to compare lower-dose multi–detector row CT colonography with conventional colonoscopy for detection of colorectal lesions.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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Patient Population
Between July 2001 and January 2002, 158 consecutive patients (88 men; mean age, 64 years; range, 52–80 years and 70 women; mean age, 63 years; range, 48–80 years) with a body weight range of 58–130 kg were recruited for the study and underwent both multi–detector row helical CT colonography and conventional colonoscopy on the same day. Patients were included either for colorectal cancer screening (n = 31) or for evaluation (stool with positive hemoccult test results, n = 52; history of polyps, n = 37; history of colorectal cancer, n = 18; hematochezia, n = 12; or iron deficiency anemia, n = 8). Patients with inflammatory bowel disease or acute diverticulitis, those who were pregnant, and those who were unable to provide written consent were excluded.
CT Colonography
Study technique.—After standard colonoscopic cathartic preparation with an oral polyethylene glycol electrolyte solution (Isocolan; Bracco, Milan, Italy) diluted in 4 L of water in the 24 hours preceding the CT examination, scanning was performed with a multi–detector row helical CT scanner (Somatom Plus 4 Volume Zoom; Siemens, Erlangen, Germany) equipped with an adaptive array matrix and a gantry rotation time of 0.5 second. CT colonography was performed with the patient in both the prone and the supine position after intravenous administration of 20 mg of butylscopolamine (Buscopan; Boehringer, Ingelheim, Germany).
Before the examination was performed, the patient was placed in the left lateral decubitus position, and the colon was gently insufflated with air by using a Foley catheter placed in the rectum, according to patient tolerance. The rectal tube was subsequently clamped during the examination. Patients were instructed to breathe deeply before scanning and to either suspend respiration or exhale slowly if they could not suspend respiration for the duration of the examination. With the patient in the prone position, an anteroposterior CT scout image (120 kV, 50 mA) was obtained to ensure adequate bowel distention. Further air insufflation was performed when collapsed bowel segments were identified. Before patients underwent scanning in the supine position, the colon was insufflated with additional air to maximum patient tolerance, and colonic distention was checked with a second anteroposterior CT scout image.
Imaging parameters for CT colonography were beam collimation, 4 x 2.5 mm; effective section thickness, 3.0 mm; reconstruction interval, 1.0 mm; table speed, 17.5 mm/sec; field of view to fit; 140 kV; and 10 mAs (effective). The ensuing beam pitch, which was calculated by multiplying table speed by gantry rotation time and dividing the product by beam collimation, was 0.875 (17.5 mm/sec x 0.5 sec/[4 x 2.5 mm]). Images were reconstructed with a standard body reconstruction algorithm (kernel B20, smooth) available on the CT scanner and routinely used for abdominal CT scanning. Acquisition time ranged from 16 to 24 seconds. In addition, a resident (S.T.) who was not involved in CT colonographic data evaluation recorded the average length of the procedure and any complication associated with CT colonography.
Radiation dose calculation.—To calculate the CT radiation dose to patients, we used the weighted CT dose index (31). The weighted CT dose index was estimated from the values of beam collimation, voltage, and effective milliampere seconds, as mentioned previously. Macari et al (27) and van Gelder et al (26) report that the effective dose is a setting used by two CT manufacturers (Siemens and Marconi) and is defined as the tube current multiplied by gantry rotation time and divided by the beam pitch ([17.5 mA x 0.5 second]/0.875). Thus, the true tube current was 17.5 mA, the effective tube current was 20 mA, and the effective dose was 10 mAs.
The measurements of weighted CT dose indices were performed with a plastic body phantom that was 32 cm in diameter and referred to as a CT dose index body phantom (32). The manufacturer of the CT scanner provided this phantom, and its dimension were consistent with those of an average-sized man. By definition, the weighted CT dose index is a weighted mixture of a pair of CT dose index100 values, where CT dose index100 represents the radiation dose absorbed in air integrated over an ionization chamber with an active length of 100 mm along the axis of the phantom (33). Specifically, the weighted CT dose index is equal to the sum of one-third of the central CT dose index100, c and two-thirds of the peripheral CT dose index100, p calculated with measurements obtained in the phantom (34).
In addition, we also estimated the effective doses in millisieverts for an average-sized man and woman on the basis of the CT parameters used in the present study (35). This was achieved by using commercially available dose calculation software (WinDose; Wellhofer Dosimetry, Schwarzenbruck, Germany) (36). This program uses organ-weighting coefficients based on International Commission on Radiological Protection recommendations (37) to estimate effective dose values for given imaging parameters and anatomic ranges (36).
Image analysis.—Two gastrointestinal radiologists (A.L. and C.C.), each with 4 years of experience in CT colonography, independently reviewed each scan at a dedicated workstation by using a software package with volume-rendering capabilities (Vitrea 2.6; Vital Images, Minneapolis, Minn). Both radiologists were unaware of the indications for scanning and the results of conventional colonoscopy.
For the purposes of image analysis, observers were asked to use a previously validated "time-efficient" technique (9,13,27). In brief, the initial image analysis consisted of review of magnified two-dimensional transverse images. When a suspected lesion was identified on two-dimensional transverse images and coronal and sagittal CT images, three-dimensional endoluminal views were evaluated to better characterize the lesion. Specifically, coronal and sagittal reformations and endoluminal images were used to differentiate hypertrophied haustral folds or residual stool from colonic lesions. In addition, when a suspected lesion was detected, the observers carefully examined the internal attenuation of the lesion by modifying window and level settings to depict small gas bubbles, a finding that is consistent with stool. To further differentiate residual stool from polyps, the radiologists could simultaneously review the transverse CT images from both the prone and the supine data set. This allowed them to evaluate whether the lesion changed position when the patient moved from the prone position to the supine position, which is another finding that is consistent with stool. If a suspected lesion was not detected after review of transverse scans, no further image analysis was performed. Disagreements between the interpreters were resolved by consensus before comparison with results of conventional colonoscopy.
The number, location, and size of all suspected lesions (either polyps or carcinomas) were recorded. A mass was considered to have CT features of a carcinoma rather than a polyp when it appeared either as a circumferential thickening of the bowel wall with luminal narrowing or as a discrete soft-tissue lesion protruding into the colonic lumen. Particular attention was paid to ensure correct classification of lesions into one of three categories: 5 mm in diameter or smaller, 6–9 mm, and 10 mm or larger. Lesions on CT images were measured by using an electronic ruler. To specify the location of each lesion, the colon was divided into eight segments: rectum, sigmoid colon, descending colon, splenic flexure, transverse colon, hepatic flexure, ascending colon, and cecum. With regard to polyps, the morphology (pedunculated, sessile, or flat) of the lesion was also noted. All lesions seen at CT colonography were photographed and stored in digital Joint Photographic Experts Group format. In addition, all CT colonographic images were stored in digital format in our picture archiving and communication system (Lifeweb; Ferrania Imaging Technologies, Milan, Italy). The reading time for all CT colonographic examinations was recorded by two observers (A.L. and C.C.).
Conventional Colonoscopy
Colonoscopy was performed immediately after CT scanning. A trained endoscopist, who had performed more than 5,000 examinations, performed conventional colonoscopy with a standard videocolonoscope (CV-1; Olympus, Tokyo, Japan) and without knowledge of the findings of CT colonography. The endoscopist was asked to pass the instrument tip to the cecum and to document the number, location, size, and morphology of all identified lesions during withdrawal of the endoscope. Lesion location was estimated by using endoscopic landmarks and insertion distances at withdrawal. Lesion size was determined by comparison with open endoscopic biopsy forceps pushed against the polyp or by direct comparison with the resected specimen when the lesion was retrieved in toto. In addition, a resident (F.M.) who was not involved in the evaluation of CT colonographic data was present to record any complication associated with conventional colonoscopy.
Data and Statistical Analysis
All lesions identified at CT colonography were compared with those identified at colonoscopy, which was considered the reference standard. Sensitivity for polyp detection was assessed on both a per-polyp and a per-patient basis. In the latter, specificity, positive predictive value, and negative predictive value were also calculated. For the per-polyp analysis, a polyp detected with CT colonography was considered a true-positive finding if it matched colonoscopic findings with regard to location (same colonic segment), size (less than a 3-mm difference between procedures), and morphology (similar morphologic features). All polyps depicted with CT colonography that were not seen at colonoscopy or that did not match a colonoscopic finding were considered false-positive. Thus, if a polyp was depicted by both CT colonography and colonoscopy but the findings did not match, it was considered false-positive.
With regard to the per-patient analysis, the findings at CT colonography and at colonoscopy were considered to match if both studies showed at least one polyp or if neither study showed a polyp. Only the presence of at least one true-positive polyp was considered, and the number, location, size, and morphology of polyps were not used in assessing concordance between the two procedures.
If colonoscopy was incomplete, the available results were compared. Lesions identified with CT colonography but located within segments not depicted by conventional colonoscopy were excluded from comparative analysis.
To determine the reasons for diagnostic errors at CT colonography, a retrospective review of all false-positive and false-negative findings was jointly performed by the two radiologists. Commercially available software (11.0.0 for Windows; SPSS, Chicago, Ill) was used to estimate sensitivity, specificity, positive and predictive values, and 95% CIs.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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Carcinomas were located within the rectum (n = 1), sigmoid colon (n = 10), descending colon (n = 2), splenic flexure (n = 2), transverse colon (n = 1), hepatic flexure (n = 1), ascending colon (n = 2), and cecum (n = 3).
Of the 74 polyps, 13 were 10 mm in diameter or larger, 24 were 6–9 mm, and 37 were 5 mm or smaller. Polyps were located within the rectum (n = 8), sigmoid colon (n = 33), descending colon (n = 11), splenic flexure (n = 2), transverse colon (n = 8), hepatic flexure (n = 1), ascending colon (n = 4), and cecum (n = 7).
CT Colonography
The average examination lasted 16 minutes and included explanation of the procedure, air insufflation, and acquisition of images with patients in the prone and supine positions. The median time required for data interpretation was 11 minutes (range, 7–15 minutes). No complications were associated with CT colonography. Complete depiction of the colon was achieved in all patients.
The total combined weighted CT dose index was 2.74 mGy. The weighted CT dose index increases to 3.64 mGy when the anteroposterior CT scout image acquisitions are included. The simulated effective dose was estimated with WinDose software and is 1.8 mSv for men and 2.4 mSv for women. The simulated effective dose increases to 2.15 mSv for men and 2.75 mSv for women, when the prone and supine anteroposterior CT scout image acquisitions are included.
Lesion Depiction
Lower-dose CT colonography correctly depicted all 22 carcinomas (sensitivity, 100%) (Fig 1) and 52 of 74 polyps (sensitivity, 70.3%; 95% CI: 59.6%, 80.9%) (Figs 2, 3). Lesions were correctly depicted with CT colonography in all 13 polyps 10 mm in diameter or larger (sensitivity, 100%); in 20 of 24 polyps 6–9 mm (sensitivity, 83.3%; 95% CI: 67.3%, 99.4%); and in 19 of 37 polyps 5 mm or smaller (sensitivity, 51.3%; 95% CI: 34.5%, 68.2%). In the evaluation of polyp morphology, CT colonography depicted 23 (92%) of 25 pedunculated lesions, 27 (60%) of 45 sessile lesions, and two (50%) of four flat lesions. With regard to the per-patient analysis, CT colonography had a sensitivity of 96.0% (95% CI: 90.4%, 100%), a specificity of 96.6% (95% CI: 92.8%, 100%), a positive predictive value of 94.1% (95% CI: 87.4%, 100%), and a negative predictive value of 97.7% (95% CI: 94.5%, 100%).
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Analysis of images obtained with CT colonography yielded 11 false-positive lesions in three patients who did not have lesions at colonoscopy and in three patients who had at least one polyp at colonoscopy. In one patient with two lesions (one carcinoma and one polyp) and in two patients with a single polyp, the location, size, or morphologic features of the lesion at CT colonography did not match that of the lesion identified at colonoscopy. Of the remaining eight lesions detected with CT colonography but not with colonoscopy, one was 12 mm, one was 8 mm, and six were 5 mm or smaller. At retrospective analysis, the 12-mm-diameter lesion detected with CT colonography was probably due to a hypertrophied fold. The seven remaining false-positive findings were attributed to adherent stool.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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Indeed, a recent study in 12 different institutions (26) showed that the median effective dose for complete CT colonography in both the prone and supine positions is 8.8 mSv. CT colonography performed with an effective dose of 8.8 mSv may result in a 0.02% risk for inducing cancer in patients older than 50 years, who are currently considered the target population for colorectal cancer screening (37). Considering these factors, increasing attention has been focused on the optimization of low-dose protocols for multi–detector row CT colonography (26,27,29).
Recently, Hara et al (29) took advantage of the faster data acquisition provided by multi–detector row CT. Their results demonstrated better bowel distention and fewer respiratory artifacts with a beam collimation (5.0 mm) and an effective radiation dose (4.7 mSv in men and 6.7 mSv in women) comparable to those used in single–detector row CT examinations.
A different approach has been proposed by Macari et al (27). These authors used a 1.0-mm beam collimation protocol to obtain images with near isotropic voxels, but they simultaneously decreased the effective milliampere second setting to 50 to keep the effective dose at a level comparable to that of a single-detector row CT examination (5.0 mSv for men and 7.8 mSv for women) (27). Such a protocol provided excellent sensitivity for the detection of polyps 10 mm in diameter or larger and improved differentiation of colorectal polyps from residual stool and hypertrophied folds. This protocol also provided consequent reduction of false-positive diagnoses (27).
In another study, van Gelder et al (26) used a 2.5-mm beam collimation protocol and demonstrated that, despite a perceptible worsening of image quality, the sensitivity for detection of polyps was equal at 100, 50, and 30 mAs (effective). These results indicate that multi–detector row CT colonography can be reliably performed with an effective dose of 3.6 mSv (26).
Our experience demonstrates that multi–detector row CT technology can be used to achieve a further substantial reduction in radiation dose for CT colonography with a total effective dose of 1.8 mSv for men and 2.4 mSv for women. These values are substantially lower than those in previously published reports of CT colonography, both as reported for single– (4.4 mSv for men; 6.7 mSv for women) (29) and multi–detector row CT (26,27,29). These values are also lower than the effective dose of a barium enema examination, which is between 3.0 and 7.0 mSv (40).
All carcinomas were correctly identified with the use of lower-dose multi–detector row helical CT colonography. Moreover, lower-dose CT colonography correctly depicted 33 of 37 polyps 6 mm in diameter or larger (sensitivity, 89.1%). This finding is of paramount importance, because lesions larger than 6 mm in diameter have the highest probability of being malignant. Our results agree with the results of several previous studies (10,14,16,27) and confirm that lesions 5 mm in diameter or smaller (in particular, those 3 mm in diameter or smaller) are difficult to detect with CT colonography. In our experience, all lesions 5 mm in diameter or smaller that are not detected on CT colonographic images after the initial image analysis could still not be detected, even after careful retrospective analysis. Given the extremely low (0.1%) incidence of carcinoma in such lesions (41) and the long growth time of adenomas (estimated in the literature as 15–18 years) (42,43), the detection of these tiny excrescences remains of uncertain clinical importance, especially if colorectal cancer screening is performed at regular intervals (44).
Regarding polyp morphology, our results indicate that CT colonography has a high sensitivity for the depiction of pedunculated polyps. This can be explained by the fact that all pedunculated polyps in our study were larger than 6 mm in diameter and, therefore, were generally easier to identify compared with sessile lesions, which tended to be smaller. Because only four flat lesions were present in our cohort, two of which were correctly identified with CT colonography, no reliable conclusion can be stated on the ability of lower-dose CT colonography to depict such lesions.
One possible limitation of our study is that CT colonographic images were interpreted with the consensus of two experienced abdominal radiologists. Thus, interobserver variability was not assessed. Interobserver agreement has been documented in other investigations and may influence the adoption of the technique into general practice (45). In addition, although a recent study demonstrated that experienced abdominal radiologists can become proficient at CT colonographic image interpretation after proper training (45), evidence suggests interpretation of CT colonographic images is associated with a long learning curve (15). Thus, we are currently investigating the accuracy of lower-dose CT colonography when images are interpreted by less experienced readers.
Another potential limitation of our study is that the majority of our patients had risk factors for polyps; therefore, the performance of lower-dose CT colonography needs to be evaluated in a large screening population.
Two further potential criticisms to our research are inherent to the use of a low effective dose. First, the use of a low effective dose may not be well suited for the assessment of low-contrast structures, such as the liver, pancreas, kidneys, and lymph nodes. This can be expected, because noise affects the image quality in low-contrast structures more than it does in high-contrast structures such as the colonic mucosa-air interface. Two reports have demonstrated the efficacy of CT colonography in the detection of lesions located outside the colon (46,47). Although the vast majority of such findings are of low clinical importance and the cost-effectiveness of reporting such findings deserves further evaluation (47), the ability to identify disease outside the colon is potentially a major advantage of CT colonography over all other colorectal cancer diagnostic tests (48). Thus, after we assess the colon, we routinely evaluate the abdomen and pelvis for the presence of incidental findings. In addition, the use of denoising filters in CT colonography has recently been demonstrated to allow adequate assessment of both high- and low-contrast structures at 35 mAs (49). It is reasonable, therefore, to predict that the forthcoming introduction of new optimized denoising filters may improve the evaluation of low-contrast regions, even at 10 mAs (effective).
A second criticism of our study may be that it is well known that artifacts increase when examining obese patients with low dose techniques. In this study, however, we did not detect a substantial loss in image quality, even when examining subjects who weighed as much as 130 kg.
Several options for further dose reduction can be explored. First, to reduce the CT dose index further, it is possible to increase the beam collimation (ie, 4 x 5.0 mm); however, this would generate a worsening of the spatial resolution, thus making the detection of smaller lesions more difficult. Second, the tube voltage can be diminished to 120 kV. Our personal experience, however, indicates that the reduction in voltage that occurs when using 10 mAs (effective) would generate severe streak artifacts, especially at the level of the bony structures of the pelvis, thus hampering assessment of this region. Third, the possibility of further dose reduction in slimmer patients cannot be ruled out. Finally, as a result of better dose efficiency (50), further reductions of CT dose index can be expected with the use of eight- and 16-section multi–detector row scanners.
In conclusion, our results indicate that lower-dose multi–detector row helical CT colonography ensures a substantial reduction in the radiation dose delivered to patients while maintaining excellent sensitivity for detection of colorectal carcinomas and polyps larger than 6 mm in diameter. Thus, lower-dose multi-detector row helical CT colonography appears to be a viable alternative to CT colonography, as performed with previously reported protocols that delivered a higher amount of ionizing radiation, especially in a screening setting, when exposure of patients to radiation is a major concern.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, R.P.; study concepts and design, R.I.; literature research, R.I., F.M., S.T., F.P.; clinical studies, R.I., F.M., S.T., F.P.; data acquisition, R.I., F.M., S.T., F.P.; data analysis/interpretation, A.L., C.C.; statistical analysis, R.I., F.M., S.T., F.P.; manuscript preparation, R.I.; manuscript definition of intellectual content, R.I., A.L., C.C., R.P.; manuscript editing and revision/review, J.A.B., A.L., C.C., R.P.; manuscript final version approval, all authors
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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