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Original Research |
1 Radiology and Imaging Sciences, National Institutes of Health Clinical Center,
Bldg. 10, Rm. 1C368X MSC 1182, Bethesda, MD 20892-1182.
2 Department of Radiology, Walter Reed Army Medical Center, Washington,
DC.
3 Department of Radiology, Uniformed Services University of the Health Sciences,
Bethesda, MD.
4 Department of Radiology, University of Wisconsin Medical School, Madison,
WI.
Received December 27, 2007;
accepted after revision May 23, 2008.
The opinions and assertions contained herein are those of the authors and
should not be construed as official or as representing the opinions of the
Department of the Army or the Department of Defense.
Abstract
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MATERIALS AND METHODS. The CTC scans of 120 patients were retrospectively selected, 30 from each of four institutions. The bowel preparation included oral contrast material for fecal and fluid tagging. Patients at one institution (institution 4) drank half the amount of oral contrast material compared with the patients at the other three institutions. Fifteen of the CTC scans were from the beginning of the protocol studied at each institution and 15 scans were from the same protocol acquired approximately 1 year later in the study. We used previously validated QA software to automatically measure the mean distention and residual fluid of each of five colonic segments (ascending, transverse, descending, sigmoid, and rectum). Adequate distention was defined as a colonic diameter of at least 2 cm. Residual fluid was determined by the percentage of colonic surface area covered by fluid. We compared how the quality varied across multiple institutions and over time within the same institution.
RESULTS. No significant difference in the amount of colonic distention among the four institutions was found (p = 0.19). However, the distention in the prone position was significantly greater than the distention in the supine position (p < 0.001). Patients at institution 4 had about half the amount of residual colonic fluid compared with patients at the other three institutions (p < 0.01). The sigmoid and descending colons were the least distended segments, and the transverse and descending colons contained the most fluid on the prone and supine scans, respectively. More recently acquired studies had greater distention and less residual fluid, but the differences were not statistically significant (p = 0.30 and p = 0.96, respectively).
CONCLUSION. Across institutions, a significant difference can exist in bowel preparation quality for CTC. This study reaffirms the need for standardized bowel preparation and quality monitoring of CTC examinations to reduce poor CTC performance.
Keywords: colon • colonography • CT • quality • virtual colonoscopy
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Residual colonic fluid and colonic distention are two important factors that have an effect on CTC quality. The minimization of residual fluid and adequate colonic distention produce higher examination quality and improve the ability of radiologists to accurately identify lesions in the colon [6–14].
Previously, we developed and validated automated quality assessment (QA) software in an effort to facilitate assessment of CTC examination quality [15]. The software automatically measures colonic distention and luminal surface area obscured by residual fluid. The purpose of this study was to use that QA software to compare the colonic distention and residual fluid present in CTC examinations at various institutions and to determine how the quality of the examinations changed over time.
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Patient Population
Patients were chosen retrospectively from the patient cohorts at four
medical centers (institu tions 1–4) to assess the quality of CTC exami
nations between the different centers and over time. Patients in institutions
1–3 were scanned between May 2002 and June 2003
[1], whereas patients in
institution 4 were scanned between April 2004 and September 2005
[2]. To determine the sample
size, we used Altman's nomogram for calculating sample size with p =
0.05, power of 0.80, target difference of 10%, and SD of 10%
[16]. In chronological order,
we used the first consecutive 15 (group A) and last 15 (group B) patients' CTC
examinations within each of the four institutions to make com parisons. Group
B patients from institution 4 were part of an ongoing study, whereas group B
patients from institutions 1–3 were scanned at the conclusion of the
study. Nine of 30 (30%), eight of 30 (27%), and eight of 30 (27%) of the
patients at institutions 1, 2, and 3 had positive findings of a 
6-mm
polyp at CTC, respectively. Patients from institution 4 were the subset of all
patients with positive findings of a 6-mm polyp at CTC taken from a
consecutive series of patients who underwent CTC. The time intervals between
the last patient scanned in group A and the first patient scanned in group B
were 308, 331, 363, and 328 days for each of the four institutions,
respectively.
Fifteen patients were excluded from the study because of the inability of preprocessing software to accurately segment the colon or create a center line for the colon. Of the excluded patients, 10 and 5 were from groups A and B, respectively. The number of patients excluded by institution was 2, 4, 7, and 2 for each of the four institutions, respectively.
The mean age of the patients kept in the study was 57.5 years (age range, 42–80 years), whereas the mean age of the patients excluded from the study was 57.1 years (age range, 50–67 years). The number of men and women kept in the study was 71 and 34, respectively, whereas the number of men and women excluded from the study was 10 and five, respectively.
Bowel Preparation
At institutions 1–3, patients underwent a 24-hour colonic preparation
that consisted of a clear liquid diet and oral administration of 90 mL of
sodium phosphate, 10 mg of bisacodyl, 500 mL of barium (2.1% by weight), and
120 mL of diatrizoate meglumine and diatrizoate sodium given in divided doses
[1]. Although considered
appropriate at the time of scanning of these patients, a double dose of sodium
phosphate was subsequently found to be associated with a rare but serious form
of renal failure; conse quently, in May 2006, the U.S. Food and Drug
Administration (FDA) issued a recommendation to health care professionals to
not exceed the single dose (45 mL)
[17].
At institution 4, patients underwent a standard 24-hour colonic preparation that consisted of oral administration of 45 mL of sodium phosphate, 10 mg of bisacodyl, 250 mL of barium (2.1% by weight), and 60 mL of diatrizoate sodium [9]. Patients were placed on a clear liquid diet on the day before colonic scanning and did not eat or drink after 12 midnight. This simplified preparation used half the amount of contrast agent and sodium phosphate as the preparation used at institutions 1–3 and has been equally effective for polyp detection [18–20].
CT Colonography
At institutions 1–3, a small, flexible rectal catheter was inserted
and pneumocolon was achieved by patient-controlled insufflation of room air.
Each patient was scanned in the supine and prone positions during a single
breath-hold using a 4- or 8-MDCT scanner (LightSpeed or Light Speed Ultra, GE
Healthcare). CT parameters included 1.25- to 2.5-mm section collimation, 15
mm/s table speed, 1-mm reconstruction interval, 100 mAs, and 120 kVp
[1].
At institution 4, pneumocolon was achieved by either patient-controlled manual insufflation of room air (n = 10) or automated insufflation of CO2 with an equilibrium pressure setting of 20–25 mm Hg (n = 18) [9]. The patients were scanned in the supine and prone positions using 8- or 16-MDCT scanners (LightSpeed series, GE Healthcare). In group A, 10 patients' colons were insufflated with room air and three with CO2. In group B, all 15 patients' colons were insufflated with CO2. CT parameters included 1.25- to 2.5-mm section collimation, 1-mm reconstruction interval, 50–70 mAs, and 120 kVp [9].
Image Analysis by Automated QA
A multistage procedure was performed that included computation of the colon
centerline, man ual correction of centerline connectivity errors, and
determination of the colonic segments. The colon centerline was computed using
an investigational, previously published automatic procedure
[21]. The version of the
centerline program used in this study was an improved algorithm from the
centerline method that was used in the previous QA study
[15]. The new centerline
algorithm was validated using the manual distention and fluid scoring results
from the prior QA study. Collapsed segments of the colon could have prevented
accurate connectivity of the centerline between discrete colonic segments. A
graphical user interface was used to manually correct these connectivity
errors.
Because the analyses in this article are reported, in part, on the basis of colonic segments, the segments of the colon needed to be determined. The locations of the colonic segments were predefined manually with the assistance of computer software that provided a graphical user interface. The graphical user interface enabled a user to manually place separators along the colon centerline to subdivide the colon into five segments (ascending colon, transverse colon, descending colon, sigmoid colon, and rectum). The cecum was considered part of the ascending colon for ease of analysis.
A medical student and a premedical student, under the supervision of a board-certified diagnostic radiologist, placed the colonic segment separators. The segments were defined using a modified version of a procedure developed by Taylor et al. [10]. The separators were placed at the proximal and distal ends of the colon and the junctions between segments. The portion of the colon from the anorectal junction proximally to the level of the acetabular roof was defined as the rectum. The portion of the colon proximal to the rectum to the level of the iliac crest at which the colon does not reenter the pelvis was defined as the sigmoid. The portion of the colon proximal to the iliac crest to the midpoint of the splenic flexure was defined as the descending colon. The portion of the colon between the midpoints of the splenic and hepatic flexures was defined as the transverse colon. The ascending colon was defined as the midpoint of the hepatic flexure to the portion of the cecum distal to the ileocecal valve. For the purposes of this project, the standardization of the colonic segment positions was more important than the precise definitions of the beginning and end of each segment.
After the separators were placed, the QA software computed the distention in each colonic segment. Each segment was subdivided into narrow slices about 1-cm wide perpendicular to the centerline. The mean diameter of the colon in each slice was computed. Finally, based on a previous study showing that a 2-cm colonic diameter indicated adequate distention [22], the percentage of centerline slices in each colonic segment that had a corresponding distention greater than or equal to 2 cm was calculated.
Next, the number of colon surface vertices abutting air or fluid was calculated by the QA software using the CT attenuation values at each point and converted into a percentage of surface area covered by fluid. The percentage of each segment obscured by fluid was the fluid surface area divided by the total colonic surface area.
Evaluation Across Multiple Institutions
We performed inter- and intrainstitution comparisons to evaluate trends in
quality. For evaluations not comparing supine and prone quality, the supine
and prone scores were first averaged per patient. For evaluations not
comparing quality by colonic segment, an overall colon score for each patient
was obtained by averaging the scores for each segment; the segment scores were
not weighted by their relative length.
Statistical Analysis
Paired Student's t tests were used to compare supine and prone
quality. Unpaired Student's t tests were used to compare group A
patients with group B patients and patients at different institu tions.
Analysis of variance was used to perform hypothesis testing across all four
institutions. Values for p less than 0.05 were considered
significant.
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Table 2 shows the average percentage of colonic wall surface area obscured by fluid for all four institutions. There was a significant difference in the amounts of residual colonic fluid at each of the institutions (p < 0.001). The patients at institution 4 had significantly less fluid obscuring the colonic surface in both prone and supine scans compared with the patients at institutions 1–3 (p < 0.01). There was no significant difference in residual fluid on the prone, 15.8% ± 8.7%, versus the supine, 16.6% ± 10.2%, scans (p = 0.16).
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Over time, the distention increased and the residual fluid decreased, although the differences were not statistically significant (p = 0.30 and p = 0.96, respectively) (Table 3). Regarding the different insufflation methods used at institution 4, the average distention of the patients' colons in group A, 87.0% ± 10.6%, was not significantly different from the distention of the patients' colons in group B, 85.0% ± 18.6% (p = 0.73).
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Quality varied by colonic segment and patient position. In the prone position, the least-distended segment was the sigmoid colon, whereas in the supine position, the least-distended segment was the descending colon (Table 4). Conversely, in the prone position, the best-distended segment was the rectum, whereas in the supine position, the best-distended segment was the ascending colon. In the prone position, the fraction of colonic surface obscured by residual fluid was greatest in the transverse colon, whereas in the supine position, it was greatest in the descending colon (Table 5).
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Although a larger percentage of patients at institution 4 used the automatic CO2 insufflator in group B than in group A, there was not a significant difference in colonic distention. In a larger sample of patients at institution 4, it has been reported that automatic CO2 insufflation may provide a small benefit over insufflation with room air in certain colonic segments on the supine view only [9].
In a busy clinical practice, quality can drift upward or downward over time, and slow drifts can be difficult to detect. However, although a slight increase in quality (better distention and less fluid) was observed over about a 1-year interval, the change was not statistically significant (Table 3). This finding suggests that in the settings of these clinical research programs, the radiologists and support staff were able to maintain consistent quality without the use of quantitative assessments of quality. Whether this would be the case in a nonresearch clinical program is unknown. This result also suggests that the patients' adherence to the bowel preparation was consistent over the 1-year period.
Colonic distention varied by segment and patient position. Overall, the prone scans were better distended than the supine scans. The distal colon was particularly poorly distended on the supine scans. In prone scans, the sigmoid was the least distended. In the supine scans, the sigmoid had lower distention than all segments except the descending colon. Radiologists must ensure adequate distention of the sigmoid and descending colons because distention was relatively poor in these areas and it is known that polyps frequently occur there.
We found that residual colonic fluid shifted between the supine and prone positions. The descending colon and ascending colon had less residual fluid in the prone position; and the rectum, sigmoid, and transverse colons had less fluid in the supine position. These fluid shifts are not unexpected because the patient must be turned 180° between the two positions and peristalsis is present. The fluid shifts can be helpful, particularly when fluid tagging is not used, because they improve visibility of previously obscured colonic mucosa and are one of the rationales behind scanning patients in two positions [14, 23].
The amount of residual stool and the thickness of the colonic wall are two other important factors in the quality assessment of CTC. Residual stool is known to be a reason for missing polyps [24]. Developing automated software to determine the quantity of residual stool is a difficult task, particularly prospectively and if fecal tagging is not used, because stool can have the appearance of polyps or masses. The thickness of the colonic wall is known to vary with differing degrees of colonic distention. In two studies, good or optimal colonic distention was defined as a pencil-thin bowel wall [10, 14]. In the future, QA software will need to assess the amount of residual stool and the thickness of the colonic wall in CTC scans.
Of the 12.5% (15/120) of patients who were excluded from analysis, 66.7% (10/15) were in group A. These patients were excluded because the colon centerline did not pass through a segment or part of a segment in the patient scan. Possible reasons the centerline missed part of the segment were that leakage occurred (organs outside the colon were segmented—for example, bone and liver), two segments were near each other and the centerline passed through the colon wall, or a segment was so collapsed that the centerline passed through to the next closest portion of the colon that was not necessarily the adjacent segment. The large number of cases excluded from institution 3 may be due to the increased amount of noise present in those CT scan data. The exclusion of 50% more patients from group A than from group B may also be an indication that the bowel preparation techniques improved over time so that the CTC scans could be processed and analyzed automatically.
Another limitation of the study is that our QA software requires the use of oral contrast material for fluid tagging because it is much more difficult to assess residual fluid without tagging. However, the best reported CTC results have been in studies that used fluid tagging [1, 19, 25].
In conclusion, the QA software can be used to assess the quality of CTC examinations between institutions and over time. This software may help improve the consistency and quality of CTC examinations and lead to better polyp detection.
Acknowledgments
We thank William O. Schindler for supplying CT colonography data.
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