Careful CT colonography technique avoids pitfalls

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Careful CT colonography technique avoids pitfalls

Although there are known risk factors for the development of colorectal cancer, such as hereditary polyposis syndromes and chronic inflammatory bowel disease, most cancers occur in patients with no risk factors other than increasing age. Screening for colorectal cancer in the large average-risk population above the age of 50 has been proven cost-effective.1 Colorectal cancer screening can be performed using a variety of tests, including fecal occult blood test, flexible sigmoidoscopy, conventional colonoscopy, double contrast barium enema, and CT colonography (CTC).

CTC was first described in 19942 as a noninvasive rapid method for imaging evaluation of the colon. Controversy has arisen recently concerning the performance characteristics of CTC for colorectal cancer screening. Pickhardt et al3 reported sensitivities and specificities of CTC that compared favorably with conventional colonoscopy. CTC sensitivity for polyps greater than or equal to 10 mm, greater than or equal to 8 mm, and greater than or equal to 6 mm was 93.8%, 93.9%, and 88.7%, respectively, compared with 87.5%, 91.5%, and 92.3% for conventional colonoscopy. The specificity of CTC for polyps greater than or equal to 10 mm, greater than or equal to 8 mm, and greater than or equal to 6 mm was 96%, 92.2%, and 71.6%, respectively.

In contrast, a study performed by Cotton et al reported much lower sensitivity and specificity for CTC (sensitivity of 55% for polyps greater than or equal to 10 mm and 39% for polyps greater than or equal to 6 mm for CTC, compared with 100% and 99% for conventional colonoscopy).4 However, the Cotton study had several shortcomings in its methodology, including outdated equipment and scanning protocols. In addition, image interpretation was not performed on dedicated workstations. Most important, radiologists reading CTC examinations did not receive adequate training. Familiarity with important technical aspects of CTC will contribute to the performance of a technically adequate examination and thus maximize its sensitivity and specificity in detecting colorectal adenomas.

PATIENT PREPARATION

We advocate the use of a low-residue diet commenced 72 hours prior to the procedure. In addition, all patients take a standard purgative 24 to 48 hours before the procedure to remove stool that may mimic a polyp or mass. Table 1 outlines our approach to bowel preparation.

A variety of purging agents are commercially available: polyethylene glycol (Golytely), sodium phosphate (Fleet Prep, ColoPrep), and magnesium citrate (LoSoPrep). Each has advantages and contraindications.

- Polyethylene glycol can be taken in 4 L of water. One day prior to CTC, the patient drinks one glass of the mixture every 15 minutes until the entire mixture is consumed. This induces an osmotic diarrhea and is feasible in patients with renal and/or congestive cardiac failure. While polyethylene glycol is a good preparation for conventional colonoscopy, it is not ideal for CT colonography because it typically results in a large volume of retained fluid within the colon, which can obscure a submerged polyp and limit CTC interpretation.

- Sodium phosphate preparations consist of 45 mL of sodium phosphate, four bisacodyl laxative tablets (Dulcolax) and one bisacodyl suppository (Dulcolax). Fleet Prep Kit 1 is mixed with a large glass of chilled Gatorade or Sprite. The mixture is consumed within 20 minutes. Sodium phosphate is an osmotic cathartic agent that results in less residual fluid than polyethylene glycol and therefore a drier mucosal surface, which is preferable for CT colonography.5 Although the preparation is better tolerated, it should be used with caution in patients with renal or congestive cardiac failure.

- Magnesium citrate kits contain magnesium citrate, four bisacodyl laxative tablets (Dulcolax), and one bisacodyl suppository (Dulcolax). This bowel preparation is well tolerated and can be used in patients with renal or congestive cardiac failure. It results in a dry colon mucosa and is thus ideal for CT colonography.

Fecal tagging has been proposed to augment or even replace the standard bowel cleansing, as bowel cleansing limits patient acceptance of and compliance with colorectal cancer screening. The use of fecal tagging has resulted in improved tolerance of CTC examination.6 Residual stool or fluid is labeled with barium or an iodine solution that is ingested several times over the 48-hour period preceding CTC. Fecal tagging has recently been performed with as little as 50 mL of a 40% barium solution taken four times prior to CTC as a 12.5-mL dose with meals.7 The labeled stool is of high density on the CT image and thus can be differentiated from polyps or masses. Polyps submerged in contrast-labeled residual fluid can be seen as defects within the contrast pool. Once the stool is labeled it can be subtracted automatically, using thresholding techniques in a process called "digital cleansing."8 No consensus exists at present about the optimal approach to fecal tagging.

DATA ACQUISITION

- Colonic distention. The colon should be emptied immediately before the examination to ensure a minimum of retained fluid within the rectum. The patient is then placed in the right lateral decubitus position on the CT table. A digital rectal examination is performed prior to insertion of a soft-tipped rectal tube if a recent colonoscopy has not been performed. When CTC is performed following an incomplete endoscopic colonoscopy, a digital rectal examination is not necessary.

Colonic distention can be achieved with gentle insufflation of room air or CO2. If room air is used, a small Foley or Robinson catheter works well. If CO2 is the colonic distention agent of choice, a rectal tube similar to that used for barium enema examination is employed. Most centers prefer room air (2 to 3L volume insufflated and titrated to patient tolerance) for CTC, as it easily and reliably achieves colonic distention.

Insufflation of CO2 has been reported to be better tolerated due to the rapid resorption of CO2 through the colon wall and blood and elimination of the gas through the respiratory tract.9,10 It can be administered using an automatic CO2 pump insufflator that is pressure-sensitive and maintains a continuous preselected pressure in the colon throughout the examination. When using the pump, balloon insufflation is maintained throughout the procedure, and CO2 may be insufflated continuously until the CT scan is finished. Constant CO2 flow rates of 3 L/min are commonly used until a pressure of 25 mm Hg is reached within the rectum. A recent study demonstrated improved colonic distention without increased patient discomfort when a ramp-up flow rate to 3 L/min and a higher pressure relief system set to 50 mm Hg was used.11

The adequacy of distention is initially judged by a scout film (Figure 1). But checking the complete supine and prone data sets by scrolling through all images prior to taking the patient off the table allows identification of underdistended segments that may require repeat scanning after more air insufflation. Repositioning may be necessary, as polyps or masses can be hidden in collapsed bowel segments (Figure 2).

- Confirmation of an adequate examination. Adequate bowel preparation is determined on the initial supine scan. Scrolling through the entire supine data set in lung windows is essential to determine whether excess residual stool is present. If it is, the patient should continue to consume clear fluids for another 24 hours and a repeat examination is performed the next day (Figure 3). If excess fluid layering is noted that obscures over 50% of the lumen of a segment, injection of intravenous contrast can be considered for the scan in the prone position, to allow complete circumferential interrogation of the colonic mucosa. Another approach in patients who cannot tolerate IV contrast may be to perform an oblique scan to displace the fluid.

- Dual positioning. Evaluation of the colon during CTC requires dual positioning. This improves colonic distention,12 redistributes intraluminal fluid, and helps differentiate adherent stool from polyps (Figure 4).13 Dual positioning increases the sensitivity for detection of polyps equal to or greater than 5 mm by 15% compared with supine scanning alone.14 Towels placed under the lower chest and pelvis limit compression of transverse colon in this position. Decubitus views may be helpful to achieve dual positioning if the patient is unable to lie prone or supine.

- Smooth muscle relaxants. Glucagon hydrochloride (1 mg) and hyoscine-N-butylbromide (Buscopan) (20 to 40 mg) have been used for CTC.12,15,16 While the ability of intravenous, intramuscular, or subcutaneous injection of glucagon to aid colonic distention remains unproven in controlled trials,12,15 smooth muscle relaxants influence patient tolerance. We currently offer glucagon to all patients with spasm, not just those with sigmoid thickening due to severe diverticulosis and those very sensitive to air insufflation.

- Intravenous contrast. As a screening test, CTC is routinely performed without contrast enhancement. In some clinical situations (about 10% to 15% of examinations), however, administration of IV contrast before the prone scan may be helpful.17 These situations include:

-Patients with residual fluid obscuring more than 50 % of the lumen of a colonic segment (particularly ascending and sigmoid colon segments) on the supine scan. In this setting, polyps may be submerged in fluid and thus be undetectable despite dual positioning. IV contrast outlines the enhancing mucosa against intraluminal fluid and is taken up by polyps larger than 6 mm, allowing visualization of submerged polyps (Figure 5).

-Problem-solving at the time of examination, particularly differentiation of polyps from stool. Stool typically has a polygonal shape and contains fat, air, or barium labeling and often changes position between the supine and prone scan. Polyps, on the other hand, have a smooth, homogeneous morphology. IV contrast may be beneficial for evaluation of lesions lacking classic features of stool or polyp, and in particular for adherent stool. This minimizes the need for repeat studies in patients with poor bowel preparation.

-Patients with suspected colonic masses for staging. IV contrast is particularly helpful for evaluation of liver metastases.

-Further characterization of incidental findings such as renal masses. In a recent study, IV contrast significantly decreased the number of patients with extracolonic findings considered to be of high clinical importance. Online monitoring of extracolonic disease with administration of IV contrast when needed decreases the need for additional imaging and minimizes patient inconvenience and concern.18

We perform contrast-enhanced examinations with 100 cc of Optiray administered at a rate of 3 cc/sec. The following empiric scan delays are used: single detector, 40-sec delay; four detector, 45-sec delay; eight detector, 60-sec delay; 16 detector, 75-sec delay.

- Scan parameters. CTC can be performed using a single- or multidetector CT scanner; no significant difference has been found in the depiction of polyps larger than 10 mm.19 Advantages of multislice CT scanners are their shorter acquisition times with higher spatial resolution, which substantially reduce the occurrence of breathing and stairstep artifacts.20 In addition, colonic distention is improved compared with single-detector-row CT. To minimize the radiation dose, efforts have been made to lower the tube current to the minimum accepted dose that does not interfere with study performance. Reducing the tube current from 140 to 70 mA using single-detector CT did not influence diagnostic efficacy.21 Low-dose CTC has been shown to have excellent sensitivity and specificity for detection of colorectal neoplasms 10 mm and larger.22 Preliminary experience suggests that tube currents as low as 10 mA may be feasible in the future.

Volumetric data sets of the entire colon are acquired in the supine and prone positions during a single breath-hold of 20 to 30 sec. Slice thickness varies between 1 mm and 5 mm. Thicker slices have been found inadequate for evaluation of small polyps.23 Table 2 lists scanning parameters, and Table 3 outlines a scanning protocol.

IMAGE POSTPROCESSING

Acquired CT volumetric data sets are transferred onto a dedicated workstation equipped with navigator software. A typical study acquired using 1 to 1.5-mm collimation consists of approximately 700 images before additional views are generated on the workstation. CTC images can be analyzed in many ways. Most centers evaluate 2D images primarily with 2D multiplanar reformations and 3D endoluminal views in antegrade and retrograde directions as a problem-solving tool.24,25

For evaluation of 2D images, the magnified axial images can be viewed in colonoscopy-adjusted lung window settings (window width 1200, window level 200) optimized for polyp visualization, and abdominal windows optimized for evaluation of extracolonic findings in abdominal and pelvic organs. Multiplanar reconstructions are particularly helpful in evaluating aspects of the wall perpendicular to the imaging plane on axial images in the transverse and sigmoid colon and the hepatic and splenic flexures.

Endoluminal navigation using surface-shaded or volume-rendered postprocessing can be used for problem solving and improves differentiation of small polyps from mucosal folds. Some authors advocate 3D endoluminal navigation as the primary viewing modality.3 The conspicuity of polyps and duration of visualization have been reported to be increased with this technique, thus improving detection.26 In addition, the primary use of 3D display may be more time-effective, as 2D display requires review of large numbers of thin-collimation images.

Computer-assisted diagnosis could potentially increase the sensitivity of CTC for polyp detection and decrease the time for interpretation of the study. Current CAD methods generally rely on shape-based algorithms to localize potential polyps.27-29 While a large number of false-positive lesions are a concern with this technique, and further investigation is necessary, a recent study demonstrated an acceptable sensitivity of 88% and a low false-positive rate. The performance of CAD slightly improved with the use of IV contrast in this study.30

CONCLUSION

CTC is a fast, safe, and highly accurate method for detection of colorectal polyps and neoplasm. Best results are achieved with small slice thickness in spiral scanning in a single breath-hold. Adequate bowel preparation with bowel cleansing or fecal tagging and optimal distention are critical to performing a thorough examination. New methods for minimizing bowel preparation are promising for increasing compliance in the screening population and are likely to make CT colonography an important tool in colorectal cancer screening.

Dr. Siewert and Dr. Morrin are assistant professors of radiology, and Dr. Raptopoulos is a professor of radiology, all at Harvard Medical School.

Dr. Siewert, Dr. Morrin, and Dr. Raptopoulos have no significant financial arrangement or affiliation with any manufacturer of any pharmaceutical or medical device and are not affiliated in any manner with any provider of any commercial medical or healthcare professional service.

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REFERENCES

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7. Lefere P, Gryspeerdt S, Baekelandt M, Van Holsbeeck B. CT colonography after fecal tagging with a reduced cathartic cleansing and reduced volume of barium. AJR 2004:182(suppl):75.

8. Zalis ME, Hahn PF. Digital subtraction bowel cleansing in CT colonography. Am J Roentgenol 2001;176:646-648.

9. Rogalla P, Schmidt E, Korves M, Hamm BK. Optimal colon distention for virtual colonoscopy: room air versus CO2 insufflation. Radiology 1999;213(suppl):342.

10. Yee J. CT colonography: examination prerequisites. Abdom Imaging 2002;27(3):244-252.

11. Fidler JL, Christensen JA, Summers RM, et al. Comparison of two mechanical insufflation devices for colonic distention in CT colonography. AJR 2004:182(suppl):76.

12. Morrin MM, Farrell RJ, Keogan MT, et al. CT colonography: colonic distention improved by dual positioning but not intravenous glucagon. Eur Radiol 2002;12:525-530.

13. Chen SC, Lu DS, Hecht JR, Kadell BM. CT colonography: value of scanning in both the supine and prone positions. AJR 1999;172:595.

14. Fletcher JG, Johnson CD, Welch TJ, et al. Optimization of CT colonography technique: prospective trial in 180 patients. Radiology 2000;216:704-711.

15. Yee J, Hung RK, Akerkar GA, Wall SD. The usefulness of glucagons hydrochloride for colonic distention in CT colonography. AJR 1999;173:169-172.

16. Goei R, Nix M, Kessels AH, Ten Tusscher MP. Use of antispasmodic drugs in double contrast barium enema examination: glucagon or buscopan? Clin Radiol 1995;50:553-557.

17. Morrin MM, Farrell RJ, Kruskal JB, et al. Utility of intravenously administered contrast material at CT colonography. Radiology 2000;217:765-771.

18. Steinberg FB, Hara AK, Hernandez JL, et al. The effect of IV contrast on extracolonic findings at CT colonography. AJR 2004;182(suppl):75.

19. Hara AK, Johnson CD, MacCarty RL, et al. CT colonography: single- versus multi-detector row imaging. Radiology 2001;219:461-465.

20. McCullough CH, Bruesewitz MR, Zink FE, Johnson CD. CT colonography (CTC) using a multislice CT scanner: optimization of scan acquisition parameters. Radiology 1999;213(suppl):197.

21. Hara AK, Johnson CD, Reed JE, et al. Reducing data size and radiation dose for CT colonography. AJR 1997;168:1181-1184.

22. Macari M, Bini EJ, Xue X, et al. Colorectal neoplasms: prospective comparison of thin-section low-dose multi-detector row CT colonography and conventional colonoscopy for detection. Radiology 2002;224:383-392.

23. Beaulieu CF, Napel S, Daniel BL, et al. Detection of colonic polyps in a phantom model: implications for virtual colonoscopy data acquisition. J Comput Assist Tomogr 1998;22:656-663.

24. Dachman AH, Kuniyoshi JK, Boyle , et al. CT colonography with three-dimensional problem solving for detection of colonic polyps. AJR 1998;171:989-995.

25. Macari M, Milano A, Lavelle M, et al. Comparison of time-efficient CT colonography with two- and three-dimensional colonic evaluation for detecting colorectal polyps. AJR 2000;174:1543-1549.

26. Pickhardt PJ. Three-dimensional endoluminal CT colonography (virtual colonoscopy): comparison of three commercially available systems. AJR 2003;181:1599-1606.

27. Summers RM, Beaulieu CF, Pusanik LM, et al. Automated polyp detector for CT colonography: feasibility study. Radiology 2000;216:284-290.

28. Summers RM, Johnson CD, Pusanik LM, et al. Automated polyp detector for CT colonography: feasibility assessment in a human population. Radiology 2001;219:51-59.

29. Yoshida H, Masutani Y, MacEneaney P, et al. Computerized detection of colonic polyps at CT colonography on the basis of volumetric features: pilot study. Radiology 2002;222:327-336.

30. Summers RM, Dempsey J, Campbell SR, et al. CT colonography with intravenous contrast enhancement: computer-aided polyp and mass detection. AJR 2004;182(suppl):75.

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