Since the introduction of computed tomographic colonography (CTC) or "virtual colonoscopy" in 1994, there has been significant progress in clinical implementation and development of practice standards. At present, CTC is being performed on an elective basis at countless institutions around the world. Recent advances in the software used for interpretation and developing advances in "prepless" CTC will continue to evolve and improve the technique.

BOWEL PREPARATION AND TAGGING

A variety of scanning techniques have been described for CTC, ^1,2 all of which apply the same basic principles of patient preparation and imaging. The accuracy of CTC is directly related to the adequacy of colon cleansing since retained stool can simulate polyps, decreasing specificity, while retained fluid can obscure polyps, decreasing sensitivity. Multiple laxative regimens for CTC such as polyethylene glycol, magnesium citrate, and sodium phosphate laxative solution have been used. ³ At the current time, full colonic cleansing with cathartics is required to achieve acceptable results in routine clinical practice.

Even following a full dose bowel-cleansing regimen, residual stool can cause problem for interpretation. Methods to improve CTC accuracy using barium or/and iodine to label or "tag" residual stool have been investigated. ^4-13 These agents are orally administrated with meals, 1 to 2 days before the CTC. They mix with the fecal material allowing the differentiation between a polyp (soft-tissue density) and stool (high density). The high-density tagging agents also mix with the residual fluid allowing for "subtraction" of the fluid layer to "uncover" submerged polyps. Many clinicians have partially attributed the excellent results achieved by Pickhardt et al to the use of fecal and fluid tagging with subtraction. ^7,11

Since the requirement of a laxative for bowel cleansing in CTC is one of the major barriers for patient compliance with screening, ¹⁴ efforts to eliminate or minimize this burden are under investigation. They include allowing the patient to consume a structured low fiber diet instead of clear liquids, reducing the laxative dose by adding fecal/fluid tagging, or completely eliminating the laxative by using fecal/fluid tagging combined with electronic stool subtraction. ^8,9,10,12,13

Figure 1. Axial CT colonographic images obtained in prone (abdomen window; upper left) and in supine (lung window; upper right) showing a lipoma (arrows) on the ileal-cecal valve. The 3D image is shown just below the corresponding 2D image. (Click the image for a larger version.)

The ultimate goal of fecal tagging is to completely eliminate the need for cathartic preparation. ⁶ Once the stool has been adequately labeled, interpretation can be improved by electronically removing the tagged fecal material prior to inspection. ^7-9 This technique has been termed "electronic cleansing" or "digital stool subtraction." This automated technique has been shown to be useful when fecal tagging is combined with cathartic bowel cleansing⁷ and also when tagging is used as a complete replacement for cathartic cleansing. ^8-10 A recent study by Iannaccone et al ¹⁰ demonstrated excellent results in polyp detection using fecal tagging without cathartic and suggests that virtual colonoscopy with a "virtual preparation" is a realizable goal in the near future.

INTERPRETATION STRATEGIES

Figure 2. Translucency rendering feature. Axial CT colonography (left) shows a sigmoid polyp. The translucency rendering applied to 3D image (right) shows typical color pattern of soft-tissue polyp, consisting in red core and gradual shift to green, light blue, and dark blue hues.

Following image acquisition, the CT data is sent to a workstation (Figures 1-4) and can be viewed using a variety of techniques. ^19-22 While most experts in CTC (as of 2004) interpreted virtual colonoscopy studies using a primary 2D approach with 3D reserved for problem solving, a growing number prefer a primary 3D approach. ³ Most studies to date have used the primary 2D interpretation method ^{4,10,12,13,23-32} ; however, recent studies have achieved excellent results with a primary 3D approach. ^11,33 Few studies have directly compared primary 2D methods against primary 3D methods. ^23,32 In addition, new software algorithms have been developed to supplement or replace traditional 2D and traditional endoluminal 3D views. ^22,33,34 As workstations continue to improve, it is almost certain that the debate between primary 2D versus primary 3D interpretation will become moot; interpretation will consist of seamless integration of both 2D and 3D images. In addition, the development of computer-aided detection (CAD) will greatly influence the reading strategies and visualization methods of the future.

CTC AS A SCREENING TOOL

Figure 3. Axial CT colonographic images obtained in supine (upper left) and in prone (upper right) and the respective 3D images show a polyp in the sigmoid. (Click the image for a larger version.)

Cancer screening has been shown to reduce mortality by detecting disease at an early asymptomatic stage with better prognosis. The goal of screening is to identify individuals who are more likely to have an unrecognized disease from among the healthy population. Once identified, these individuals can undergo the more invasive, expensive, but definitive diagnostic procedure. ³⁵ At the moment, endoscopic colonoscopy has come to be used for screening but, since it is both invasive and expensive, it is not a true screening tool. ³⁵ Virtual colonoscopy or CT colonography is a true screening tool with minimal invasiveness and acceptable cost. Multiple studies have shown that it has accuracy similar to that of conventional colonoscopy both in high-risk groups ^27,28,36 and, more recently, in a low-prevalence screening population. ¹¹

LIIMITATIONS

While CT colonography has numerous advantages in comparison to other leading screening tests, such as high patient acceptance, safety, lack of sedation, and extracolonic review during polyp screening, it also has several limitations, including inability to perform simultaneous detection and removal of polyps, exposure to ionizing radiation, and requirement of colonic cleansing with cathartics. Although several studies suggest CTC has a higher accuracy than any colorectal screening test other than endoscopic colonoscopy, the accuracy of CTC is still being debated and studied. Moreover, there is conflicting data establishing the accuracy of CTC in a true screening population. ^11,23,25,37 A large multicenter trial (ACRIN) is now under way, and results of this asymptomatic cohort are eagerly anticipated. The risk/benefit balance of virtual versus endoscopic colonoscopy will need reevaluation once these results are available.

ISSUES IN IMPLEMENTATION

Figure 4. Three-dimensional image shows a flat lesion near the rectal tube (arrow; bottom right). Coronal (upper left), sagittal (upper right), and axial (bottom left) reconstructions are shown. (Click the image for a larger version.)

Issues relating to clinical implementation of CTC continue to be solved as virtual colonoscopy moves from being a research technique to becoming a generally accepted screening test. These include development of practice guidelines (technique, reporting, and training), as well as securing adequate reimbursement for the procedure. Traditionally, development of such practice guidelines follows from an accumulation of clinical data, literature reviews, and consensus opinions from knowledgeable experts in the field. ³ Three recent manuscripts have reported (or will report) on the developing consensus in technique, ³ training, ³⁸ and reporting. ³⁹

In the survey by Barish et al, ³ oral sodium phosphate solution without routine fecal tagging was the preferred preparation regimen. All surveyed experts recommended both prone and supine imaging, with most believing neither IV contrast material nor spasmolytics are necessary. Recommended scan technique included a minimum acceptable slice thickness of 3 mm and a low dose technique. There was agreement that there is a threshold size below which polyps are not clinically important. ³

Soto et al³⁸ reported on the developing consensus in training. Recommendations included attending a formal VC training course consisting of didactic lectures and review of clinical cases on a workstation. Readers of CTC studies should review a library of at least 50 cases with known pathology prior to performing interpretation on clinical patient studies. Fortunately, several formal training courses now exist throughout the country. Information regarding these training courses is readily available on the Internet (eg, www.virtualcolonoscopy.org/Training.html ).

Zalis et al ³⁹ will publish the reporting standards (C-RADS) for CTC in a manuscript by the end of the summer. The manuscript describes the development of a standardized reporting lexicon and recommendations for follow-up. C-RADS recommends that polyps less than 6 mm need not be reported due to the low prevalence of advanced adenomas or cancers in polyps of this size. Additional recommendations regarding follow-up and reporting of larger polyps and extra-colonic findings are included.

The last step in the clinical implementation of CTC as a screening test for colo-rectal cancer involves securing adequate reimbursement for the procedure. Early strides in this area have been successful in several states. In the Northeast, several Medicare carriers now cover CTC following a failed endoscopic colonoscopy. In addition, several third-party payors are now considering adding coverage for CTC for limited indications, including screening of patients who are unable to undergo endoscopic colonoscopy.

CONCLUSION

Advances in CTC techniques such as fecal tagging, software visualization strategies, and development of CAD will continue to improve accuracy and reduce current limitations. Development of consensus opinions in the areas of training, technique, and reporting will aid in the formulation of practice standards and guidelines. Finally, third-party payors will recognize the growing body of evidence in support of CTC and begin to offer coverage for screening.

Tatiana C. Rocha, MD, is research fellow at Brigham and Women's Hospital, Boston.

Matthew A. Barish, MD, is director, 3D Image Processing Center, Department of Radiology, Brigham and Women's Hospital, Boston.

References:

1. Johnson CD, Hara AK, Reed JE. Computed tomographic colonography (virtual colonoscopy): a new method for detecting colorectal neoplasms. Endoscopy. 1997;29:454-61.
2. Fenlon HM, Barish MA, Ferrucci JT. Virtual colonoscopy—technique and applications. Ital J Gastroenterol Hepatol. 1999;31:713-20.
3. Barish MA, Soto JA, Ferrucci JT. Consensus on current clinical practice of virtual colonoscopy. AJR Am J Roentgenol. 2005;184:786-92.
4. Fletcher JG, Johnson CD, Welch TJ, et al. Optimization of CT colonography technique: prospective trial in 180 patients. Radiology. 2000;216:704-11.
5. Bielen D, Thomeer M, Vanbeckevoort D, et al. Dry preparation for virtual CT colonography with fecal tagging using water-soluble contrast medium: initial results. Eur Radiol. 2003;13:453-8.
6. Lefere PA, Gryspeerdt S, Baekelandt M, Van Holsbeeck B. Laxative-free CT colonography. AJR Am J Roentgenol. 2004;183:945-8.
7. Pickhardt PJ, Choi JR. Electronic cleansing and stool tagging in CT colonography: advantages and pitfalls with primary three-dimensional evaluation. AJR Am J Roentgenol. 2003;181:799-805.
8. Zalis ME, Hahn PF. Digital subtraction bowel cleansing in CT colonography. AJR Am J Roentgenol. 2001;176:646-8.
9. Zalis ME, Perumpillichira J, Del Frate C, Hahn PF. CT colonography: digital subtraction bowel cleansing with mucosal reconstruction—initial observations. Radiology. 2003;226:911-7.
10. Iannaccone R, Laghi A, Catalano C, et al. Computed tomographic colonography without cathartic preparation for the detection of colorectal polyps. Gastroenterology. 2004;127:1300-11.
11. Pickhardt PJ, Choi JR, Hwang I, et al. Computed tomographic virtual colonoscopy to screen for colorectal neoplasia in asymptomatic adults. N Engl J Med. 2003;349:2191-200.
12. Callstrom MR, Johnson CD, Fletcher JG, et al. CT colonography without cathartic preparation: feasibility study. Radiology. 2001;219:693-8.
13. Lefere PA, Gryspeerdt SS, Dewyspelaere J, Baekelandt M, Van Holsbeeck BG. Dietary fecal tagging as a cleansing method before CT colonography: initial results—polyp detection and patient acceptance. Radiology. 2002;224:393-403.
14. Gluecker TM, Johnson CD, Harmsen WS. et al. Colorectal cancer screening with CT colonography, colonoscopy, and double-contrast barium enema examination: prospective assessment of patient perceptions and preferences. Radiology. 2003; 227:378-84.
15. Yee J, Hung RK, Akerkar GA, Wall SD. The usefulness of glucagon hydrochloride for colonic distention in CT colonography. AJR Am J Roentgenol. 1999;173:169-72.
16. Morrin MM, Farrell RJ, Keogan MT, Kruskal JB, Yam C, Raptopoulos V. CT colonography: colonic distention improved by dual positioning but not intravenous glucagon. Eur Radiol. 2002;12:525-30.
17. Taylor SA, Halligan S, Goh V, et al. Optimizing colonic distention for multi-detector row CT colonography: effect of hyoscine butylbromide and rectal balloon catheter. Radiology. 2003;229:99-108.
18. Bruzzi JF, Moss AC, Brennan DD, MacMathuna P, Fenlon HM. Efficacy of IV Buscopan as a muscle relaxant in CT colonography [published correction appears in Eur Radiol. 2004;14:756]. Eur Radiol. 2003;13:2264-70.
19. Paik DS, Beaulieu CF, Jeffrey Jr RB, Karadi CA, Napel S. Visualization modes for CT colonography using cylindrical and planar map projections. J Comput Assist Tomogr. 2000;24:179-88.
20. Fletcher JG, Johnson CD, Reed JE, Garry J. Feasibility of planar virtual pathology: a new paradigm in volume-rendered CT colonography. J Comput Assist Tomogr. 2001;25:864-9.
21. Ji H, Haker SJ, Barish MA, Kim J, Kikinis R, Ros PR. Area preserving colon unfolding: a novel display for CT colonography. Radiology. 2002;225:746.
22. Sorantin E, Werkgartner G, Balogh E, et al. Virtual dissection and automated polyp detection of the colon based on spiral CT—techniques and preliminary experience on a cadaveric phantom. Eur Surg. 2002;34:143-9.
23. Macari M, Milano A, Lavelle M, Berman P, Megibow AJ. Comparison of time-efficient CT colonography with two- and three-dimensional colonic evaluation for detecting colorectal polyps. AJR Am J Roentgenol. 2000;174:1543-9.
24. Macari M, Bini EJ, Jacobs SL, et al. Colorectal polyps and cancers in asymptomatic average-risk patients: evaluation with CT colonography. Radiology. 2004; 230:629-36.
25. Cotton PB, Durkalski VL, Pineau BC, et al. Computed tomographic colonography (virtual colonoscopy): a multicenter comparison with standard colonoscopy for detection of colorectal neoplasia. JAMA. 2004;291:1713-9.
26. Johnson CD, Harmsen WS, Wilson LA, et al. Prospective blinded evaluation of computed tomographic colonography for screen detection of colorectal polyps. Gastroenterology. 2003;125:311-9.
27. Fenlon HM, Nunes DP, Schroy PC III, Barish MA, Clarke PD, Ferrucci JT. A comparison of virtual and conventional colonoscopy for the detection of colorectal polyps. N Engl J Med. 1999;341:1496-503.
28. Yee J, Akerkar GA, Hung RK, Steinauer-Gebauer AM, Wall SD, McQuaid KR. Colorectal neoplasia: performance characteristics of CT colonography for detection in 300 patients. Radiology. 2001;219:685-92.
29. Iannaccone R, Laghi A, Catalano C, Mangiapane F, Piacentini F, Passariello R. Feasibility of ultra-low-dose multislice CT colonography for the detection of colorectal lesions: preliminary experience. Eur Radiol. 2003;13:1297-302.
30. Cohnen M, Vogt C, Beck A, et al. Feasibility of MDCT colonography in ultra-low-dose technique in the detection of colorectal lesions: comparison with high-resolution video colonoscopy. AJR Am J Roentgenol. 2004;183:1355-9.
31. Iannaccone R, Laghi A, Catalano C, et al. Detection of colo- rectal lesions: lower-dose multi-detector row helical CT colonography compared with conventional colonoscopy. Radiology. 2003;229:775-81.
32. Dachman AH, Kuniyoshi JK, Boyle CM, et al. CT colonography with three-dimensional problem solving for detection of colonic polyps. AJR Am J Roentgenol. 1998;171:989-95.
33. Vos FM, van Gelder RE, Serlie IWO, et al. Three-dimensional display modes for CT colonography: conventional 3D virtual colonoscopy versus unfolded cube projection. Radiology. 2003;228:878-85.
34. Pickhardt PJ. Translucency rendering in 3D endoluminal CT colonography: a useful tool for increasing polyp specificity and decreasing interpretation time. AJR Am J Roentgenol. 2004;183:429-36.
35. Helm J, Choi J, Sutphen R, et al. Current and evolving strategies for colorectal cancer screening. Cancer Control. 2003;10:193-204.
36. 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;224:383-92.
37. Edwards JT, Mendelson RM, Fritschi L, et al. Colorectal neoplasia screening with CT colonography in average-risk asymptomatic subjects: community-based study. Radiology. 2004;230:459-64.
38. Soto JA, Barish MA, Ferrucci J. CT Colonography Interpretation: Guidelines for Training Courses. Presented at: RSNA annual meeting; November 28-December 3, 2004: Chicago.
39. Zalis ME et al. CT Colonography Reporting and Data System (C-RADS): a Consensus Statement. Radiology. In press.