US Gastroenterology & Hepatology Review, 2007;(2):78-82
Virtual colonoscopy was first described in 1994.1 More properly termed ‘computerized tomographic colonography’ (CTC), this technique uses sophisticated post-processing software to generate 2D and 3D reconstructions of the colon based on data from spiral computerized tomography of the abdomen and pelvis. There has been intense interest in CTC because of its potential to complement or replace conventional colonoscopy as a less invasive means of visualizing the colon.
Scanning Technique
CTC technique is still evolving and has not yet been completely standardized. In most institutions, bowel preparation is currently required for CTC because retained stool in the lumen may be mistaken for polyps. The colon is distended with gas during the scanning procedure, as visualization is compromised in underinflated segments. The degree of insufflation is controlled by the technician, the patient, or an electronic insufflation device. In most centers, room air is used for colonic distension, although carbon dioxide may be better tolerated.2 Smooth-muscle relaxants such as intravenous glucagon are occasionally administered before scanning in an attempt to minimize colonic motility-related artifacts; however, studies have shown that smooth-muscle relaxants do not significantly improve colonic distension or polyps detection.3 The use of intravenous contrast results in increased attenuation of mass lesions and improved differentiation of polyps from colonic fluid.4 In addition, oral iodinated contrast may be ingested in order to change the attenuation of residual colonic fluid; however, no significant improvement in sensitivity was seen in studies.5 In Pickhardt’s landmark study (see below), the investigators used combined solid-stool tagging with barium and luminal fluid opacification with oral iodinated contrast in order to optimize results.6 Fecal tagging is particularly important when CTC is performed without cathartic preparation.7
During colonography, the abdomen is scanned during one or two breath-holds lasting under two minutes. Scans are performed in the craniocaudal direction, with the patient in both prone and supine positions. The addition of the prone position improves distension of colonic segments and allows for displacement of fluid and stool, aiding visualization.5 Scanning in the supine and left lateral decubitus positions may improve visualization even further.8 The best results for CTC have been reported using multidetector (four- or eight-channel) scanners with 1.25–2.5mm collimation and reconstruction intervals of 1mm. Standard helical images of the colon are processed by imaging software using one of three rendering techniques: surface rendering, volume rendering, or perspective rendering. In addition to 2D axial, coronal, and sagittal images, 3D-rendered views of the colon that simulate endoluminal views during colonoscopy can be reproduced. These allow both anterograde and retrograde ‘fly-throughs’ of the colon, with the ability to examine the proximal aspect of haustral folds—a potential blind-spot for conventional colonoscopy.
References:
1. Vining DJ, Virtual endoscopy: is it reality?, Radiology,
1996;200(1):30–31.
2. Pineau BC, Paskett ED, Chen GJ, et al., Virtual colonoscopy
using oral contrast compared with colonoscopy for the
detection of patients with colorectal polyps, Gastroenterology,
2003;125(2):304–10.
3. Morrin MM, Farrell RJ, Keogan MT, et al., CT colonography:
colonic distention improved by dual positioning but not
intravenous glucagon, Eur Radiol, 2002;12(3):525–30.
4. Oto A, Gelebek V, Oguz BS, et al., CT attenuation of colorectal
polypoid lesions: evaluation of contrast enhancement in CT
colonography, Eur Radiol, 2003;13(7):1657–63.
5. Fletcher JG, Johnson CD, Welch TJ, et al., Optimization of CT
colonography technique: prospective trial in 180 patients,
Radiology, 2000;216(3):704–11.
6. 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(23):2191–2200.
7. Iannaccone R, Laghi A, Catalano C, et al., Computed
tomographic colonography without cathartic preparation for the
detection of colorectal polyps, Gastroenterology, 2004;127(5):
1300–11.
8. Gryspeerdt SS, Herman MJ, Baekelandt MA, et al., Supine/left
decubitus scanning: a valuable alternative to supine/prone
scanning in CT colonography, Eur Radiol, 2004;14(5):768–77.
9. Fenlon HM, Nunes DP, Schroy PC 3rd, et al., A comparison of
virtual and conventional colonoscopy for the detection of
colorectal polyps, N Engl J Med, 1999;341(20):1496–1503.
10. Pickhardt PJ, Nugent PA, Mysliwiec PA, et al., Location of
adenomas missed by optical colonoscopy, Ann Intern Med,
2004;141(5):352–9.
11. Johnson CD, Harmsen WS, Wilson LA, et al., Prospective blinded
evaluation of computed tomographic colonography for screen
detection of colorectal polyps, Gastroenterology, 2003;125(2):
311–19.
12. 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(14):1713–19.
13. Rockey DC, Paulson E, Niedzwiecki D, et al., Analysis of air
contrast barium enema, computed tomographic colonography,
and colonoscopy: prospective comparison, Lancet,
2005;365(9456):305–11.
14. Sosna J, Morrin MM, Kruskal JB, et al., CT colonography of
colorectal polyps: a metaanalysis, AJR Am J Roentgenol,
2003;181(6):1593–8.
15. Halligan S, Altman DG, Taylor SA, et al., CT colonography in the
detection of colorectal polyps and cancer: systematic review,
meta-analysis, and proposed minimum data set for study level
reporting, Radiology, 2005;237(3):893–904.
16. Mulhall BP, Veerappan GR, Jackson JL, Meta-analysis: computed
tomographic colonography, Ann Intern Med, 2005;142(8):
635–50.
17. Rosman AS, Korsten MA, Meta-analysis comparing CT
colonography, air contrast barium enema, and colonoscopy, Am
J Med, 2007;120(3):203–10, e4.
18. van Gelder RE, Birnie E, Florie J, et al., CT colonography and
colonoscopy: assessment of patient preference in a 5-week
follow-up study, Radiology, 2004;233(2):328–37. Epub 2004
Sep 9.
19. Angtuaco TL, Banaad-Omiotek GD, Howden CW, Differing
attitudes toward virtual and conventional colonoscopy for
colorectal cancer screening: surveys among primary care
physicians and potential patients, Am J Gastroenterol,
2001;96(3):887–93.
20. Svensson MH, Svensson E, Lasson A, Hellstrom M, Patient
acceptance of CT colonography and conventional colonoscopy:
prospective comparative study in patients with or suspected of
having colorectal disease, Radiology, 2002;222(2):337–45.
21. Thomeer M, Bielen D, Vanbeckevoort D, et al., Patient
acceptance for CT colonography: what is the real issue?, Eur
Radiol, 2002;12(6):1410–15.
22. Akerkar GA, Yee J, Hung R, McQuaid K, Patient experience and
preferences toward colon cancer screening: a comparison of
virtual colonoscopy and conventional colonoscopy, Gastrointest
Endosc, 2001;54(3):310–15.
23. Ristvedt SL, McFarland EG,Weinstock LB, Thyssen EP, Patient
preferences for CT colonography, conventional colonoscopy, and
bowel preparation, Am J Gastroenterol, 2003;98(3):578–85.
24. Taylor SA, Halligan S, Saunders BP, et al., Acceptance by
patients of multidetector CT colonography compared with
barium enema examinations, flexible sigmoidoscopy, and
colonoscopy, AJR Am J Roentgenol, 2003;181(4):913–21.
25. Bosworth HB, Rockey DC, Paulson EK, et al., Prospective
comparison of patient experience with colon imaging tests, Am
J Med, 2006;119(9):791–9.
26. Gluecker TM, Johnson CD, Harmsen WS, et al., Colorectal cancer
screening with CT colonography, colonoscopy, and doublecontrast
barium enema examination: prospective assessment of
patient perceptions and preferences, Radiology, 2003;227(2):
378–84.
27. Sosna J, Sella T, Bar-Ziv J, Libson E, Perforation of the colon and
rectum—a newly recognized complication of CT colonography,
Semin Ultrasound CT MR, 2006;27(2):161–5.
28. Coady-Fariborzian L, Angel LP, Procaccino JA, Perforated colon
secondary to virtual colonoscopy: report of a case, Dis Colon
Rectum, 2004;47(7):1247–9.
29. Wong SH, Wong VW, Sung JJ, Virtual colonoscopy-induced
perforation in a patient with Crohn’s disease, World J
Gastroenterol, 2007;13(6):978–9.
30. Triester SL, Hara AK, Young-Fadok TM, Heigh RI, Colonic
perforation after computed tomographic colonography in a
patient with fibrostenosing Crohn’s disease, Am J Gastroenterol,
2006;101(1):189–92.
31. Kamar M, Portnoy O, Bar-Dayan A, et al., Actual colonic
perforation in virtual colonoscopy: report of a case, Dis Colon
Rectum, 2004;47(7):1242–4, discussion 1244–6.
32. Young BM, Fletcher JG, Earnest F, et al., Colonic perforation at
CT colonography in a patient without known colonic disease,
AJR Am J Roentgenol, 2006;186(1):119–21.
33. Burling D, Halligan S, Slater A, et al., Potentially serious adverse
events at CT colonography in symptomatic patients: national
survey of the United Kingdom, Radiology, 2006;239(2):464–71.
34. Sosna J, Blachar A, Amitai M, et al., Colonic perforation at CT
colonography: assessment of risk in a multicenter large cohort,
Radiology, 2006;239(2):457–63.
35. Kay CL, Kulling D, Hawes RH, et al., Virtual endoscopy—
comparison with colonoscopy in the detection of spaceoccupying
lesions of the colon, Endoscopy, 2000;32(3):226–32.
36. Brenner DJ, Radiation risks potentially associated with low-dose
CT screening of adult smokers for lung cancer, Radiology,
2004;231(2):440–45.
37. Ladabaum U, Song K, Fendrick AM, Colorectal neoplasia
screening with virtual colonoscopy: when, at what cost, and
with what national impact?, Clin Gastroenterol Hepatol,
2004;2(7):554–63.
38. Heitman SJ, Manns BJ, Hilsden RJ, et al., Cost-effectiveness of
computerized tomographic colonography versus colonoscopy for
colorectal cancer screening, CMAJ, 2005;173(8):877–81.
39. Sonnenberg A, Delco F, Bauerfeind P, Is virtual colonoscopy a
cost-effective option to screen for colorectal cancer?, Am J
Gastroenterol, 1999;94(8):2268–74.
40. Hur C, Gazelle GS, Zalis ME, Podolsky DK, An analysis of the
potential impact of computed tomographic colonography
(virtual colonoscopy) on colonoscopy demand, Gastroenterology,
2004;127(5):1312–21.
41. Lin OS, Kozarek RA, Schembre DB, et al., Risk stratification for
colon neoplasia: screening strategies using colonoscopy and
computerized tomographic colonography, Gastroenterology,
2006;131(4):1011–19.
42. Tsai CJ, Lu DK, Small colorectal polyps: histopathology and
clinical significance, Am J Gastroenterol, 1995;90(6):988–94.
43. Pickhardt PJ, Hassan C, Laghi A, et al., Cost-effectiveness of
colorectal cancer screening with computed tomography
colonography: the impact of not reporting diminutive lesions,
Cancer, 2007;109(11):2213–21.
44. Hur C, Chung DC, Schoen RE, Gazelle GS, The management of
small polyps found by virtual colonoscopy: results of a decision
analysis, Clin Gastroenterol Hepatol, 2007;5(2):237–44.
45. Gluecker TM, Johnson CD, Wilson LA, et al., Extra-colonic
findings at CT colonography: evaluation of prevalence and cost
in a screening population, Gastroenterology, 2003;124(4):
911–16.
46. Hellstrom M, Svensson MH, Lasson A, Extra-colonic and
incidental findings on CT colonography (virtual colonoscopy),
AJR Am J Roentgenol, 2004;182(3):631–8.
47. Rajapaksa RC, Macari M, Bini EJ, Prevalence and impact of
Extra-colonic findings in patients undergoing CT colonography,
J Clin Gastroenterol, 2004;38(9):767–71.
48. Morrin MM, Kruskal JB, Farrell RJ, et al., Endoluminal CT
colonography after an incomplete endoscopic colonoscopy, AJR
Am J Roentgenol, 1999;172(4):913–18.
49. Fenlon HM, McAneny DB, Nunes DP, et al., Occlusive colon
carcinoma: virtual colonoscopy in the preoperative evaluation of
the proximal colon, Radiology, 1999;210(2):423–8.
50. Halligan S, Altman DG, Mallett S, et al., Computed tomographic
colonography: assessment of radiologist performance with and
without computer-aided detection, Gastroenterology,
2006;131(6):1690–99.
51. Summers RM, Yao J, Pickhardt PJ, et al., Computed tomographic
virtual colonoscopy computer-aided polyp detection in a
screening population, Gastroenterology, 2005;129(6):1832–44.
