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CLINICAL GASTROENTEROLOGY AND HEPATOLOGY 2005;3:S37–S41

olon Imaging: Computed Tomographic Colonography

ON C. ROCKEY

uke University Medical Center, Durham, North Carolina

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omputed tomographic colonography (CTC, virtualolonoscopy) is an attractive modality with which tomage the colon. Many different techniques are avail-ble; moreover, during the last several years, advances

n hardware and software have been remarkable. Evi-ence to this date suggests that CTC has varying sensi-ivity for detection of large colonic lesions, largely de-endent on technique and the method of study. A varietyf issues related to CTC are reviewed, including evolu-ion of CTC, sensitivity and specificity of CTC, patientxperience, extracolonic lesions, advances in colon prep-ration, and training. It is clear that CTC has greatromise, but also that many questions about its useemain to be answered.

istorically, the most commonly used diagnostictools to image the colon include air contrast bar-

um enema (ACBE) and colonoscopy, each of which isssociated with specific potential advantages and disad-antages. On one hand, ACBE is relatively inexpensive,imple to perform, and safe. However, some investigatorsave reported that it has excellent sensitivity,1 whereasthers have reported that it is not highly sensitive foretection of polypoid lesions.2,3 Recently, computed to-ographic colonography (CTC) (also CT colonography,T colography, or virtual colonoscopy) has been intro-uced as a method to examine the colon. Since its initialntroduction in 1994,4 it has undergone enormous mod-fications.

CTC (as for ACBE) is attractive because it is nonin-asive, is relatively simple to perform, and in theoryhould be accurate. CTC is currently performed by ca-hartic cleansing followed by air insufflation of the colon,nd CT scanning of the abdomen and pelvis, typicallyuring a single breath hold and usually in prone andupine positions; two-dimensional (2-D) axial images areaptured. Multiple variations in the way the CT scan iserformed are possible and include variations in collima-ion, slice thickness, reconstruction interval, table speed,cquisition time, and effective milliamperes. Capturedmages are then downloaded to a workstation that isquipped with software programs that allow a range ofanipulations of the data, eg, multiplanar reformation,

-D, three-dimensional (3-D) rendering, virtual dissec-

ion, computer-aided diagnosis. Variations in hardware,oftware, and general technique abound. For example,any different bowel preparations can be used, including

imple cathartic preparation and those including contrastgents. Many types of scanners are available. In addition,atients can be given air or CO2 to insufflate the colon.ir might be given by the person performing the CT

can or might be self-administered by the patient. An-ispasmodics such as glucagon or hyoscine-N-butylbro-ide might be given to try to reduce colon spasm.erhaps the most important point for the clinician toecognize is that this area is rapidly evolving and willontinue to undergo rapid changes during the next sev-ral years.

Computed TomographicColonography AccuracySince the introduction of CTC, a major endeavor

n the field has been to investigate the sensitivity ofTC.5–13 Early reports typically involved small popula-

ions at high risk for colorectal pathology and usedingle-row scanners.14 In these studies, the sensitivity ofTC compared with colonoscopy revealed that on a perolyp basis, the sensitivity of CTC was excellent forarger lesions (up to 100%) but was poor for smalleresions (11%–55% sensitivity). A number of variablesere present in these earlier studies, including differ-

nces in patient cohorts, methodology, and training ofT colonographers. The accuracy of CTC and details of

everal of these studies are provided in Table 1.A group of larger, single-center studies demonstrated

mproved detection sensitivity for polypoid lesions butotably continued to reveal wide variation in results. Inhese studies, the per polyp sensitivities for lesions inifferent categories on the basis of polyp size were thereatest for larger lesions and were as follows: �5 mm29%–59%), 6–9 mm (47%–82%), and �10 mm

Abbreviations used in this paper: ACBE, air contrast barium enema;TC, computed tomographic colonography; 3-D, three-dimensional;-D, two-dimensional.

© 2005 by the American Gastroenterological Association1542-3565/05/$30.00

PII: 10.1053/S1542-3565(05)00260-0

Table 1. Published Studies Describing Comparisons of CTC and Colonoscopy

Study, firstauthor

Yearpublished Study group

Total no.subjectsexamined

No. polyps� 1 cm/no.

cancers Preparation

Primarytechnicalmethod

Polypa Cancerb

Reconciliation CommentsSensitivity

(%)Specificity

(%)Sensitivity

(%)

Rockey et al5 2005 FOBT, IDA,BRBPR, FH

614 67/9 NaPhos 2-D 59 96 78 Yes CTC was moresensitive thanACBE

Cotton, et al6 2004 Asyx, FOBT, IDA,miscellaneous

600 46/8 NaPhos 2-D 55 96 75 Yes CTC missed onlysmall cancers

Pickhardt et al7 2003 Asyx screening 1233 82/0c NaPhos/stooltagging

3-D fly-through 94 96 — Yes 1 cancerous polypmissed bycolonoscopy

Johnson et al8 2003 Hx of polyps, FH,or anemia

703 59/NA Mixed/notagging orcontrast

2-D, 3-Dproblemsolving

63c 95 NA No

Pineau et al9 2003 Asyx, FOBT, IDA,miscellaneous

205 19/8 NaPhos/oralcontrast

NA 78 95 NA Yes 1 cancer missedby colonoscopy

McFarland et al10 2002 NA 70 40/NA NA 2-D, 3-Dproblemsolving

68 76 NA Yes 4 CTC readerscombined

Yee et al11 2001 Asyx, FOBT, IDA,miscellaneous

300 82/8 Mg citrate �PEGsolution

2-D and 3-D 90 94 100 No 2 CTC readerscombined

Fletcher et al12 2000 Hx of polyps, FH 180 121/14 PEG �biscadoyl/oralcontrast

2-D, 3-Dproblemsolving

75 90/97d NA No

Fenlon et al13 1999 Asyx, FOBT, IDA,miscellaneous

100 19/3 PEG or Mgcitrate �biscadoyl

2-D and 3-D 90 98 100 No 2 CTC readerscombined

Abbreviations: Hx, history; Asyx, asymptomatic; FH, family history of colon cancer; NaPhos, sodium phosphate; NA, not available; FOBT, fecal occult blood test positive, IDA, iron deficiency anemia;PEG, polyethylene glycol; BRBPR, bright red blood per rectum.aSensitivity/specificity for detection of lesions �1 cm in size are provided on a per patient basis.bDefined as mass lesion with adenocarcinomatous histology or in some studies as a polypoid lesion �20 mm.c2 polyps had carcinoma in situ and were less than 20 mm in size.dWithout or with oral contrast.

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July Supplement 2005 COLON IMAGING: CT COLONOGRAPHY S39

63%–92%). The specificity of CTC was generally in the0%–95% range. When analyzed on a per-patient sen-itivity basis, the sensitivity consistently increased (for 6-o 9-mm polyps, 65%–93%, and �10 mm,4%–100%). Most recently, results of several largerulti-center studies have been presented. The results ofof these studies demonstrated that CTC was signifi-

antly less sensitive than colonoscopy,5,6 whereas anotheremonstrated that CTC was more sensitive than colonos-opy for detection of lesions �10 mm.7

A number of issues contribute to the wide degree ofariability in the reported sensitivity of CTC. First,ifferent technologies have been used; multi-slice scan-ers allow more accurate detection of smaller lesions thaningle-slice scanners; software used to analyze images andven the types of images used to examine the colon varyidely. For example, 2 large multi-center studies5,6 thatsed primary 2-D reading demonstrated sensitivity ofTC that was substantially lower than with primary 3-D

eading7 (Table 1). Second, bowel preparation methodsave ranged from polyethylene glycol–electrolyte lavageolutions to oral sodium phosphate solution, magnesiumitrate, and/or bisacodyl tablets. Some studies have usedral contrast, and others have not. An additional criticalariable is the cohort of individuals examined. Sometudies have examined patients at high risk for colonbnormalities, whereas others have examined cohorts atow risk. Most studies have examined highly variableohorts. Finally, the method in which it was ascertainedhat lesions detected by CTC were accurately assessed hasaried as well. For example, colonoscopy has typicallyeen used as the gold standard; however, colonoscopyoes not detect all lesions, including large polyps.15

Evolution of Computed TomographicColonography TechniquesA number of innovations in CTC have occurred

uring the last several years. Perhaps the most criticaldvance in the CTC field has been in hardware improve-ent. Single-section helical CT technology was intro-

uced in 1988, dual-section scanners in 1992, and multi-ection scanners in 1998. Currently, extremelyophisticated multidetector (ie, 16–64 section) scannersre routinely available and allow more rapid scanning,mproved temporal resolution, and reduced motion arti-acts. This will lead to further improvement in theerformance of CTC. Indeed, preliminary data suggesthat CTC specificity was improved when CT images werebtained with 1.25-mm sections reconstructed every 1m compared with 5-mm sections reconstructed every

mm.16 l

CTC software also has evolved rapidly. An apparentritical advance has been the development of 3-D endo-copic volume-rendered views (ie, “fly-through view-ng”). Although most experts in the field believe thatoth 3-D and 2-D endoluminal views are required toptimally image the colon, 3-D viewing of the colonight be superior to 2-D reading.7,17 The 2-D multi-

lanar reformation views allow evaluation of lesions inhe context of an extraluminal orientation (an advantagen tortuous colons) and provide better information onall characteristics (very helpful for advanced lesions)

nd the density of lesions (to identify fat in lipomas ortool adherent to the wall). The 3-D views allow im-roved surface area and morphologic visualization of theolon compared with 2-D views.

Two large multicenter studies5,6 that used primary-D reading demonstrated that the sensitivity of CTCas substantially less than another study7 that usedrimary 3-D reading (Table 1). This latter study wasarticularly noteworthy because it reported that the sen-itivity of CTC was significantly greater than that forolonoscopy.7 Most clinicians currently performing CTCse the 2-D reading modality. Experts in the field sug-est that the 2 different image display techniques (2-Dnd 3-D) are complementary.

In addition, new modifications in software analysis/endering of images, including so-called virtual dissec-ion,18 computer-aided diagnostics, and more, are on theorizon. In computer-aided diagnosis, volumetric dataets are generated from transverse CT sections, and volu-etric features characterizing polyps are computed. Pol-

ps can then be detected by means of sophisticatedhresholding, followed by mathematical rule–based test-ng on the basis of feature values.19–21

There are active investigation and ongoing progress inptimizing preparation of the colon. Methods to label orag the stool (and to remove it from analysis) have beenntroduced and might improve the sensitivity of CTC.urthermore, considerable study has been directed ateveloping a minimal preparation CTC examination22,23

r no cathartic preparation.24 If proven to be highlyensitive and safe, such an approach would have majormplications for the entire field.

Miscellaneous IssuesCTC has the capability of detecting extracolonic

esions25–28 of varying importance (calcifications, gall-tones, hernias, bone lesions, abdominal aortic aneu-ysms, benign and malignant tumors). Although mostrequently of minimal importance, important extraco-

onic lesions such as renal cell carcinomas or large ab-

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ominal aortic aneurysms have been identified in asymp-omatic patients.5,28 Evaluation and management ofxtracolonic findings have been found to lead to signif-cant additional cost.27

Several studies have examined patient experience withnd preference for colon imaging procedures. Patientxperience with CTC was compared with that for otherolon imaging tests (typically colonoscopy)6,7,24,29–32

Table 2). The data are mixed. Some studies demon-trated a strong preference for CTC, whereas others in-icated that colonoscopy is preferable. All of these stud-es performed colonoscopy with sedation. “Prepless” CTCffers a substantial advance in this area, because thispproach appears to be preferable among patients.24

CTC is a new technique, and therefore specific readerraining and experience are thought to be important. Inddition, reading and interpretation of CTC by physi-ians can be tiring. Currently, little data about CTCraining or experience are available with which to makerm conclusions about this aspect of CTC. However,uidelines about CTC standards and expectations foreader experience are expected in the future.

FutureMany questions remain about the use of CTC. Is

-D reading superior to 2-D reading? Is “prepless” CTCeasible? Must CTC be able to detect small lesions (�6m in size) to be valuable for colorectal screening pro-

rams? What kind of training should those who performTC have? Can non-radiologists (ie, gastroenterologists)erform the CTC? What will the cost of CTC be? Wouldmplementing CTC be cost-effective if used as a screen-

able 2. Patient Experience and Preference Studies

Study (first authorand year) Year Study design Na

P

kerkar et al29 2001 Patient preference 295ngtuaco et al30 2001 Patient preference 323ickhardt et al7 2003 CTC vs colon

comparison1233

istvedt et al31 2003 CTC vs coloncomparison

120

otton et al6 2004 CTC vs coloncomparison

600

annaconne et al24b 2004 CTC vs coloncomparison

203

ockey et al32c 2004 CTC vs coloncomparison

614

S, not stated.Total number of subjects enrolled; in most studies not all patients cThis study used fecal tagging without cathartic cleansing for CTC, anThis study used a validated questionnaire; study included ACBE also

ng method for colorectal cancer? If CTC is implemented

s a common method of colorectal cancer screening, whatmplications does this have for the gastroenterology andadiology communities? These and other questions muste studied during the next several years, so that theastroenterology, radiology, and practicing communitiesre able to determine the role of CTC among the avail-ble colon imaging tests. It is clear that this effort shoulde a joint one, including individuals from various back-rounds.

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Address requests for reprints to: Don C. Rockey, MD, Room 336,ands Building, Box 3083, Duke University Medical Center, Durham,

C 27710. e-mail: dcrockey@acpub.duke.edu; fax: (919) 684-4983.