A

Cip

fp©

K

1

aecnndcs3

irpwTpr

0d

European Journal of Radiology 61 (2007) 388–399

CT colonography: Techniques, indications, findings

Thomas Mang a,∗, Anno Graser b, Wolfgang Schima a, Andrea Maier a

a Department of Radiology, Medical University of Vienna, Waehringer Guertel 18-20, A-1090 Vienna, Austriab Department of Clinical Radiology, Grosshadern Campus, University of Munich,

Marchioninistrasse 15, 81377 Munich, Germany

Received 21 October 2006; received in revised form 29 October 2006; accepted 2 November 2006

bstract

Computed tomographic colonography (CTC) is a minimally invasive technique for imaging the entire colon. Based on a helical thin-sectionT of the cleansed and air-distended colon, two-dimensional and three-dimensional projections are used for image interpretation. Several clinical

mprovements in patient preparation, technical advances in CT, and new developments in evaluation software have allowed CTC to develop into aowerful diagnostic tool. It is already well established as a reliable diagnostic tool in symptomatic patients.

Many experts currently consider CTC a comparable alternative to conventional colonoscopy, although there is still debate about its sensitivityor the detection of colonic polyps in a screening population. This article summarizes the main indications, the current techniques in patientreparation, data acquisition and data analysis as well as imaging features for common benign and malignant colorectal lesions.

2007 Published by Elsevier Ireland Ltd.

epara

cinnrpcfaib

tyc

eywords: CT colonography; Polyps; Colorectal cancer; Indications; Bowel pr

. Introduction

Computed tomographic colonography (CTC), also knowns virtual colonoscopy (VC), is a powerful technique for thevaluation of the entire colon. It has potential advantages overonventional colonoscopy because of its minimally invasiveature and no need for sedation and recovery time. The exami-ation is based on a helical, thin-section CT of the cleansed andistended colon. Data evaluation is performed with commer-ially available CT colonography post-processing software withimultaneously available multiplanar 2D and virtual endoscopicD image displays.

The ability of CTC to detect colorectal polyps has been testedn a multitude of studies. CTC appeared to be promising in high-isk populations, with a reported sensitivity greater than 90% forolyps ≥10 mm. Recent results in the low prevalence populationere more heterogeneous and less impressive (34–93.8%) [1–4].his wide range of results is likely caused by differences in

atient selection, examination and evaluation techniques, andeader experience [5,6].

∗ Corresponding author. Tel.: +43 1 40400 4893; fax: +43 1 40400 4898.E-mail address: thomas.mang@meduniwien.ac.at (T. Mang).

2

pcea

720-048X/$ – see front matter © 2007 Published by Elsevier Ireland Ltd.oi:10.1016/j.ejrad.2006.11.019

tion

Currently, there are several clinical indications for CTolonography. They include evaluation of the colon after anncomplete or unsuccessful conventional colonoscopic exami-ation and evaluation of the colon proximal to an obstructingeoplasm [7–11]. Another potential indication for CT colonog-aphy is in the evaluation of frail and elderly patients oratients who would have an increased risk with conventionalolonoscopy. The use of CTC to monitor patients after surgeryor colorectal cancer is currently under investigation [12,13]. Inddition, CT colonography may contribute to colorectal screen-ng by providing a safe, effective, and rapid examination that cane used to evaluate the entire colon for clinically relevant lesions.

This article summarizes the main indications, the currentechnique in patient preparation, data acquisition and data anal-sis, and the imaging features of common benign and malignantolorectal lesions.

. Indications

Currently, there are several indications where CT colonogra-

hy may play an important role in patient care. These includelinical indications, such as the evaluation of patient symptoms,valuation after an incomplete colonoscopy, and screening insymptomatic high- and average-risk patients.

al of Radiology 61 (2007) 388–399 389

2

otcdcsdtcaaua

uhrbocTD1b1asnaedt

aat5iIo

poaicfCc

wiaa

Fig. 1. Incomplete colonoscopy because of a stenotic cancer of the transversecolon. (a) Surface-rendered global CT image shows the stenosis of the transversecolon (arrow) and a second stenosis in the ascending colon (arrowhead). (b)Supine 2D axial contrast-enhanced CT image shows circular wall thickeningact

eabt

i

T. Mang et al. / European Journ

.1. Clinical indications

The most widely accepted clinical indication is incompleter failed colonoscopy. An incomplete colonoscopic examina-ion is defined as a failure to intubate the caecum. Incompleteolonoscopy may be the result of poor bowel preparation, redun-ant colon, and patient intolerance to the procedure, spasm, orolonic obstruction caused by a neoplastic or non-neoplastictenosis. The CTC examination can be performed on the sameay directly after conventional colonoscopy, and thus, no addi-ional bowel preparation is needed [14]. CTC can complete theolonic evaluation in the majority of patients, which can alsonalyze the cause of incomplete endoscopy [7,8]. Patients withhistory of incomplete colonoscopy are at higher risk for fail-re of a second attempt. Therefore, CTC, rather than a secondttempt at conventional colonoscopy, may be recommended.

Double-contrast barium enema (DCBE), traditionally beingsed to evaluate the proximal colon in incomplete colonoscopy,as been outperformed by CTC in some institutions [15]. Theesidual colonic air distention after a failed colonoscopy maye an impediment to a successful, immediate, follow-up DCBEn the same day, but an immediate follow-up CTC is benefi-ial because the requisite colonic distention is already present.wo recent large prospective clinical trials show a sensitivity forCBE of only 50% for “significant” polyps, measuring at least0 mm in diameter [4,16]. At the same time, CTC has shown aetter performance in several studies in patients with polyps0 mm or larger [17]. In addition, patient preference studiesre also discouraging for DCBE. In two recent questionnairetudies, patients ranked DCBE a distant last among three alter-ative colon cancer screening tests CTC, optical colonoscopy,nd DCBE [4,18]. Last, but not least, in the future, radiologicxperience in performing DCBE studies will fade because of theecreasing volume of investigations and fewer knowledgeableeachers.

In cases of an obstructing cancer, CTC offers informationbout the pre-stenotic colon, local tumor invasion, lymph nodes,nd distant metastases [19–21]. CTC reportedly allowed detec-ion of synchronous carcinomas, which occur in approximately% of cases, as well as synchronous adenomas, with a reportedncidence of 27–55% proximal to the stenosis [7,9,11] (Fig. 1).n this setting, IV contrast is helpful to enable a complete stagingf the patient [11].

Another potential indication for CTC is in the evaluation ofatients with contraindications to colonoscopy or who refusether screening options [22]. This includes patients in need ofnticoagulation, a history of difficult or incomplete colonoscopyn the past, and patients who cannot be sedated due to medicalonditions. In addition, in cases of advanced patient age, and inrail or immobile patients and patients with severe comorbidities,TC can be safely performed to exclude neoplastic or stenoticonditions [22].

Taylor et al. reported on the use of diagnostic CTC in patients

ith positive symptoms of colorectal cancer, such as a change

n bowel habits, lower gastrointestinal bleeding, iron deficiencynemia, or abdominal mass and abdominal pain [23]. Theuthors suggested that CTC may be a valuable tool for the rapid

CliC

nd contrast enhancement in the transverse (arrow) as well as in the ascendingolon (arrowhead). Histology revealed a second cancer in the ascending colonhat was not diagnosed by endoscopy.

valuation of symptomatic patients. In addition, CTC could haven extra benefit for the local staging of tumors (T3, T4 stages)ecause of its high spatial resolution and excellent depiction ofhe colonic wall, as well as adjacent pericolonic fat.

With regard to post-surgical conditions in the colon, theres no general agreement about the use of CT colonography.

ontrast-enhanced CT colonography has the potential to detect

ocal recurrence, metachronous disease, and distant metastasesn patients with a history of invasive colorectal cancer [12,13].urrently, endoscopy or barium enemas are performed in many

3 al of

ctpcmcaa

ccocoediowspsc

cre

2

ph

mtClsc

laa[cr

sAnCbeaif

pti[

2

restiptgp

3

cmad

3

pflTl

ppmCP(ser

rmlntffcc

90 T. Mang et al. / European Journ

ases after colonic surgery for routine surveillance, to detectumour recurrence, or to discover a metachronous cancer. Afterartial colonic resection, some of these follow-up examinationsould be replaced by contrast-enhanced CT colonography. Inost cases, CT colonography allows visualization of the entire

olon, which is important for the demonstration of post-surgicalnatomic conditions. Two-dimensional views offer informationbout the wall morphology of the anastomosis [4,16].

In addition to these main indications, there are otheronditions where the role of CT colonography is not yetlearly defined. Some of these conditions may lead to colonicbstruction, where CTC may be performed after incompleteolonoscopy. However, CTC may also be used for surveillancef these conditions, as an alternative to colonoscopy or bariumnema. Diverticular disease is the most common colonic disor-er in the Western world and often leads to diverticulitis. CTCs not only helpful in the assessment of the lumen and the extentf diverticulosis, but also for any extramural changes in casesith the chronic stenotic stages of diverticulits [24]. At chronic

tages of inflammatory bowel diseases, CT colonography canrovide information on the extent of the disease and about steno-is and prestenotic regions, as well as the extracolonic extent andomplications of the diseases [25–27].

At the very least, patients who are unwilling to undergoonventional colonoscopy because of anxiety, fear, or embar-assment may profit from a relatively painless and safe CTCxamination [22].

.2. Colorectal cancer screening

In contrast to symptomatic patients, CTC is also considered aotential screening tool for colorectal neoplasia in asymptomaticigh- and average-risk patients.

The goal of colorectal cancer screening is a reduction in theorbidity and the mortality associated with colorectal cancer

hrough early detection and resection of adenomas and cancer.TC as a screening tool has the potential for a wider pub-

ic acceptance compared to conventional colonoscopy. Severaltudies have shown that patients prefer CTC over conventionalolonoscopy [18,28,29].

Although a recent study by Pickhardt et al. showed an excel-ent performance for CTC in an average-risk population, with

reported sensitivity of 93.8% for polyps ≥10 mm [3], otheruthors achieved less impressive results ranging from 34 to 53%1,2,4]. This wide scatter in sensitivity was attributed to techni-al differences with regard to the examinations, evaluations, andeader experience.

Therefore, the current use of VC generally does not includecreening of asymptomatic individuals, as suggested by themerican Cancer Society [30] and the American Gastrointesti-al Association [31]. Both of these organizations decided thatTC should not yet be used for colorectal cancer screening,ecause data on true screening populations are missing. How-

ver, further attempts to validate this rapidly evolving techniquere ongoing. Currently, CTC is the second best imaging modal-ty for complete colonic evaluation, being superior to DCBEor detection of colorectal polyps and cancer [79]. Therefore, a

Ea

o

Radiology 61 (2007) 388–399

ractical approach is to consider CTC a reasonable alternativeo the other colorectal cancer screening tests when a patients unable or unwilling to undergo conventional colonoscopy32].

.3. Contraindications

Contraindications to CTC include acute abdominal pain,ecent abdominal or pelvic surgery, abdominal wall hernia withntrapment of colonic loops, and acute inflammatory conditions,uch as acute diverticulitis, acute active stage of ulcerative coli-is or Crohn’s disease, and toxic megacolon. In these conditions,nsufflation of the colon can lead to perforation and widespreaderitonitis [33–35]. In addition, there are also general CT con-raindications that matter in CTC as well, such as weight andirth limitations of the scanner, artifacts from metal prosthesis,regnancy, and patients with claustrophobia.

. CT colonography technique

CT colonography is based on a helical, thin-section CT of theleansed and distended colon. The examination consists of threeajor steps: patient preparation, including cathartic cleansing

nd distention of the bowel; data acquisition with MDCT; andata evaluation.

.1. Bowel preparation

The key element of a high quality CTC examination is a well-repared clean, and well-distended colon. Residual stool anduid may lead to a false-negative or false-positive diagnosis.herefore, CTC, at present, requires full bowel preparation, just

ike colonoscopy and DCBE.Patients usually follow a clear liquid diet starting about 24 h

rior to the CTC examination. Cathartic cleansing is usuallyerformed by oral administration of laxatives. Different com-ercially available bowel preparations have been described forTC, including cathartics, such as magnesium citrate (LoSoreparation, EZ-Em Inc, Westbury, USA) and phosphosodaFleet Pharmaceuticals, Lynchburg, VA), and colonic lavageolutions, such as polyethylene glycol (PEG). Radiologists gen-rally need a dry colon because of the inability to aspirateesidual fluid during the examination.

It has been reported that phosphosoda and magnesium cit-ate result in an adequately prepared and dry colon for theajority of patients, whereas PEG preparations frequently leave

arge amounts of residual fluid in the colon and, therefore, areot optimal for CTC [36]. However, it should be mentionedhat phosposoda is contraindicated in patients with known renalailure, preexisting electrolyte abnormalities, congestive heartailure, ascites, or ileus [37]. Therefore, because PEG is notontraindicated in patients with renal or cardiac insufficiency, itan be used alternatively in these cases [22]. In addition, some

uropean centers have reported the use of other laxatives, suchs Picolax [38].

As opposed to a standard bowel preparation, the additionf oral contrast agents (small amounts of barium or iodine

T. Mang et al. / European Journal of

Fs(

chdtyasrbeo1SFic

palopccfcataop

3

C

li

atcrsdrcpbCbdawratcaoipgaudgvatltsi

NdidgttcipIc[

ig. 2. Fecal tagging with orally ingested barium: prone 2D axial CT imagehows high attenuation of the contrast marked fecal residuals and residual fluidarrows).

ontrast) will tag residual stool or fluid (Fig. 2). The resultingigher attenuation of fecal and fluid residues simplifies theiristinction from colonic abnormality. Although commercialagging agents (Tagitol, EZ-Em Inc., Westbury, USA) are notet available in the EU, there is no general consensus about thegent or the amount preferable for use in fecal tagging. Whereasome authors prefer tagging with barium only [39], others haveeported good results with iodine [40] or a combination ofoth to achieve fecal and fluid tagging [3]. In brief, Pickardt al. achieved excellent results with a combination of 500 mlf 2.1% barium contrast material for solid stool tagging and20 ml of an ionic water-soluble contrast agent (Gastrografin,chering AG, Berlin, Germany) for fluid opacification [3].ecal tagging is now advocated by an increasing number of

nvestigators as the method of choice to prepare the colon for CTolonography.

Compared to cathartic preparations, so called prepless bowelreparation is a laxative-free regimen where patients may eatnd drink normally or are asked to maintain a low residue orow fiber diet while fecal tagging is achieved by multiple dosesf oral contrast (barium or iodinated contrast) over a 24–72 heriod prior to CTC. With computer-generated electronicleansing techniques, either by using thresholding or using spe-ific subtraction computer software, the high attenuation taggedecal material can be subtracted from the data, leaving only theolonic wall and colonic lesions [41,42]. Although initial resultsre promising for detection of lesions of 1cm or larger [40,41],his method is still at the research stage and must be validated inscreening population. In the future, prepless CTC may increaseverall patient compliance by replacing the cumbersome bowelreparation.

.2. Bowel distention

Optimal colonic distention is a fundamental prerequisite forTC data evaluation that allows intraluminal evaluation of the

ricb

Radiology 61 (2007) 388–399 391

arge bowel. Underdistended or collapsed segments may hidentraluminal lesions.

Before beginning bowel distention, patients should bedvised to empty their rectum. Bowel distention is performed inhe left decubitus or supine position with a thin, flexible rubberatheter placed in the rectum (e.g., thin plastic or rubber 14Fectal tube, small gauge Foley catheter) [43]. Larger catheters,uch as those used in DCBE, are not required to successfullyistend the colon in CTC [44]. For bowel distention, eitheroom air or carbon dioxide (CO2) can be used. The easiest andheapest method is manual room air distention via a handheldlastic bulb insufflator, an effective technique that can evene performed by patients themselves. Some authors argue thatO2 is superior to air, largely based on colonoscopy and thearium enema literature, which suggests that it causes lessiscomfort because of its rapid mucosal absorption. CO2 can bedministered either manually, over a standard enema bag filledith approximately 3 l of gas (via a gas cylinder) attached to a

ectal catheter over a connecting tube, or automatically, usingdedicated insufflation device (Protocol colon insufflation sys-

em, EZ-Em Inc., Westbury, USA). This device electronicallyontrols the flow rate of CO2, the total administered gas volume,nd the intracolonic pressure (which is limited up to a maximumf 25 mmHg). Advocates of automated insufflation suggest thatntroducing CO2 at controlled flow rates and pressures improvesatient compliance and overall distention [45,46]. During theas insufflation, the patient is placed in the supine positionnd gentle insufflation is continuous until the patient feelsncomfortable or bloated. This generally will take 2–4 l of gas,epending on the patient’s individual colonic anatomy. Fixedas amounts may be impractical because of different colonicolumes. After distention is completed, a CT scout view of thebdomen is obtained to ensure optimal colonic distention ando add additional gas if collapsed segments are identified. Fol-owing the supine axial image acquisition, the patient is turnedo the prone position. Before prone image acquisition, anothercout scan is obtained with additional gaseous insufflationf needed.

The i.v. administration of antispasmodic agents (hyoscin--butylbromide, buscopan, or glucagon) may improve colonicistention and reduce spasms. However, the use of spasmolyticss controversial; i.v. hyoscin-N-butylbromide likely improvesistention, but is not licensed in the United States [44], whereaslucagon hydrochloride, its licensed alternative, does not appearo be as beneficial [47,48]. In addition, some authors arguehat administration of an IV drug adds to patient discomfort,reates an opportunity for possible new side effects, andncreases examination time and cost. The general opinion,rovided by a recently published, consensus statement is thatV spasmolytics should not be administered routinely, butan be used if patients experience pain, discomfort, or spasm32].

Bowel distention may lead to perforation of the bowel in

are cases. In most of the reported cases, perforation occurredn symptomatic patients with acute inflammatory or stenoticolons, the bowel distention was performed manually, and rectalalloon catheters were used [33–35].

3 al of

3

ntsavossoidsconpc

rbn1to4aWfiis

rfniswer“tneteamsC

3

et

malpbaoprtp

4

cdapsr

3mavea

aasattpt3bsAibfli

3rAtpb

92 T. Mang et al. / European Journ

.3. CT scanning

CT scanning is ideally performed on an MDCT scan-er in both the supine and then the prone positions with ahin collimation. MDCT has several technical advantages overingle-detector-row CT, including faster imaging times andcquisition of multiple [4–64] thin sections with nearly isotropicoxels. The higher speed and spatial resolution of MDCT shouldffer improved sensitivity and specificity for CTC compared toingle-detector CT. As mentioned before, acquisition of an initialcout view before each scan helps to ensure adequate distentionf the colonic segments with additional CO2 or room air beingnsufflated, if required. The use of both the supine and prone CTatasets helps to differentiate mobile stool from fixed pathology,uch as polyps or cancers, allows more even distention of theolon because of gas redistribution, and improves visualizationf segments of the colon that may be obscured by intralumi-al fluid [49,50]. To avoid breathing artifacts, which are morerominent in the upper abdomen, scanning is performed in theephalo-caudal direction.

Thin sections are a prerequisite for high-quality multiplanareformations (MPR) and 3D reconstructions. As recommendedy a recently published consensus statement, collimation shouldot exceed 3 mm when using MDCT [32]. However, with6-row and 64-row MDCT, thinner, sub-millimeter, collima-ions are preferable. Near-isotropic imaging is already providedn a 4-row MDCT scanner with a detector configuration ofmm × 1 mm (minimal slice thickness of 1.25 mm), whichllows scanning of the abdomen during a 30-s breath-hold.ith a 16-row or 64-row MDCT scanner, and a detector con-

guration of 16 mm × 0.75 mm or 64 mm × 0.6 mm, scannings completed in 11–12 s or 6–7 s. Such datasets can be recon-tructed as 1 mm sections overlapped every 0.7 mm.

One of the major limitations of CTC is the relatively highadiation exposure, and therefore, increasing attention has beenocused on low-dose protocols. Because a thin collimation isecessary for CTC, dose reduction is widely achieved by reduc-ng the milliampere-seconds level. A recent MDCTC studyhowed excellent results for the detection of polyps >10 mm,ith thin beam collimation and an effective 50 mAs [51]. How-

ver, lower mAs values (10 mAs, ultra low-dose) have also beeneported to be feasible for polyp detection [52]. However, suchultra low-dose” protocols may not be feasible for the evalua-ion of extracolonic structures (because of the increased imageoise,) or for obese patient, but further research is needed. Gen-rally useful exposure settings are 120 kVp and 50–100 mAs inhe prone and in the supine positions. A recent analysis of differ-nt CTC research institutions showed a median mAs value of 67t a median collimation of 2.5 mm [53]. Use of automated doseodulation techniques that adapt mAs values to patient anatomy

hould always be used, if these techniques are available on theT scanner [80].

.3.1. Contrast mediaIntrinsic colonic lesions and extracolonic pathology can be

nhanced by IV injection of iodine contrast media (IV CM) inhe second scan [54,55]. Previous studies have shown contrast

rDu

Radiology 61 (2007) 388–399

edia to be of benefit in cases with clinical indications, suchs in patients with symptoms of colorectal cancer, or to detectocal recurrence, metachronous disease, or distant metastasis inatients with prior colorectal cancer [11,12,20,54]. However,ecause of the increased cost, the need for intravenous access,nd the risk of allergic anaphylactic reactions the applicationf IV contrast media might be contraindicated in a screeningopulation and limited to clinical indications. As reported by aecently published consensus statement most of CTC investiga-ors do not routinely administer i.v. contrast media for screeningurposes [32].

. Data analysis

Image processing and interpretation is performed on aommercially available computer workstation equipped withedicated CT colonography software. In addition to 2D axialnd multiplanar reformations in a cine mode, such systemsrovide an interactive, manual, mouse-driven, automated oremi-automated, virtual “fly-through” of the surface- or volume-endered 3D intraluminal images.

CTC datasets can be evaluated by a primary 2D or a primaryD approach. In either case, the alternative viewing techniqueust be available for rapid correlation and characterization of

ny suspicious findings. The combined use of both, 2D and 3Disualization techniques has been shown to be superior to thevaluation of 3D or 2D views only, with regard to sensitivitynd specificity [56,57].

Primary 2D evaluation is based on “lumen tracking,” by inter-ctively tracing through the 2D dataset, focusing on only their-distended colonic lumen from one end to the other, withpecial focus on the cross-section of one colonic segment attime [57]. Primary 2D evaluation provides information about

he attenuation of findings during the search process and is moreime-efficient [56,58]. Additional 3D views are often used forroblem solving. Primary 3D evaluation is based on 3D vir-ual endoscopy in an antegrade and retrograde fashion. PrimaryD evaluation was shown to be sensitive for polyp detectionecause both, the conspicuity, especially of small and medium-ized polyps, and the duration of visualization, are increased [3].dditional 2D views are necessary for characterization of find-

ngs. However, the primary 3D evaluation is time-consumingecause it must be performed in an antegrade and retrogradeashion for the perception of lesions behind haustral folds. Col-apsed segments must be evaluated alternatively, by 2D planarmages.

There is no general consensus on whether a primary 2D orD approach should be employed. It depends primarily on theadiologist’s preferences and the capabilities of the workstation.t present, the most commonly used platform for data interpre-

ation of CTC is a primary 2D interpretation, with 3D used forroblem solving, while primary 3D interpretation is preferredy an increasing minority [32].

In 2005, the working group on virtual colonoscopy proposed aeporting scheme, “C-RADS—CT Colonography Reporting andata System,” that includes recommendations for the follow-p of colonic polyps [59]. In brief, this scheme is based on

T. Mang et al. / European Journal of Radiology 61 (2007) 388–399 393

FCt

lrnsr

4

st3lbcv

d(jTautfm

pdfclin

ori

Fig. 4. Standard virtual endoscopy versus panoramic view for visualization oftwo colonic polyps: (a) standard virtual endoscopic CT image shows one 7 mmpolyp in the background of the image. (b) Panoramic view CT image of thesame area unfolds the colonic wall on a disc, increasing the field of view. Noteaw

aoed

ig. 3. Virtual colonic dissection in a patient with two polyps: virtual dissectedT image shows the colon fully dissected longitudinally and unfolded, similar

o a pathological preparation. Two polyps are seen in the sigmoid colon (arrows).

esion size, morphology, location, attenuation, and preliminaryecommendations for lesion surveillance. Although this efforteeds to be refined with a multi-disciplinary consensus, it repre-ents an important step toward more consistent and reproducibleeporting in CTC.

.1. Improvements in 3D visualization

Current developments in 3D visualization include virtual dis-ection, or panoramic, as well as unfolded, cube projections andranslucency rendering. These views differ from conventionalD simulation of endoscopic examinations, a technique that hasimitations with regard to blind spots, as well as time-consuming,i-directional evaluation. These new projections have beenreated to overcome this limitation by increasing surfaceisibility.

With “virtual dissection,” the colon is fully dissected longitu-inally and unfolded, similar to a pathological preparation [60]Fig. 3). The “unfolded cube” projection renders six planar pro-ections at 90◦ viewing angles from points on the central path.he unfolding of such a cube shows the complete field of viewt a path position [61]. The panoramic view is a variation of thenfolded cubic view, which renders five faces of a cubic view inhe plane in a continuous fashion. To minimize distortions, theront view is mapped into a square while the other four faces areapped around it into a disk [62] (Fig. 4).Initial results for some algorithms tested in small series of

atients showed a reduced reading time without a significantifference in sensitivity [60,61,63]. However, increasing sur-ace visibility, by flattening a 3D structure in a 2D image or byhanging viewing angles, is likely to suffer from the fact thatuminal anatomy is distorted, especially in areas of flexures orn suboptimal distended segments. Therefore, further studies areeeded to evaluate the feasibility of such tools.

“Translucency rendering” is a 3D tool that has been devel-ped to provide information on lesion attenuation beneath theendered surface by an attenuation-dependent color scale super-mposed on a 3D endoluminal view [64] (Fig. 5). These views

uwtg

n additional 6 mm polyp can be seen with the panoramic view (arrowhead),hich was hidden between haustral folds on standard virtual endoscopy.

ssign different colors (e.g., blue, green, red, and white) to areasf increasing attenuation, and therefore, provide a rapid andffective differentiation between soft-tissue lesions and pseu-olesions, such as stool. True colorectal polyps will show a

niform concentric ring pattern with a homogenous red core,hile stool, if tagged, will have a white core. If stool is not

agged, because of small gas bubbles, it will display an inhomo-eneous translucent pattern.

394 T. Mang et al. / European Journal of

Fig. 5. Colon polyp versus residual stool with translucency view. (a) 3D virtualendoscopic CT image shows a polypoid filling defect between two haustralfolds (arrow). Translucency view shows a homogenous, uniform, concentricring pattern with a homogenous red core revealing the polyp. (b) 3D virtuale(g

4

gtmmd

ppab

rthaamrdoCta“oa

5

o[

daeaptcma

TtOafidpGbidub

cr

ndoscopic CT image shows a polypoid filling defect in the transverse colonarrow). Translucency view shows an inhomogeneous pattern because of smallas bubbles, revealing untagged stool.

.2. Role of CAD

Computer-aided detection (CAD) systems are software pro-rams that automatically highlight polyp “candidates” and

herefore support the radiologist by pointing out possible abnor-

alities that may otherwise have been missed. Based onorphologic and attenuation characteristics, the reader then

ecides whether the “candidate lesion” is a true- or a false-

csll

Radiology 61 (2007) 388–399

ositive finding. Recent CAD algorithms showed a promisingerformance, with a reported a CAD sensitivity of 89.3% fordenomas ≥1 cm [65]. Some of these systems have recentlyecome available on CTC workstations.

CAD algorithms can be used as a “first reader,” as a “concur-ent reader,” or as a “second reader.” In a first-reader paradigm,he observer only ever reads with CAD-assistance and confinesis/her attention to CAD marks alone, ignoring unpromptedreas of the colon. However, aside from ethical issues, suchseparation is likely to come with an increased number ofissed abnormalities [66]. If CAD is used as a concurrent

eader, pre-calculated CAD results are displayed in the imageata while performing the primary data evaluation. In the sec-nd reader scheme, a reader reviews the case, first blinded to theAD results, and then evaluates the CAD findings to integrate

hem with his/her own findings. There are relatively little databout the potential benefit of CAD on reader performance. Thepotential” contribution of a CAD algorithm to the sensitivityf different readers without CAD was reportedly between 10%nd 38% [67,68].

. Findings

Findings of the colon are characterized by their morphol-gy, by their attenuation characteristics, and by their mobility69,70].

One of the most common findings detected with CTC isiverticular disease. On 2D CTC images diverticula appear asir-filled outpouchings of the colonic wall. On the 3D virtualndoscopic images, the diverticular orifice can be recognized ascomplete dark ring [71] (Fig. 6). Due to the increased risk oferforation CTC has no role in the diagnosis of acute diverticuli-is. However, in chronic stages of diverticulitis, CTC may showone-shaped stenosis with mild wall thickening with involve-ent of a long segment (>10 cm) with pericolic fat stranding,

nd fluid at the root of the mesentery.Polyps are the most common benign lesions of the colon.

he risk of malignant transformation increases with the size ofhe polyp. Most polyps are sessile although some have a stalk.n 3D virtual endoscopic images, polypoid lesions present

s a sessile or stalked, round, oval, or lobulated intraluminallling defect. Typically, the margin to the normal mucosa isisplayed as an incomplete ring shadow. On 2D plane images,olyps have homogenous, soft tissue attenuation (Fig. 7) [69].enerally, polyps maintain their position with respect to theowel surface, with the exception of stalked polyps or polypsn mobile colon segments [72]. CTC is not able to reliablyistinguish between hyperplastic and adenomatous polypssing morphological features alone, although 50% of polypselow 5 mm are hyperplastic [73].

Lipomas are the most common submucosal lesions in theolon (especially common on the ileocecal valve). CT colonog-aphy allows confident diagnosis of lipomas based on their

haracteristic fatty attenuation. A lipoma, in general, is 1–3 cm inize and rarely exceeds 4 cm. On 3D virtual endoscopic images,ipomas are present as a sessile or pedunculated polypoid intra-uminal filling defect, most often with a smooth surface. On

T. Mang et al. / European Journal of Radiology 61 (2007) 388–399 395

Fsi

2tcIli

lbl5

Fig. 7. Large, 2.6 cm stalked polyp and 1.1 cm sessile polyp in the sigmoidcolon. (a) 3D virtual endoscopic CT image shows a large polypoid filling defectwith a stalk (arrow) adjacent to a smaller, sessile, round, and smooth fillingdtr

bPnflta mild nodularity is found on the surface by 3D endoluminal

ig. 6. Diverticula in the sigmoid colon. (a) 3D virtual endoscopic CT imagehows a complete dark rings at the diverticula (arrows). (b) Prone 2D axial CTmage shows multiple gas-filled outpouchings of the colon wall.

D plane images, lipomas show a homogenous fatty attenua-ion (Fig. 8). Because lipomas are soft lesions, their shape mayhange when moving from the prone to the supine position.n general, small lipomas need no further treatment; only largeipomas require endoscopic resection because they can lead tontusseption [69].

Whereas truly flat adenomas, not arising over the mucosalevel, are almost impossible to detect at CTC, flat lesions can

e depicted if they are slightly raised over the colonic mucosaevel. Flat polyps are defined as lesions with a height less than0% of the lesion width [74]. Their clinical importance has

i

t

efect (arrowhead). (b) Supine 2D coronal CT image shows a homogenous softissue attenuation of the two intraluminal lesions (arrow, arrowhead). Histologyevealed two adenomas of the sigmoid colon.

een a topic of controversy, and recent data from the Nationalolyp Study indicate that the flat lesions that were found areot more aggressive than sessile or stalked polyps [81]. In CTC,at polyps appear as a fairly circumscribed area of mild wall

hickening with homogenous soft tissue attenuation. Sometimes

mages (Fig. 9).Colorectal cancer is the most common colonic primary

umour. Most carcinomas show an exophytic, polypous type of

396 T. Mang et al. / European Journal of Radiology 61 (2007) 388–399

Fig. 8. Large flat adenoma of the transverse colon. (a) 3D virtual endoscopicCT image shows a flat filling defect of the colon adjacent to a haustral fold(ht

gtftcwbTts

Fig. 9. Large, pedunculated lipoma of the ascending colon. (a) 3D virtual endo-sas

ptabtb

arrow) (b) Prone 2D axial CT image shows a slight focal wall thickening withomogenous soft tissue attenuation. Histology revealed a villous adenoma ofhe transverse colon.

rowth with frequent central degeneration. Adenocarcinomasend to infiltrate the bowel wall circumferentially and 50% areound in the rectum, and 25% in the sigmoid. Colorectal cancerypically shows extensive focal polypoid, asymmetric, or cir-ular wall thickening with short extension (<5 cm), especiallyith shoulder formation (Fig. 10) [71,75]. CT differentiation

etween stages T1 (invasion of mucosa and/or submucosa) and2 (invasion of the muscularis propria) is not feasible, but

umor extension beyond the colon wall (T3), characterized by fattranding, an indistinct boundary, and nodular protrusions into

siol

copic CT image shows a a large polypoid filling defect with a smooth surfacedjacent to a haustral fold (arrow). (b) Prone 2D axial CT image shows aubmucosal lesion with a homogenous fatty attenuation, revealing a lipoma.

ericolic fat tissue, is readily appreciated by CT. Tumor infiltra-ion to adjacent organs (T4) is most likely if the carcinoma showsbroad-based contact, no intervening fat planes, and indistinctoundaries to other organs. Pericolonic lymph nodes and dis-ant metastases are signs of progression of the disease and cane evaluated using 2D axial source images and MPR views.

Since the whole abdomen and pelvis is scanned, extracolonic

tructures can be evaluated. The incidence of extracolonic find-ngs has been reported to be high (69%), but only about 10% aref high clinical importance, such as large aneurysms, masses,ung nodules, and large lymph nodes (Fig. 10b) [76,77]. Elderly

T. Mang et al. / European Journal of

Fig. 10. Stenotic cancer of the transverse colon. (a) 3D endoluminal imageshows an irregular, circular, stenotic filling defect (arrow). (b) Supine sagittalcbfi

aellc

6

o

stpCcoppiae

R

[

[

[

[

[

[

[

ontrast-enhanced CT image shows circular wall thickening and tumor extensioneyond the colon wall (T3) (arrow). Note: liver metastasis as an extracolonicnding (arrowhead).

nd symptomatic patients are more likely to have significantxtracolonic findings compared to a younger screening popu-ation. It has been noted that the lack of IV contrast, and theow-dose technique used for VC, limit the evaluation of extra-olonic findings [78].

. Conclusion

In summary, CTC is a valuable diagnostic tool for evaluationf the entire colon. Accurate diagnosis relies on high quality

[

Radiology 61 (2007) 388–399 397

tudies, which require adequate bowel cleansing and distention,hin-section MDCT scans performed in the prone and supineositions, and a combined evaluation of 2D and 3D images.TC is widely accepted as a powerful tool for evaluation of theolon after incomplete colonoscopy, or in patients with knownr suspected colorectal cancer. The role of CTC in a screeningopulation remains to be determined, with encouraging resultsublished in the last two years. Ongoing research in CAD andmproved software products have the potential to increase theccuracy and ease of interpretation of CTC, which would accel-rate its acceptance as a colorectal screening tool.

eferences

[1] Cotton PB, Durkalski VL, Pineau BC, et al. Computed tomographiccolonography (virtual colonoscopy): a multicenter comparison withstandard colonoscopy for detection of colorectal neoplasia. JAMA2004;291:1713–9.

[2] Johnson CD, Harmsen WS, Wilson LA, et al. Prospective blinded eval-uation of computed tomographic colonography for screen detection ofcolorectal polyps. Gastroenterology 2003;125:311–9.

[3] Pickhardt PJ, Choi JR, Hwang I, et al. Computed tomographic virtualcolonoscopy to screen for colorectal neoplasia in asymptomatic adults.N Engl J Med 2003;349:2191–200.

[4] Rockey DC, Paulson E, Niedzwiecki D, et al. Analysis of air contrastbarium enema, computed tomographic colonography, and colonoscopy:prospective comparison. Lancet 2005;365:305–11.

[5] Ferrucci JT. Colonoscopy: virtual and optical—another look, another view.Radiology 2005;235:13–6.

[6] Halligan S, Atkin W. Unbiased studies are needed before virtualcolonoscopy can be dismissed. Lancet 2005;365:275–6.

[7] Fenlon HM, McAneny DB, Nunes DP, Clarke PD, Ferrucci JT. Occlusivecolon carcinoma: virtual colonoscopy in the preoperative evaluation of theproximal colon. Radiology 1999;210:423–8.

[8] Macari M, Berman P, Dicker M, Milano A, Megibow AJ. Usefulness ofCT colonography in patients with incomplete colonoscopy. AJR Am JRoentgenol 1999;173:561–4.

[9] Morrin MM, Farrell RJ, Raptopoulos V, McGee JB, Bleday R, KruskalJB. Role of virtual computed tomographic colonography in patients withcolorectal cancers and obstructing colorectal lesions. Dis Colon Rectum2000;43:303–11.

10] Morrin MM, Kruskal JB, Farrell RJ, Goldberg SN, McGee JB, Rap-topoulos V. Endoluminal CT colonography after an incomplete endoscopiccolonoscopy. AJR Am J Roentgenol 1999;172:913–8.

11] Neri E, Giusti P, Battolla L, et al. Colorectal cancer: role of CT colonog-raphy in preoperative evaluation after incomplete colonoscopy. Radiology2002;223:615–9.

12] Fletcher JG, Johnson CD, Krueger WR, et al. Contrast-enhanced CTcolonography in recurrent colorectal carcinoma: feasibility of simultaneousevaluation for metastatic disease, local recurrence, and metachronous neo-plasia in colorectal carcinoma. AJR Am J Roentgenol 2002;178:283–90.

13] Laghi A, Iannaccone R, Bria E, et al. Contrast-enhanced computed tomo-graphic colonography in the follow-up of colorectal cancer patients: afeasibility study. Eur Radiol 2003;13:883–9.

14] Laghi A. Virtual colonoscopy: clinical application. Eur Radiol2005;15(Suppl 4):D138–41.

15] Ferrucci JT. Double-contrast barium enema: use in practice and implica-tions for CT colonography. AJR Am J Roentgenol 2006;187:170–3.

16] Winawer SJ, Stewart ET, Zauber AG, et al. A comparison of colonoscopyand double-contrast barium enema for surveillance after polypectomy.

National Polyp Study Work Group. N Engl J Med 2000;342:1766–72.

17] Halligan S, Altman DG, Taylor SA, et al. CT colonography in the detec-tion of colorectal polyps and cancer: systematic review, meta-analysis,and proposed minimum data set for study level reporting. Radiology2005;237:893–904.

3 al of

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

98 T. Mang et al. / European Journ

18] Gluecker TM, Johnson CD, Harmsen WS, et al. Colorectal cancer screeningwith CT colonography, colonoscopy, and double-contrast barium enemaexamination: prospective assessment of patient perceptions and prefer-ences. Radiology 2003;227:378–84.

19] Chung DJ, Huh KC, Choi WJ, Kim JK. CT colonography using 16-MDCT in the evaluation of colorectal cancer. AJR Am J Roentgenol2005;184:98–103.

20] Filippone A, Ambrosini R, Fuschi M, Marinelli T, Genovesi D, BonomoL. Preoperative T and N staging of colorectal cancer: accuracy ofcontrast-enhanced multi-detector row CT colonography—initial experi-ence. Radiology 2004;231:83–90.

21] Iannaccone R, Laghi A, Passariello R. Colorectal carcinoma: detectionand staging with multislice CT (MSCT) colonography. Abdom Imaging2005;30:13–9.

22] Macari M, Bini EJ. CT colonography: where have we been and where arewe going? Radiology 2005;237:819–33.

23] Taylor SA, Halligan S, Saunders BP, et al. Use of multidetector-row CTcolonography for detection of colorectal neoplasia in patients referredvia the Department of Health “2-week-wait” initiative. Clin Radiol2003;58:855–61.

24] Lefere P, Gryspeerdt S, Baekelandt M, Dewyspelaere J, van HolsbeeckB. Diverticular disease in CT colonography. Eur Radiol 2003;13(Suppl4):L62–74.

25] Biancone L, Fiori R, Tosti C, et al. Virtual colonoscopy compared with con-ventional colonoscopy for stricturing postoperative recurrence in Crohn’sdisease. Inflamm Bowel Dis 2003;9:343–50.

26] Tarjan Z, Zagoni T, Gyorke T, Mester A, Karlinger K, Mako EK. Spiral CTcolonography in inflammatory bowel disease. Eur J Radiol 2000;35:193–8.

27] Ota Y, Matsui T, Ono H, et al. Value of virtual computed tomographiccolonography for Crohn’s colitis: comparison with endoscopy and bariumenema. Abdom Imaging 2003;28:778–83.

28] Svensson MH, Svensson E, Lasson A, Hellstrom M. Patient acceptance ofCT colonography and conventional colonoscopy: prospective comparativestudy in patients with or suspected of having colorectal disease. Radiology2002;222:337–45.

29] van Gelder RE, Birnie E, Florie J, et al. CT colonography and colonoscopy:assessment of patient preference in a 5-week follow-up study. Radiology2004;233:328–37.

30] Smith RA, Cokkinides V, Eyre HJ. American Cancer Society guidelinesfor the early detection of cancer, 2003. CA Cancer J Clin 2003;53:27–43.

31] Winawer S, Fletcher R, Rex D, et al. Colorectal cancer screening andsurveillance: clinical guidelines and rationale-update based on new evi-dence. Gastroenterology 2003;124:544–60.

32] Barish MA, Soto JA, Ferrucci JT. Consensus on current clinical practice ofvirtual colonoscopy. AJR Am J Roentgenol 2005;184:786–92.

33] Burling D, Halligan S, Slater A, Noakes MJ, Taylor SA. Potentially seri-ous adverse events at CT colonography in symptomatic patients: nationalsurvey of the United Kingdom. Radiology 2006;239:464–71.

34] Pickhardt PJ. Incidence of colonic perforation at CT colonography: reviewof existing data and implications for screening of asymptomatic adults.Radiology 2006;239:313–6.

35] Sosna J, Blachar A, Amitai M, et al. Colonic perforation at CT colonog-raphy: assessment of risk in a multicenter large cohort. Radiology2006;239:457–63.

36] Macari M, Lavelle M, Pedrosa I, et al. Effect of different bowel preparationson residual fluid at CT colonography. Radiology 2001;218:274–7.

37] Fass R, Do S, Hixson LJ. Fatal hyperphosphatemia following Fleet Phospo-Soda in a patient with colonic ileus. Am J Gastroenterol 1993;88:929–32.

38] Taylor SA, Halligan S, Goh V, Morley S, Atkin W, Bartram CI. Optimiz-ing bowel preparation for multidetector row CT colonography: effect ofCitramag and Picolax. Clin Radiol 2003;58:723–32.

39] Lefere PA, Gryspeerdt SS, Dewyspelaere J, Baekelandt M, Van HolsbeeckBG. Dietary fecal tagging as a cleansing method before CT colonog-

raphy: initial results polyp detection and patient acceptance. Radiology2002;224:393–403.

40] Iannaccone R, Laghi A, Catalano C, et al. Computed tomographic colonog-raphy without cathartic preparation for the detection of colorectal polyps.Gastroenterology 2004;127:1300–11.

[

Radiology 61 (2007) 388–399

41] Callstrom MR, Johnson CD, Fletcher JG, et al. CT colonography withoutcathartic preparation: feasibility study. Radiology 2001;219:693–8.

42] Zalis ME, Hahn PF. Digital subtraction bowel cleansing in CT colonogra-phy. AJR Am J Roentgenol 2001;176:646–8.

43] Dachman AH. Advice for optimizing colonic distention and minimizingrisk of perforation during CT colonography. Radiology 2006;239:317–21.

44] Taylor SA, Halligan S, Goh V, et al. Optimizing colonic distention formulti-detector row CT colonography: effect of hyoscine butylbromide andrectal balloon catheter. Radiology 2003;229:99–108.

45] Burling D, Taylor SA, Halligan S, et al. Automated insufflation of carbondioxide for MDCT colonography: distension and patient experience com-pared with manual insufflation. AJR Am J Roentgenol 2006;186:96–103.

46] Shinners TJ, Pickhardt PJ, Taylor AJ, Jones DA, Olsen CH. Patient-controlled room air insufflation versus automated carbon dioxide deliveryfor CT colonography. AJR Am J Roentgenol 2006;186:1491–6.

47] Rogalla P, Lembcke A, Ruckert JC, et al. Spasmolysis at CT colonography:butyl scopolamine versus glucagon. Radiology 2005;236:184–8.

48] Yee J, Hung RK, Akerkar GA, Wall SD. The usefulness of glucagonhydrochloride for colonic distention in CT colonography. AJR Am JRoentgenol 1999;173:169–72.

49] Chen SC, Lu DS, Hecht JR, Kadell BM. CT colonography: value ofscanning in both the supine and prone positions. AJR Am J Roentgenol1999;172:595–9.

50] Yee J, Kumar NN, Hung RK, Akerkar GA, Kumar PR, Wall SD. Com-parison of supine and prone scanning separately and in combination at CTcolonography. Radiology 2003;226:653–61.

51] Macari M, Bini EJ, Xue X, et al. Colorectal neoplasms: prospective com-parison of thin-section low-dose multi-detector row CT colonography andconventional colonoscopy for detection. Radiology 2002;224:383–92.

52] Iannaccone R, Laghi A, Catalano C, Mangiapane F, Piacentini F, Pas-sariello R. Feasibility of ultra-low-dose multislice CT colonography forthe detection of colorectal lesions: preliminary experience. Eur Radiol2003;13:1297–302.

53] Jensch S, van Gelder RE, Venema HW, et al. Effective radiation doses inCT colonography: results of an inventory among research institutions. EurRadiol 2006;16:981–7.

54] Morrin MM, Farrell RJ, Kruskal JB, Reynolds K, McGee JB, RaptopoulosV. Utility of intravenously administered contrast material at CT colonog-raphy. Radiology 2000;217:765–71.

55] Oto A, Gelebek V, Oguz BS, et al. CT attenuation of colorectal poly-poid lesions: evaluation of contrast enhancement in CT colonography. EurRadiol 2003;13:1657–63.

56] Dachman AH, Kuniyoshi JK, Boyle CM, et al. CT colonography with three-dimensional problem solving for detection of colonic polyps. AJR Am JRoentgenol 1998;171:989–95.

57] Royster AP, Fenlon HM, Clarke PD, Nunes DP, Ferrucci JT. CTcolonoscopy of colorectal neoplasms: two-dimensional and three-dimensional virtual-reality techniques with colonoscopic correlation. AJRAm J Roentgenol 1997;169:1237–42.

58] Macari M, Milano A, Lavelle M, Berman P, Megibow AJ. Compari-son of time-efficient CT colonography with two- and three-dimensionalcolonic evaluation for detecting colorectal polyps. AJR Am J Roentgenol2000;174:1543–9.

59] Zalis ME, Barish MA, Choi JR, et al. CT colonography reporting and datasystem: a consensus proposal. Radiology 2005;236:3–9.

60] Juchems MS, Fleiter TR, Pauls S, Schmidt SA, Brambs HJ, Aschoff AJ.CT colonography: comparison of a colon dissection display versus 3Dendoluminal view for the detection of polyps. Eur Radiol 2006;16:68–72.

61] Vos FM, van Gelder RE, Serlie IW, et al. Three-dimensional display modesfor CT colonography: conventional 3D virtual colonoscopy versus unfoldedcube projection. Radiology 2003;228:878–85.

62] Geiger B, Chefd’hotel C, Sudarsky S. Panoramic views for virtualendoscopy. Med Image Comput Comput Assist Interv Int Conf Med Image

Comput Comput Assist Interv 2005;8:662–9.

63] Hoppe H, Quattropani C, Spreng A, Mattich J, Netzer P, Dinkel HP. Virtualcolon dissection with CT colonography compared with axial interpretationand conventional colonoscopy: preliminary results. AJR Am J Roentgenol2004;182:1151–8.

al of

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

T. Mang et al. / European Journ

64] Pickhardt PJ. Translucency rendering in 3D endoluminal CT colonography:a useful tool for increasing polyp specificity and decreasing interpretationtime. AJR Am J Roentgenol 2004;183:429–36.

65] Summers RM, Yao J, Pickhardt PJ, et al. Computed tomographic virtualcolonoscopy computer-aided polyp detection in a screening population.Gastroenterology 2005;129:1832–44.

66] Yoshida H, Dachman AH. CAD techniques, challenges, and controversiesin computed tomographic colonography. Abdom Imaging 2005;30:26–41.

67] Summers RM, Jerebko AK, Franaszek M, Malley JD, Johnson CD. Colonicpolyps: complementary role of computer-aided detection in CT colonog-raphy. Radiology 2002;225:391–9.

68] Taylor SA, Halligan S, Burling D, et al. Computer-assisted reader softwareversus expert reviewers for polyp detection on CT colonography. AJR AmJ Roentgenol 2006;186:696–702.

69] Macari M, Bini EJ, Jacobs SL, Lange N, Lui YW. Filling defects atCT colonography: pseudo- and diminutive lesions (the good), polyps(the bad), flat lesions, masses, and carcinomas (the ugly). Radiographics2003;23:1073–91.

70] Pickhardt PJ. Differential diagnosis of polypoid lesions seen at CT colonog-raphy (virtual colonoscopy). Radiographics 2004;24:1535–56, discussion

1557-1539.

71] Fenlon HM, Clarke PD, Ferrucci JT. Virtual colonoscopy: imaging featureswith colonoscopic correlation. AJR Am J Roentgenol 1998;170:1303–9.

72] Laks S, Macari M, Bini EJ. Positional change in colon polyps at CTcolonography. Radiology 2004;231:761–6.

[

Radiology 61 (2007) 388–399 399

73] Winawer SJ, Fletcher RH, Miller L, et al. Colorectal cancer screen-ing: clinical guidelines and rationale. Gastroenterology 1997;112:594–642.

74] Rembacken BJ, Fujii T, Cairns A, et al. Flat and depressed colonic neo-plasms: a prospective study of 1000 colonoscopies in the UK. Lancet2000;355:1211–4.

75] Taylor SA, Halligan S, Bartram CI. CT colonography: methods, pathologyand pitfalls. Clin Radiol 2003;58:179–90.

76] Gluecker TM, Johnson CD, Wilson LA, et al. Extracolonic findings at CTcolonography: evaluation of prevalence and cost in a screening population.Gastroenterology 2003;124:911–6.

77] Hara AK, Johnson CD, MacCarty RL, Welch TJ. Incidental extracolonicfindings at CT colonography. Radiology 2000;215:353–7.

78] Pickhardt PJ. Virtual colonoscopy: issues related to primary screening. EurRadiol 2005;15(Suppl. 4):D133–7.

79] Johnson CD, MacCarty RL, Welch TJ, et al. Comparison of the relativesensitivity of CT colonography and double-contrast barium enema forscreen detection of colorectal polyps. Clin Gastroenterol Hepatol 2004;2:314–21.

80] Graser A, Wintersperger BJ, Suess C, Reiser MF, Becker CR. Dose

reduction and image quality in MDCT colonography using tube currentmodulation. AJR Am J Roentgenol 2006;187:695–701.

81] O’Brien MJ, Winawer SJ, Zauber AG, et al. Flat adenomas in the NationalPolyp Study: is there increased risk for high-grade dysplasia initially orduring surveillance? Clin Gastroenterol Hepatol 2004;2:905–11.