CT colonography: avoiding traps and pitfalls

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1 Insights Imging (2011) 2:57 68 DOI /s PICTORIAL REVIEW CT colonogrphy: voiding trps nd pitflls Philippe Lefere & Stefn Gryspeerdt Received: 5 August 2010 / Accepted: 18 Novemer 2010 / Pulished online: 4 Jnury 2011 # Europen Society of Rdiology 2010 Astrct Computed tomogrphic colonogrphy (CTC) is relile technique for detecting tumorl lesions in the colon. However, good performnce of polyp detection is only chieved if experienced CTC rdiologists comine meticulous interprettion with stte-of-the-rt CTC technique. To rech this experience level, CTC trining is mndtory. With considerly long nd steep lerning curve, it hs een demonstrted tht in inexperienced hnds oth technicl filure nd oserver errors stnd for the mjority of missed lesions. The purpose of this pictoril review is to give n overview of trps nd pitflls in CTC imging resulting in flse negtive nd positive findings, nd how to void them y ppliction of stte-of-the-rt CTC technique nd interprettion. Keywords Computed tomogrphic colonogrphy. CTC. 2D. 3D. Pitflls Introduction Computed tomogrphic colonogrphy (CTC) is relile technique for detecting tumorl lesions in the colon [1 4]. However, good performnce in polyp detection is only chieved if experienced CTC rdiologists comine meticulous interprettion with stte-of-the-rt CTC technique [5]. To rech this experience level, CTC trining is mndtory. With considerly long nd steep lerning curve, it hs een demonstrted tht in inexperienced hnds oth P. Lefere (*) : S. Gryspeerdt VCTC Virtul Colonoscopy Teching Centre, Akkerstrt 32c, 8830 Hooglede, Belgium e-mil: info@vctc.eu technicl filure nd oserver errors ccount for the mjority of missed lesions [6, 7]. The purpose of this pictoril review is to give n overview of trps nd pitflls in CTC imging resulting in flse-negtive nd flsepositive findings, nd how to void them y ppliction of stte-of-the-rt CTC technique nd interprettion. Pitflls in imging re relted to the technicl spects of CTC, the ntomicl spect of the colon nd two- (2D) nd three-dimensionl (3D) imging. Technique-relted pitflls Preprtion CTC requires the colon to e s clen nd dry s possile for dequte interprettion [8]. Together with preprtion comining low residue diet with lxtives or cthrtics, fecl tgging is n essentil prt of this process [9]. By impregnting the fecl residue, stool tgging not only llows for differentition etween solid residue nd true polypoid lesion (Fig. 1), fluid tgging lso enles detection of polyps s negtive filling defects in tgged fluid (Fig. 2). Even if these requirements for preprtion re fulfilled, prolems my still occur with non-tgged stool mimicking polypoid lesion. Using dul positioning (= supine/prone cquisition) nd 2D imging most prolems re solved, s the stool cn e correctly identified in the cse of: A luminl defect with ir inclusion: mottled ppernce Positionl shift (Fig. 3) A defect floting in pool of rium (Fig. 3) A defect with hyperdense peripherl ring nd hypodense centre

2 58 Insights Imging (2011) 2:57 68 loting. Tips to void prticulr prolems will e given in the ntomy section. One prticulr entity needs our ttention: pneumtosis coli [11]. This is n uneventful enign compliction of the colonic insuffltion. It presents s sumucosl nd/or suserosl ir ules or cysts with polypoid ppernce in 3D. In 2D, the dignosis of pneumtosis coli is mde strightforwrd y identifying the ir collections (Fig. 5). Acquisition Dose-relted prolems Fig. 1 Ascending colon: sessile 9-mm polyp with soft tissue density (white rrowhed) in contrst mteril with tgged residue on the posterior wll (white rrow) In out 50% of cses tumourl lesions re covered y contrst mteril cusing n spect of heterogeneous tgging [10]. Correct identifiction cn e otined y compring the supine nd prone cquisitions nd y ppreciting the typicl chrcteristics of the tumourl lesions eneth the contrst mteril lyer (Fig. 4). Distension Optiml colonic distension is the second cornerstone of good CTC prctice. This is chieved y comining smooth muscle relxtion, colonic infltion with cron dioxide (preferly with CO 2 injector) nd dul positioning. Using CO 2 injector llows monitoring the pressure in the colon. It is importnt to keep pressure in the colon etween 20 nd 25 mmhg. Besides this, it is very importnt to wit until the ptient mentions dominl prolems such s CTC is prticulrly well designed for low-dose cquisition ecuse of the high contrst etween the luminl gs nd the colonic wll. In some cses low dose cuses excess imge noise resulting in pseudo-enhncement: n rtificil increse in the tissue density sometimes resulting in pseudo-tgging, i.e. mking true lesion look like tgged stool (Fig. 6) [12]. This prolem cn e solved y compring the lesion with tgged stool elsewhere in the colon or y smoothing the dominl window. When strting CTC s novice, dose of 80 mas should e used. This voids imge noise. With growing experience the dose cn e decresed to 50 mas. Idel for 64-slice systems, this dose produces considerle noise in 16-slice systems. Motion rtefcts Although recent systems llow for very short cquisition times nd require only short reth-hold, motion rtefcts my deteriorte imge qulity or cuse pseudo-lesions ecuse of: A twin fold spect: two longitudinl defects close to ech other (Fig. 7) Fig. 2 Ascending colon, xil view: thickened fold in tgged fluid (white rrowhed). Corresponding coronl imge shows 12-mm lesion with flt morphology s negtive filling defect in the tgged fluid (lck rrowhed)

3 Insights Imging (2011) 2: c Fig. 3 Three-dimensionl view of the rectum showing smll polypoid defect surrounded y some fluid (white rrowhed). The corresponding xil imge in the supine position shows negtive filling defect floting in contrst mteril pool (white rrowhed). White rrow hyperdense tip of the rectl ctheter. c In the prone position the filling defect hs moved to the nterior prt of the rectum (white rrowhed). Both positionl shift nd floting of the defect in contrst mteril llow for correct dignosis of non-tgged stool Curved or serpiginous ppernce of fold Pseudopolypoid spect Motion rtefcts cn e confirmed in 2D y looking for stir steps on the dominl wll on the sgittl reformts. As generl rule it is mndtory to lwys compre 3D with 2D when detecting murl normlity (Fig. 7c). Antomy-relted pitflls Generl remrks Some pitflls occur in ny of the colonic segments. Segmentl moility Due to long mesentery, some segments of the colon frequently chnge in position with dul positioning [13]. This occurs in ll segments nd more specificlly t the level of ll flexures, the sigmoid, the trnsverse colon nd the cecum. These movements re vrile: turning over its longitudinl xis (flipping), rotting or more complex movements. They crete n spect of pseudo-stool s polypoid lesions chnge in position together with the moving segment. Compring the 2D imges in the different reformts nd referring to other structures such s the ileocecl vlve, diverticul nd diverticulr fecliths llows for detecting the positionl chnge of the colonic segment (Fig. 8). Flexurl pseudo-tumour At ech flexure of the colon, pseudo-thickening of the colonic wll t the inner side of the flexure my occur ecuse of compression of the colonic wll. This is sometimes ccentuted y the pericolonic ft nd/or vsculr structures. This phenomenon is clled the flexurl pseudo-tumour [14]. In 3D thickened fold is detected sometimes with pseudo-polypoid spect. Frequently there is lso convergence of multiple folds. The thickened fold hs smooth ppernce nd mostly chnges in shpe with dul positioning. In 2D thickened fold situted t the inner prt of flexure is shown (Fig. 9). Extrinsic impressions Fig. 4 Axil view of the heptic flexure showing lrge defect with solid components nd contrst mteril (white rrowhed). Imge of lrge tumourl lesion covered y contrst mteril Extr-colonic structures occur nywhere in the colon. They re more frequent when colonic distension is optiml. They

4 60 Insights Imging (2011) 2:57 68 Fig. 5 Three-dimensionl view of the splenic flexure revels multiple polypoid defects. Some present with shine-through phenomenon (white rrows). The corresponding xil imge shows sumucosl (white rrows) nd suserosl (white rrowhed) ir collections consistent with pneumtosis coli my e cused y norml or pthologicl structures. In 3D they sometimes mimic polypoid structure. Agin, correltion with the corresponding 2D imges is mndtory to detect the origin of the 3D ump (Fig. 10). Rectum The rectum is prticulrly difficult to exmine. Anl mrgin The rectum is closed structure ending t the nl mrgin. Tumourl lesions my e locted t the nl mrgin. It is mndtory to inspect the nl mrgin corresponding to the region where the rectl ctheter enters the rectum. Peri-nl lesions my e true tumourl lesions. They re to e differentited from hypertrophied nl ppill. CTC cnnot differentite etween the two entities nd once lesion mesures 6 mm, opticl colonoscopy is necessry to rule out significnt lesion (Fig. 11). Internl hemorrhoids represent nother cuse of pseudopolypoid lesion close to the nl mrgin. They look like rod-sed, grpe-like defects surrounding the ctheter. They usully chnge in shpe with dul positioning [15]. Frequently, longitudinl folds converging on the nl cnl re present. The most prominent or rectl r is locted nteriorly nd my pper s polyp on 2D imges. In 3D this is clerly depicted it s fold. Rectl mpull The rectl mpull is the segment of the rectum ove the nl cnl. It is the widest portion, mking it more difficult to cover its entire wll. Furthermore, some prts my e hidden y the rectl ctheter. A comined 2D-3D pproch using coronl nd sgittl imges is to e considered. Detiled inspection with 3D requires the widest ngle of the virtul cmer: 120. Rectl vlves The rectl vlves or vlves of Houston, mostly three (inferior, middle, superior) re prominent trnsverse folds supporting the weight of the fecl mtter [16]. As they re prominent, they my hide lesions nd need specil ttention y using the 120 cmer ngle, y looking well ehind them nd relying upon coronl nd sgittl imges. Fig. 6 Ultr-low dose imge (140 kv, 10 mas) of the sigmoid: pedunculted polyp with incresed density due to imge noise (pseudoenhncement) (white rrow). Sme ptient imged with norml dose for stging of mlignnt tumour elsewhere in the colon. The sme polyp shows norml soft tissue density (white rrow)

5 Insights Imging (2011) 2: The rectl tue my compromise good nd/or the entire visulistion of the rectum nd hence oscure lesion. Hving good look round the rectl ctheter, where it uts the rectl wll, is mndtory. As it is for rectl use, the ctheter is provided with n infltle lloon prevent c Fig. 7 Three-dimensionl view of the scending colon showing longitudinl luminl defect long semi-lunr fold (lck rrow): flt lesion?, c The corresponding xil nd sgittl views show sutle stir step rtefct on the dominl wll (white rrowheds) nd douled or twin fold spect of the semilunr fold explining the smll longitudinl defect s sutle rething rtefct Rectl distension The rectum is situted posteriorly in the pelvis, mking dequte distension in the supine position rduous nd optiml distension in the prone position esy. The sic principles of distension, s mentioned ove, need specil ttention. Rectl tue Fig. 8 Axil view of the cecum in the supine position showing n 8-mm polyp on the left lterl side (white rrowhed). The corresponding view in the prone position shows the sme defect on the nterior cecl wll, sumerged in slightly tgged fluid (white rrowhed). Residul stool with positionl shift? c, d There is rottion of the cecum over 90 s is shown y the different positions of the ileo-cecl vlve (white rrowhed) nd the terminl ileum (white rrow) in oth cquisitions: imge of true polyp or pseudo-stool due to segmentl moility c d

6 62 Insights Imging (2011) 2:57 68 c Fig. 9 Three-dimensionl view of the sigmoid in ptient with dolichocolon: imge suggestive of lrge polypoid mss (lck rrowhed). Corresponding sgittl view in n dominl window setting showing n cute flexure of the sigmoid over 180 with thickening of the colonic wll nd compression of the inner structures. c The corresponding 3D view in the prone position shows norml fold ing disloction when turning the ptient into the prone position; it my lso compromise rectl exmintion y compressing lesions ginst the colonic wll. It is importnt to hve good look round the rectl tue. Deflting the lloon fter turning the ptient into the prone position nd efore strting the second cquisition voids compression of lesion ginst the rectl wll (Fig. 11). Rectl vrix A rectl vrix is dilted sumucosl vein tht cuses serpiginous extrinsic impression in the rectum [15]. It chnges in ppernce etween the two cquisitions s with etter distension in the prone position it tends to fltten out. Perirectl vsculr structures cn e recognised on the 2D imges. Do not mistke polyp with serpiginous spect for rectl vrix. Sigmoid Distension The teni coli re three longitudinl musculr nds, strting in the sigmoid nd converging t the ppendiculr orifice in the cecum nd define the ppernce of the colonic lumen y their contrction sttus [17]. With incresed contrction, the hustrl folds locted etween the semilunr folds ecome deeper, mking inspection of the colonic wll more rduous. In the sigmoid, they re not prominent nd give round or ovl shpe to the sigmoidl Fig. 10 Three-dimensionl view t the heptic flexure showing tumourl mss with overhnging edges (lck rrow). There is sutle ump on the colonic wll more proximlly (white rrow). The corresponding 2D imge shows the lrge tumour with overhnging edges nd shoulder formtion (lck rrow). The sutle ump ws cused y smll pericolonic nodule (white rrow) suggestive of mesenteric metstsis

7 Insights Imging (2011) 2: c Fig. 11 The rectum nd its peri-ctheter segment need specil ttention! Three-dimensionl view of the nl mrgin shows interprettion is hmpered y the rectl ctheter (lck rrow). The impression of the inflted lloon on the rectl wll is nicely pprecited (white rrows). A flt luminl defect is detected t the level of the nl mrgin, utting the rectl ctheter (white rrowhed)., c The corresponding prone view fter defltion of the lloon of the rectl ctheter shows lrge sessile polypoid defect (white rrowhed) ginst the rectl ctheter (lck rrow) confirmed on 2D s solid lesion >10 mm, prompting opticl colonoscopy lumen. In the cse of spsm the sigmoidl lumen shows round nrrowing with smooth, slightly thickened folds. On 2D CTC, spsm my pper s kissing folds: two thickened folds touching ech other. Diverticulosis In Western countries, diverticulosis is the most common colonic disese nd hrours mny trps in colonic imging with CTC. Becuse of myochosis there is reduced distension of the sigmoid, mking the semilunr folds more prominent with restricted visulistion of the sigmoidl lumen. Sometimes n dditionl right/left decuitus cquisition is required to otin optiml distension [18]. It is ovious tht diverticulosis is not restricted to the sigmoid nd tht the imging chrcteristics s descried elow my pper nywhere in the colon. Diverticulum In 3D the diverticulum my mimic polypoid lesion. Mostly it presents s completely drk ring, compred with the incomplete drk ring of the polyp when viewed en fce. In 2D the diverticulr outpouchings re clerly visulised [19]. Diverticulr feclith A diverticulum my ecome impcted with residul stool. As the diverticulum lcks musculr lyer, the stool is not expelled, remins in the diverticulum, dries nd hrdens into feclith [19]. In 3D it frequently presents s polypoid imge, while in 2D it hs typicl ppernce nd presents s luminl defect with hypodense centre nd hyperdense peripherl ring (Fig. 12). Whenever luminl Fig. 12 Three-dimensionl view of the sigmoid showing polypoid defect (white rrow). The corresponding xil imge shows tht the defect is cused y nodulr structure consisting of hypodense centre nd peripherl hyperdense ring typicl of diverticulr feclith (white rrow)

8 64 Insights Imging (2011) 2:57 68 defect is encountered in 3D, chrcteristion hs to e performed with 2D. If there re too mny fecliths, 3D ecomes time-consuming nd it is etter to switch to primry 2D red. Exceptionlly the diverticulr feclith inverts into the colonic lumen. Inverted diverticulum Although rre, wherever diverticulum ppers it my invert into the colonic lumen [20]. Three-dimensionl CTC shows sessile polypoid lesion. As the diverticulum usully tkes some luminl ir or perisigmoidl ft when inverting, it presents in 2D with n ir nd/or ft inclusion. Agin, comprison of the 3D nd 2D findings is mndtory to come to the right dignosis (Fig. 13). Wll thickening Chronic diverticulosis with periods of diverticulitis my result in wll thickening. Differentil dignosis with tumour is not lwys strightforwrd nd this entity is known with confusion in 10-50% of cses with n exct dignosis in one cse out of two [21]. Tle 1 shows the imging chrcteristics of the two entities. Polypoid mucosl prolpse syndrome With progression of diverticulr disese there is sometimes cretion of n excess of mucos ecuse of further shortening nd contrction of the musculr lyer [22]. This mucos prolpses into the colonic lumen nd cretes polypoid imge on oth 2D nd 3D. This pseudopolypoid imge cnnot e differentited from true polyp. So whenever its size exceeds 5 mm, opticl colonoscopy is needed. Sometimes the lesion chnges in spect with dul positioning. In tht cse, dignosis cn only e suggested nd opticl colonoscopy remins mndtory. Miscellneous Metllic rtefcts cuse lurring of the structures with multiple strek rtefcts, mking interprettion in oth 2D nd 3D difficult. In 2D it is helpful in smoothing the window. Descending colon As the teni coli ecomes somewht more prominent, the shpe of the colonic lumen vries from tuulr to slightly tringulr. This shpe is ccentuted in cse of spsm. Sometimes differentition from stenosing tumour is difficult. A tumour typiclly hs overhnging edges, shoulder formtion or my present with ull s eye phenomenon (Fig. 10). Trnsverse colon The teni coli re now relly prominent, giving typicl tringulr spect to the colonic lumen. This is ssocited with deep hustrtions. This necessittes good inspection of the colonic wll etween the semilunr folds using 120 viewing ngle of the virtul cmer. In 2D, coronl nd sgittl imges re helpful in chieving complete visulistion of the colonic wll. Agin in the cse of spsm, the nrrowed segment presents usully with tringulr shpe with smooth orders. Typiclly, kissing folds re distinguished (Fig. 14). As the trnsverse colon is situted nteriorly in the domen, it tends to e compressed y the weight of the domen in the prone position. As importnt pthologicl fetures my e hidden, specil cre should e tken to c Fig. 13 Three-dimensionl view of the scending colon in the supine position showing 9-mm polypoid defect (white rrow). The corresponding 2D view confirms the prole polyp (white rrow). c However, the prone cquisition revels diverticulum t the sme loction (white rrow). Review of the 2D imge in the dominl window shows centrl hypodensity of -100 to 0 HU, consistent with sutle mount of ft. These two findings confirm the dignosis of inverted diverticulum

9 Insights Imging (2011) 2: Tle 1 Differentil dignosis etween firosis nd denocrcinom. In the cse of mild cone-shped wll thickening with pericolonic strnding nd without denopthies, there is 90% specificity for firosis. In the cse of n pple core lesion or short wll thickening with shoulder formtion nd denopthies specificity for mlignnt tumour is 92% otin good distension of the trnsverse colon in the prone position. Besides the requirements of good distension, it cn e helpful putting pillow under the rest to decrese pressure on the trnsverse colon (supermn position). Exmintion of oth cquisitions is of course mndtory. Ascending colon The teni coli remin prominent, giving the typicl tringulr shpe to the colonic lumen (Fig. 13). Cecum As with the rectum, the cecum is closed structure or pouch needing specil ttention. Furthermore, the ileo-cecl vlve nd the ppendiculr orifice my render interprettion complicted. As the length of the cecum vries, it is frequently suject to flexurl pseudo-tumour nd to segmentl moility with cecum recurvtum internum nd externum. Ileo-cecl vlve It is mndtory to lwys loclise the ileo-cecl (IC) vlve when inspecting the cecum. The norml ileo-cecl vlve presents s mouth-like structure with n upper nd lower lip, converging in lrger fold, the frenulum. A centrl slit or opening represents the trnsition to the lst ilel loop. This opening is lwys directed towrds the cecl tip. Defining the norml morphology of the IC vlve excludes tumourl pthologicl fetures nd voids mistking the IC vlve for tumour. Compring 3D with 2D imges in oth dominl nd intermedite window settings is gin mndtory. Coronl nd sgittl imges re lso helpful in ssessing its structure. The IC vlve hs different spects. Lipomtous IC vlve Infiltrtion with lipomtous tissue cuses n enlrgement of the IC vlve without ltering the norml structure. Dignosis is

10 66 Insights Imging (2011) 2:57 68 Fig. 14, Three-dimensionl view of the trnsverse colon shows luminl nrrowing with tringulr shpe: imge typicl of spsm cused y contrction of the teni coli (white rrows). Thickened semilunr folds with smooth spect cusing kissing folds on the corresponding xil imge (white rrows). c Sgittl imge showing the teni coli (lck rrows) with the tringulr luminl nrrowing c confirmed y detecting ftty tissue in the dominl window setting or y checking its density (-100 to 0 HU). Ppillry trnsformtion With ppillry trnsformtion, the terminl ileum protrudes or invgintes into the IC vlve. This is norml functionl sttus with the IC vlve cting s sphincter preventing reflux of colonic contents to the smll owel. The IC vlve ecomes more prominent nd my pper s polypoid or even tumourl structure. Frequently the norml lip-like structure ecomes distorted with the IC vlve ppering s lrge, more or less rounded structure with smooth orders, with folds converging to centrl depression, representing the connection with the terminl ileum, when ssessed en fce (Fig. 15). In 2D coronl or sgittl imges llow for depiction of the norml lip-like structure. Tumourl trnsformtion With tumourl trnsformtion the IC vlve enlrges nd shows distortion of its norml structure. Differentition from norml ppillry vlve is not lwys strightforwrd nd needs inspection in oth 2D nd 3D imging modes. Sometimes infiltrtion of the pericolonic ft with or without lymph nodes is detected. Fig. 15 Axil imge of the cecum: lrge >1 cm polypoid structure (white rrow). The corresponding 3D imge shows the defect s lrge structure protruding into the colonic lumen. En fce ssessment shows converging folds towrds centrl depression: typicl imge of ppillry trnsformtion of the IC vlve (white rrow)

11 Insights Imging (2011) 2: Appendix The ppendicel orifice is situted t the convergence of the three teni coli. It ppers s smll opening frequently prtly covered y smll fold. The ppendix my prolpse into the colonic lumen. In this cse it my pper s smll polypoid structure with centrl depression representing the ppendicel orifice. Sometimes it ppers s smll defect utting the ppendix in 2D. If the defect looks polypoid nd exceeds 5 mm, opticl colonoscopy is necessry to exclude significnt tumourl lesion. An ppendectomy stump my e nother cuse of pseudopolypoid lesion in the cecum. CTC is not le to differentite this entity from regulr polyp, necessitting opticl colonoscopy whenever the defect exceeds 5 mm. 2D nd 3D imging-relted pitflls 2D-relted pitflls Polyp on fold Polyps situted on fold re prticulrly difficult to detect in 2D s sometimes they look like norml extension of the semi-lunr fold. Three-dimensionl CTC clerly shows the polyp prolpsing into the colonic lumen. The moving polyp A polyp with long pedicle my undergo importnt positionl shifts with dul positioning nd mimic nontgged stool. Three-dimensionl CTC clerly shows the pedunculted ppernce of the luminl defect. Specil cre hs to e tken if the pedicle is sumerged y contrst mteril pool. Polyp mesurement Underestimtion if the polyp size occurs when the detected polyp is only inspected on the xil imges. Very frequently, the polyp ppers lrger on the coronl nd sgittl imges. This my chnge the polyp from nonsignificnt (<6 mm) to significnt 6 mm lesion, necessitting opticl colonoscopy (Fig. 16). 3D-relted pitflls Polyp in fluid In 3D polyp is of course invisile in fluid, while it is esy to detect s negtive filling defect in 2D (Fig. 2). Some softwre is provided with electronic clensing removing the tgged residue in the colon. However this is not lwys relile tool nd polyp my e clensed in the cse of very dense tgging or insufficient clensing my occur in the cse of suoptiml tgging. Lesion chrcteristion A luminl defect detected in 3D lwys needs chrcteristion using 2D imging. Indeed 3D ump my e true lesion. However, it my lso correspond to residul stool, diverticulr feclith, lipom nd other entities s descried ove. Two-dimensionl imging using dominl window settings revels the exct nture of the luminl defect. Prominent folds In the cse of prominent semi-lunr folds the hustrtions ecome deeper nd re more difficult to inspect in 3D. This is prticulrly the cse in the scending nd trnsverse Fig. 16 Axil imge of the sigmoid reveling sessile diminutive 4-mm polyp (white rrow). The corresponding sgittl imge shows tht the identicl lesion is lrger nd ecomes significnt s it mesures 7 mm (white rrow). This finding should prompt opticl colonoscopy

12 68 Insights Imging (2011) 2:57 68 colon, nd in the cse of diverticulosis. Using 2D llows complete visulistion. Using 3D y turning the virtul cmer etween the folds, ensuring complete visulistion, is mndtory. Some softwre is provided with missed region tool. This tool utomticlly shows the regions not covered during the regulr fly-through in n ntegrde nd retrogrde direction. Other softwre llows for lterntive visulistion methods such s virtul pthologicl fetures depictinging the colon s if it were lid open showing ll the hidden spots [14]. Conclusion A flow chrt for dequte CTC interprettion: 1. Perform stte-of-the-rt CTC () Preprtion sed on fecl tgging () Colonic distension with CO 2 (+ injector), dul positioning nd smooth muscle relxtion 2. Generl principles of interprettion () Use the comined pproch: 2D-3D i. With clen, well-distended colon: use 3D ii. With filed preprtion or suoptiml distension, diverticulosis: use 2D () Alwys exmine oth supine nd prone cquisitions (c) Use ll dignostic tools: 3D, MPRs (d) Use oth the dominl nd intermedite window settings 3. Tke specil cre when exmining the rectum nd cecum References 1. Johnson CD, Chen MH, Toledno AY et l (2008) Accurcy of CT colonogrphy for detection of lrge denoms nd cncers. N Engl J Med 359: Kim DH, Pickhrdt PJ, Tylor AJ et l (2007) CT colonogrphy versus colonoscopy for the detection of dvnced neoplsi. N Engl J Med 357: Regge D (2007) the IMPACT Study Group Investigtors (2007) Accurcy of CT colonogrphy in sujects t incresed risk of colorectl crcinom: multicenter study on 1000 ptients. Presented t nnul meeting of the Rdiologicl Society of North Americ, Chicgo 4. Grser A, Stieer P, Ngel D, Schfer C, Horst D, Becker CR et l (2009) Comprison of CT colonogrphy, colonoscopy, sigmoidoscopy nd fecl occult lood tests for the detection of dvnced denom in n verge risk popultion. Gut 58 (2): Burling D, Interntionl collortion for CT colonogrphy stndrds (2010) CT colonogrphy stndrds. Clin Rdiol 65: Rockey DC, Pulson E, Niedzwiecki D, Dvis W, Bosworth HB, Snders L et l (2005) Anlysis of ir contrst rium enem, computed tomogrphic colonogrphy, nd colonoscopy: prospective comprison. Lncet 365(9456): Doshi T, Rusink D, Hlvorsen RA, Rockey DC, Suzuki K (2007) Dchmn AH (2005) CT colonogrphy: flse-negtive interprettions. Rdiology 244(1): Brish MA, Soto JA, Ferrucci JT (2005) Consensus on current clinicl prctice of virtul colonoscopy. AJR Am J Roentgenol 184: Lefere P, Gryspeerdt S, Dewyspelere J et l (2002) Dietry fecl tgging s clensing method efore CT colonogrphy: initil results polyp detection nd ptient cceptnce. Rdiology 224: O Connor SD, Summers RM, Choi JR et l (2006) Orl contrst dherence to polyps on CT colonogrphy. J Comput Assist Tomogr 30: Pickhrdt PJ, Kim DH, Tylor AJ (2008) Asymptomtic pneumtosis t CT colonogrphy: enign self-limited imging finding distinct from perfortion. AJR Am J Roentgenol 190:W112 W Yoshid H, Näppi J (2007) CAD in CT colonogrphy without nd with orl contrst gents: progress nd chllenges. Comput Med Imging Grph 31: Chen JC, Dchmn AH (2006) Cecl moility: potentil pitfll of CT colonogrphy. AJR Am J Roentgenol 186: Dchmn AH, Lefere P, Gryspeerdt S, Morrin M (2007) CT colonogrphy: visuliztion methods, interprettion, nd pitflls. Rdiol Clin North Am 45: Reeders JW, Tytgt GN (1994) Clinicl Rdiology nd endoscopy of the colon. Thieme, Stuttgrt, pp Gry H (1918) Gry s ntomy. Antomy of the humn ody. XI Splnchnology, 2H The Lrge Intestine The Brtley project t Floch HM, Floch NR, Kowdley K et l (2005) Netter s Gstroenterology. V. Colon, rectum nd nus. Icon Lerning Systems, Teteroro 18. Gryspeerdt SS, Hermn MJ, Bekelndt MA et l (2004) Supine/ left decuitus scnning: vlule lterntive to supine/prone scnning in CT colonogrphy. Eur Rdiol 14: Lefere P, Gryspeerdt S, Bekelndt M et l (2003) Diverticulr disese in CT colonogrphy. Eur Rdiol 13:L62 L Glick SN (1991) Inverted colonic diverticulum: ir contrst rium enem findings in six cses. AJR Am J Roentgenol 156: Chintplli KN, Chopr S, Ghits AA et l (1999) Diverticulitis vs colon cncer: differentition with helicl CT findings. Rdiology 210: Kelly JK (1991) Polypoid prolpsing mucosl folds in diverticulr disese. Am J Surg Pthol 15: