Causes and Solutions to Overcome False Negatives and False Positives

Stefaan Gryspeerdt and Philippe LeferE

CONTENTS

8.1 Introduction 87

8.2 False Negative Diagnosis 87

8.2.1 Failure to Detect the Lesion 88

8.2.1.1 Preparation Related False Negative Diagnosis 88

8.2.1.2 Technical Artefacts 88

8.2.1.3 Normal Anatomy and Areas of Danger 89

8.2.1.4 Diverticular Disease 90

8.2.1.5 Sessile Polyps 94

8.2.1.6 Small Lesions and Small Flat Lesions 94

8.2.2 Failure to Characterise the Lesions 96

8.2.2.1 Annular Stricturing Lesions 96

8.2.2.2 Larger Flat Lesions 96

8.2.2.3 Small Sessile Polyps 96

8.2.2.4 Sessile Cancers 96

8.2.2.5 Pedunculated Lesions 99

8.3 False Positive Diagnosis 99

8.3.1 Preparation Related False Positive Findings 99

8.3.2 Technical Artefacts Causing False Positive Findings 101

8.3.2.1 Breathing Artefacts 101

8.3.2.2 Spasm 104

8.3.3 Pitfalls Related to Normal Anatomy and Non-Tumoral Lesions 104

8.3.3.1 Ileocecal Valve 104

8.3.3.2 Extrinsic Impression 106

8.3.3.3 Complex or Thickened Folds 106

8.3.3.4 Lipoma 107

8.3.3.5 Vascular Lesions 107

8.3.3.6 Appendiceal Orifice 107

8.3.3.7 Scar after Polypectomy 107

8.3.3.8 Spasm of the Internal Sphincter 110

8.3.3.9 Intermittently Prolapsing Rectal Mucosa 110

8.3.3.10 Diverticular Disease 110 References 114

Introduction

A major requisite, prior to the use of CTC as a screening tool, is to achieve an accuracy level comparative to that of conventional colonoscopy. Until

Stedelijk Ziekenhuis, Bruggesteenweg 90, 8800 Roeselare, Belgium now, a wide range of sensitivities has been reported (Fletcher et al. 2005), even for the largest lesions (>9 mm): Johnson et al. (JohNsoN et al. 2003) reported a major inter-observer variability, with sensitivity ranges between 32% and 73%, whereas Pickhardt et al. (PickharDt et al. 2003) reported excellent sensitivities of 93.8%. This wide range of reported sensitivities is one of the major reasons for gastrointestinal endoscopists not to advocate the technique as screening tool yet (HwaNg and WoNg 2005; Pickhardt 2005).

In depth analysis of the different results has shown numerous possible causes for the reported differences in accuracy, including a learning curve, influencing sensitivity (Spinzi et al. 2001), as well as specificity (Gluecker et al. 2002). This learning curve includes the whole process of CTC: patient preparation, scanning technique (including patient positioning, colon insufflation, and scanning parameters), image manipulation and interpretation (Taylor et al. 2004).

Each step in the process of a CTC examination has its own potential dangers of hindering a final correct diagnosis.

In this chapter, different causes of false positive and false negative diagnosis are reviewed, and possible solutions to overcome these problems are proposed.

In the first part, false positive diagnosis will be reviewed, in the second part, false negative diagnosis are reviewed.

The figures represent a pictorial review of the different pitfalls, with lessons to overcome those pitfalls, marked italic in the figure legend.

False Negative Diagnosis

Overall, false negative diagnosis can be related to errors in characterisation, failure to detect the lesions, or a combination (FiDler et al. 2004).

Failure to Detect the Lesion 8.2.1.1

Preparation Related False Negative Diagnosis

There are two main bowel preparations available: cathartics, such as magnesium citrate and phos-phosoda, and gut lavage solutions, such as polyethylene glycol (PEG).

PEG is known as a "wet prep", leaving a clean colon filled with residual fluid. Residual fluid does not hinder colonoscopic evaluation because of the ability of the colonoscopist to aspirate the residual fluid. In CTC however, PEG results in fluid filled segments, impeding full mucosal visibility, resulting in false negative diagnosis.

Prone supine imaging might overcome this problem in some way (Fig. 8.1), but is still insufficient to guarantee a complete colon evaluation (Gryspeerdt et al. 2002).

Fluid tagging has been proposed to overcome the problem of drowned segments: the fluid can be tagged either by barium, iodinated contrast material, or a combination (Lefere et al. 2002; Zalis et al. 2003; PickhardT et al. 2003; Pineau et al. 2003). As a result, the polyp can be detected as a hypodense structure in the tagged, hyperdense fluid levels (Fig. 8.2). Additionally, there is the possibility of electronic cleansing, removing the tagged fluid level, resulting in complete mucosal visibility (Zalis et al. 2003; Pickhardt et al. 2003).

Preparation without cathartic cleansing, so-called "dry preparation" is currently being evaluated as the ultimate reduced preparation, further improving patient compliance, and almost eliminating residual fluid (Bielen et al. 2003; Callstrom et al. 2001; Lefere et al. 2004; McFarland and Zalis 2004; Pickhardt et al. 2003; Zalis et al. 2003).

Technical Artefacts

A major technical artefact is caused by suboptimal distended or even undistended segments, possibly hiding polyps.

Spasmolytic agents can be used to improve colonic distention.

There are two main spasmolytic agents: glucagon (a single chain polypeptide hormone that increases blood glucose and relaxes the smooth muscle of the gastrointestinal tract), and butylhyoscine (Busco-pan) used in Europe and Asia to induce bowel hypo-tonia.

The rationale to use spasmolytic agents is a possible improved colonic distention, and reduced procedural pain.

Goei et al. (Goei et al. 1995) found Buscopan to be more effective in distending the colon than glu-cagon, which is in agreement with the findings of both Yee et al. (Yee et al. 1999 ) and Morrin et al. (Morrin et al. 2002) who found that colonic disten-tion was not improved after glucagon administra

Fig. 8.1a,b. False negative diagnosis due to a non-tagged, fluid drowned segment: a prone image shows a fluid level (arrows), impeding visualisation of a stalked polyp; b the stalked polyp is clearly seen on the supine image (arrow). Lesson: Prone/supine imaging is useful to prevent false negative diagnosis in case of fluid filled segments

Fig. 8.1a,b. False negative diagnosis due to a non-tagged, fluid drowned segment: a prone image shows a fluid level (arrows), impeding visualisation of a stalked polyp; b the stalked polyp is clearly seen on the supine image (arrow). Lesson: Prone/supine imaging is useful to prevent false negative diagnosis in case of fluid filled segments a

Why People Have Nasal Polyps Negative Colonoscopy Pictures

Fig. 8.2a-c. True positive diagnosis in a tagged, fluid drowned segment: a supine image shows a hypodense structure in a tagged, hyperdense fluid level (arrows), suspicious for a sessile polyp; b,c the presence of a 5-mm sessile polyp (arrows) is confirmed on prone (b) and endoluminal 3D image (c). Lesson: Fluid tagging can be used to overcome the problem of drowned segments: polyps can be detected as a hypodense structure in a tagged, hyperdense fluid level tion. Therefore, the use of glucagon is abandoned at this moment.

Bruzzi et al. (Bruzzi et al. 2003) found that Buscopan should not be used routinely, but is useful in patients with diverticular disease. Taylor et al. (Taylor et al. 2003a) on the other hand found the effect of Buscopan also extends to those without diverticular disease.

Orally administered Buscopan has also proven useful during barium enema (Bova et al. 1999).

In our institution, we routinely use Buscopan: 10 mL diluted in 100 mL of 0.9% sodium chloride and administered intravenously at a rate of 10 mL/ min.

The reason is the subjective impression of reduced procedural pain, and the fact that procedural spasm can mimick tumor (see below) or impedes adequate evaluation.

A persistent spasm can be differentiated from tumoral lesions by its smooth contours, and the absence of surrounding lymph nodes. In some instances, additional inflation might be necessary to solve the problem (see Sect. 8.3.2.2).

The problem of segmental collapse can also be solved by prone-supine imaging (Fig. 8.3).

Normal Anatomy and Areas of Danger

Normal anatomy may cause false negative diagnosis in that normal anatomical structures can hide polyps: thickened semilunar folds typical hide polyps in either antegrade or retrograde three-dimensional evaluation (Fig. 8.4). The same holds true for complex folds at the hepatic or splenic flexure, possibly hiding small polyps, impossible to detect using standard antegrade and retrograde three-dimensional views. To overcome the problems of inadequate visualisation of the colonic lumen, b a c

Fig. 8.3-c. False negative diagnosis due to incomplete distension: a supine image shows a suboptimal distended sigmoid (arrows in a); b,c prone image shows a good distension of the sigmoid, revealing the presence of a tumoral lesion (arrows in b). The tumoral lesion is better appreciated using abdominal window settings (arrows in c), compared to intermediate window settings (arrows in b). Lesson: Optimal distension is a prerequisite to detect colonic lesions. Optimal distension can be achieved by dual positioning and routine use of butylhyo-scine (Buscopan)

different three-dimensional reconstruction methods have been developed, improving the detection of polyps on three-dimensional endoscopic views such as virtual colonic dissection or unfolded cube (Hoppe et al. 2004; Vos et al. 2003; Luo et al. 2004). Each of these different viewing modes aim to display the whole colonic lumen at one view, obviating the need of turning the virtual camera in different angles.

The cecum, hepatic flexure, transverse colon, splenic flexure, and sigmoid colon, are to be considered as "areas of danger" because of the convoluted and mobile nature.

The mobile nature of these segments mimicks positional change of lesions, possibly causing erroneous diagnosis of "mobile" residual stool (Fig. 8.5) (Park et al. 2005).

Although the rectum is straight and not mobile, one has to take care of not missing rectal lesions. False negative diagnosis of rectal lesions may be caused by "readers fatigue" if one starts at the cecal level, or by rectal balloon catheter hiding rectal lesions (Pickhardt and Choi 2005) (Fig. 8.6).

The ileocecal valve is an important "mimicker" of pathology (see below), but one has to keep in mind that the ileocecal valve might hide polyps (Figs. 8.7 and 8.8), or can even be cancerous (Fig. 8.9). Can b b

cers of the ileocecal valve should not be mistaken for lipomatous or papillary transformations. Different window settings are helpful in revealing the cancerous nature of the lesions.

Diverticular Disease

In case of diverticular disease, prominent semicircular folds, luminal narrowing and distortion impede good visualisation of the colonic surface resulting in difficult detection of polypoid lesions. In fact, as optimal detection of polyps is only achieved in well-distended segments of the colon, special care has to be taken when examining the involved segments with shortened haustrations and increased luminal tortuosity. In order not to interpret a polyp as a thickened fold, or vice versa, it is important to examine each semicircular fold by scrolling back and forth through the axial images. Imaging in both abdominal and lung window settings is mandatory to detect focal wall thickenings and luminal filling defects, respectively. (LeferE et al. 2003) Frequent comparison between 2D and 3D images is recommended (McFarland 2002) (Fig. 8.10).

Fig. 8.4a-d. False negative diagnosis: small sessile lesions, located between haustral folds: a,b antegrade (a) and retrograde endo-luminal views (b) show normal haustral folds in the ascending colon; c corresponding coronal MPR image shows a small sessile lesion, located between two haustral folds (arrow); d lateral endoluminal 3D image reveals the polyp, located between haustral folds.(arrow). Lesson: Primary 3D read should include different viewing angles, either by turning the virtual camera, either by using dedicated software, offering different three dimensional reconstruction methods, thus showing the whole colonic wall

Fig. 8.4a-d. False negative diagnosis: small sessile lesions, located between haustral folds: a,b antegrade (a) and retrograde endo-luminal views (b) show normal haustral folds in the ascending colon; c corresponding coronal MPR image shows a small sessile lesion, located between two haustral folds (arrow); d lateral endoluminal 3D image reveals the polyp, located between haustral folds.(arrow). Lesson: Primary 3D read should include different viewing angles, either by turning the virtual camera, either by using dedicated software, offering different three dimensional reconstruction methods, thus showing the whole colonic wall

Fig. 8.5a,b. False negative diagnosis: polyps simulating fecal residue in mobile segments. Differential diagnosis of mobile stool or small sessile lesions in a mobile transverse colon: a supine scan shows two lesions in the transverse colon. (arrows); b prone scan shows the lesions in the transverse colon in an apparent different position (arrows). Conventional colonoscopy revealed the presence of two small sessile polyps. Lesson: Polyps, located in mobile colonic segments such as the transverse colon can cause erroneous diagnosis of "mobile" residual stool

Fig. 8.5a,b. False negative diagnosis: polyps simulating fecal residue in mobile segments. Differential diagnosis of mobile stool or small sessile lesions in a mobile transverse colon: a supine scan shows two lesions in the transverse colon. (arrows); b prone scan shows the lesions in the transverse colon in an apparent different position (arrows). Conventional colonoscopy revealed the presence of two small sessile polyps. Lesson: Polyps, located in mobile colonic segments such as the transverse colon can cause erroneous diagnosis of "mobile" residual stool b

Fig. 8.6a,b. False negative diagnosis: rectal balloon catheter hiding rectal polyp: a prone scan after removing the rectal balloon clearly shows a large stalked polyp (arrow); b the polyp is hidden by the rectal balloon on supine image.(arrow). Lesson: Thick rectal balloon catheters can hide rectal lesions. Therefore, remove rectal balloon catheter on prone scan

Fig. 8.6a,b. False negative diagnosis: rectal balloon catheter hiding rectal polyp: a prone scan after removing the rectal balloon clearly shows a large stalked polyp (arrow); b the polyp is hidden by the rectal balloon on supine image.(arrow). Lesson: Thick rectal balloon catheters can hide rectal lesions. Therefore, remove rectal balloon catheter on prone scan a

Fig. 8.7a-c. False negative diagnosis: differentiate small sessile polyps located on the ileocecal valve from normal variations of the ileocecal valve. Although the ileocecal valve is an important "mimicker" of pathology, one has to keep in mind that polyps can arise on the ileocecal valve (arrows). Evaluation in: a "intermediate" window setting as well as; b "abdominal window setting", combined with: c 3D endolu-minal view are helpful to differentiate polyps from tumoral (see Fig. 8.9) or lipomatous transformation of the ileocecal valve (see Fig. 8.26). Lesson: For the evaluation of pathology of the ileocecal valve, always use different window settings, in combination with endoluminal 3D evaluation

c

Fig. 8.8. Small sessile polyp located in the ileocecal valve. Although most polyps are located on the ileocecal valve, polyps can also arise in the ileocecal valve (arrow)

Fig. 8.9a-c. False negative diagnosis: patient with Crohns' disease: tumoral transformation of the ileocecal valve to be differentiated from lipomatous transformation of the valve: a axial 2D image shows a dense, extremely enlarged ileocecal valve on abdominal window settings (thus excluding the possibility of lipomatous transformation) (arrows); b a hyper-trophic ileocecal valve is also seen on endoluminal 3D images (arrows); c corresponding conventional colonoscopic image shows tumoral transformation of the ileocecal valve.(arrows). Lesson: In case of an extremely enlarged and dense ileocecal valve, keep in mind the possibility of tumoral transformation, to be differentiated from papillary transformation or lipomatous infiltration of the ileocecal valve. (Fig. 8.27). Abdominal window settings are helpful in excluding the possibility of lipomatous transformation of the ileocecal valve

Fig. 8.10a-d. False negative diagnosis: thickened folds in diverticular disease, hiding a small sessile polyp. Diverticular disease is characterised by thickened semilunar folds. The luminal narrowing and the thickened semilunar folds make primary three dimensional evaluation extremely difficult: a,b there is a normal antegrade (a) and retrograde view (b) of the narrow lumen with thickened folds in a patient with diverticular disease; c corresponding axial 2D image shows a small polyp, interspaced between two thickened semilunar folds. (arrow); d tailored endoluminal 3D image shows the small polyps (arrow) interspaced between thickened haustral folds. Lesson: In case of diverticular disease, frequent comparison between 2D and 3D images is necessary, in order not to miss small polyps, interspaced between thickened haustral folds

Fig. 8.10a-d. False negative diagnosis: thickened folds in diverticular disease, hiding a small sessile polyp. Diverticular disease is characterised by thickened semilunar folds. The luminal narrowing and the thickened semilunar folds make primary three dimensional evaluation extremely difficult: a,b there is a normal antegrade (a) and retrograde view (b) of the narrow lumen with thickened folds in a patient with diverticular disease; c corresponding axial 2D image shows a small polyp, interspaced between two thickened semilunar folds. (arrow); d tailored endoluminal 3D image shows the small polyps (arrow) interspaced between thickened haustral folds. Lesson: In case of diverticular disease, frequent comparison between 2D and 3D images is necessary, in order not to miss small polyps, interspaced between thickened haustral folds

Sessile Polyps

Small Lesions and Small Flat Lesions

Although sessile polyps have a high conspicuity, if located between folds (Fig. 8.11), those lesions may remain undetected in case the lesions are located on a semilunar fold (Fig. 8.12).

A thickened fold in an otherwise well distended colon might therefore point to the correct diagnosis of a sessile polyp on a haustral fold (Fidler et al. 2004) (see also Fig. 8.18).

Park et al. (Park et al. 2005) found that for the lesions that were not detected for reasons not apparent on retrospective analysis, size of the lesion was the only significant factor associated with lesion detectability. Lesions 5 mm or smaller are more difficult to visualize than those 6 mm or larger. Up to 50% or more of those lesions smaller than 5mm are, however, non-adenomatous, and the need to detect a d c

Endoluminal
Fig. 8.11a,b. Sessile polyp located between a haustral fold. Polyps located between normal haustral folds are easy to detect on: a axial 2D image (arrow), and; b corresponding endoluminal 3D image (arrow)
Endoluminal

Fig. 8.12a-d. False negative diagnosis : sessile polyp located on a haustral fold. Polyps located on a haustral fold are difficult to detect on: a axial 2D images (arrow); b sagittal 2D images; c coronal 2D images, showing a thickened haustral fold in an otherwise well distended segment; d corresponding endoluminal 3D image nicely shows a polyp on a haustral fold (arrow). Lesson: A thickened haustral fold in an otherwise well-distended segment is suspicious for a polyp on a haustral fold

Fig. 8.12a-d. False negative diagnosis : sessile polyp located on a haustral fold. Polyps located on a haustral fold are difficult to detect on: a axial 2D images (arrow); b sagittal 2D images; c coronal 2D images, showing a thickened haustral fold in an otherwise well distended segment; d corresponding endoluminal 3D image nicely shows a polyp on a haustral fold (arrow). Lesson: A thickened haustral fold in an otherwise well-distended segment is suspicious for a polyp on a haustral fold those lesions is therefore questionable (Macari et al. 2004; Pickhardt et al. 2004a).

Flat lesions are defined as lesions with a height less than half the lesion diameter (Dachman and Zalis 2004). This definition includes a wide range of flat lesions, including small as well as large lesions.

Small flat lesions will be missed, even on retrospective analysis, for the same reason as small sessile lesions: small lesions are just more difficult to visualise (Macari et al. 2003) (Fig. 8.13).

Larger flat lesions may also cause false negative diagnosis, because of failure to correctly characterise the lesion (see 8.2.2.2).

Failure to Characterise the Lesions 8.2.2.1

Annular Stricturing Lesions

Annular structuring lesions may be misinterpreted as either spasm (Figs. 8.14 and 8.15) or residual fecal material. The use of fecal tagging with an oral contrast agent (Thomeer et al. 2003; Zalis et al. 2003; Pickhardt et al. 2005) seems to help in avoiding interpretive errors caused by residual fecal material.

Larger Flat Lesions

Flat lesions can be divided into flat adenomas, flat depressed adenomas, plaque-like carcinomas and carpet lesions (Galdino and Yee 2003).

As discussed, small flat lesions are difficult to detect for the same reason as small sessile lesions: detection is hampered by resolution.

Larger flat lesions however are also difficult to detect and to characterize for several reasons.

First of all, there is the problem of insufficient awareness and familiarity with those lesions: surveillance programs, based on the known adenoma-carcinoma sessile or pedunculated lesion, have mainly focused on identifying sessile of peduncu-lated polyps. This explains why flat lesions are frequently characterised as normal folds. As a rule, a thickened fold in an otherwise well distended colon should raise the question whether or not this lesion could represent a flat lesion.

Second, the plaque-like morphology is likely to be mistaken for residual fecal material (Park et al. 2005): the use of oral contrast media may there fore help in detecting flat lesions. Third, the size and morphology of the lesions explain the necessity of different window settings (Dachman et al. 2004; Fidler et al. 2002): detection of flat lesions is improved by using abdominal window settings, rather than the routinely used intermediate window settings (Fig. 8.16). The necessity to change window settings also explains the low conspicuity of flat lesions, even the larger ones.

Reviewing the literature shows two different morphological characteristics for flat lesions.

If the lesions are located between haustral folds, they appear as a small flat protuberance; if they are located on haustral folds, or near haustral folds, they are associated with minimal fold irregularity; if they arise from a haustral fold, they project into the lumen, creating a cigar-like appearance.

Optimal bowel preparation and distention are therefore prerequisites to detect flat lesions.

Flat adenomas measuring 6 mm or greater are, however, uncommon in a typical Western screening population, and advanced neoplasms are rare. Flat lesions should therefore not be considered a significant drawback for virtual colonoscopy screening (Pickhardt et al. 2004b).

Small Sessile Polyps

Small sessile polyps frequently represent hyper-plastic polyps. Hyperplastic lesions tend to flatten out in well distended segments, explaining the fact that those lesions might only be visible in somewhat underdistended segments. In that way, those lesions are frequently only recognised on either prone or supine position, and can therefore be mistaken as residual stool.

Hyperplastic lesions however are not to be considered precancerous, and should therefore be considered as "leave-alone" lesions. Misinterpreting those lesions as residual stool is therefore rather beneficial for the patients, avoiding unnecessary conventional colonoscopy (Pickhardt et al. 2004a) (Fig. 8.17).

Sessile Cancers

Sessile cancers, if detected, may remain unrecognised by the fact that the lesions are characterised as normal fold; correlating axial 3D images with endo-luminal views is helpful in this respect (Fig. 8.18).

Fig. 8.13a-c. False negative diagnosis: small (<5 mm) sessile lesion. The figure shows a small sessile 3-mm polyp, prospectively missed. Retrospectively, the lesion was identified on: a axial image (arrow); b endoluminal 3D image (arrows); c corresponding conventional colonoscopic image shows a small polyp, representing a hyperplastic polyp on pathological examination (arrows). Lesson: Small polyps (<5 mm) are difficult to detect. However, up to 50% of those lesions are non-adenomatous, and the necessity to detect those lesions is therefore questionable b a c

Fig. 8.14a,b. False positive diagnosis: spasm mimicking annular structuring lesion. Spasm can closely resemble annular stric-turing lesions (see Fig. 15): a a non-distended sigmoid region (arrow) in prone position; b corresponding supine image shows a normal sigmoid

Fig. 8.15a,b. False negative diagnosis - failure to characterise lesions: annular structuring lesions compared to spasm: a a non-distended sigmoid region (arrow) in supine position; b corresponding prone images shows a persistent wall thickening with shoulder forming (arrows). Conventional colonoscopy showed a tumoral lesion. Lesson: Annular structuring lesions can closely resemble spasm. Dual positioning is mandatory to avoid those pitfalls (compare Figs. 14 and 15)

Fig. 8.15a,b. False negative diagnosis - failure to characterise lesions: annular structuring lesions compared to spasm: a a non-distended sigmoid region (arrow) in supine position; b corresponding prone images shows a persistent wall thickening with shoulder forming (arrows). Conventional colonoscopy showed a tumoral lesion. Lesson: Annular structuring lesions can closely resemble spasm. Dual positioning is mandatory to avoid those pitfalls (compare Figs. 14 and 15)

Fig. 8.16a-c. False negative diagnosis - failure to characterise lesions: flat lesions. Flat lesion in the caecum at the level of the ileocecal valve: a,b axial image at the level of the ileocecal valve shows subtle wall thickening on intermediate window settings (arrows in a), better appreciated on abdominal window settings (arrows in b); c 3D endo-view image shows subtle wall thickening (arrows). Pathology showed a tubular adenoma. Lesson: Flat lesions have been defined as lesions with a height less than half the lesion diameter. This nature makes them difficult to recognize. As in this patient, changing the window settings is helpful in diagnosing these lesions b a c a

Negative Colonoscopy Pictures

Fig. 8.17a-c. False negative diagnosis - failure to characterise lesions: hyperplastic lesions: a axial, supine 2D image shows a small polyp-like lesion (arrow) in a suboptimal distended segment; b corresponding prone scan shows a better distended descending colon, and does not show the lesion anymore. Differential diagnosis was made between hyperplastic polyp and fecal residue; c corresponding conventional colonos-copy reveals a small hyperplastic polyp. Lesson: Hyperplastic lesions tend to flatten out in well-distended segments, impeding visualisation in prone or supine position. Therefore, they are frequently misinterpreted as mobile fecal residue b a c

Sessile cancers may also be mistaken for residual stool because of marked surface irregularity, usually attributed to residual stool (Gluecker et al. 2004).

Pedunculated Lesions

Pedunculated lesions may remain undetected because of mischaracterisation as fecal residues or even residual fluid.

Mischaracterisation as fecal residues is caused by the fact that there are three observations that are made to distinguish stool from polyps: presence of gas, morphology (polyps and small cancers have rounded and lobulated smooth borders), and the mobility. In particular, the mobility of the lesions is used in favour of residual stool, analogue to the findings on double contrast barium enema (Laks et al. 2004).

Pedunculated polyps however change in position between prone and supine images, and may moreover include gas between the stalk and the bowel wall, mimicking residual stool (Fig. 8.19).

A pedunculated polyp can also mimick residual fluid (Fig. 8.20).

False Positive Diagnosis 8.3.1

Preparation Related False Positive Findings

One of the major reasons why virtual colonoscopy is attractive to the patients is its ability to evaluate the colon without the need for an intensive colon cleansing regimen. Different reduced preparations have been evaluated: reduced amount of PEG in

Fig. 8.18a-d. False negative diagnosis - failure to characterise the lesions: sessile cancer: a,c broad based thickened haustral fold on supine (arrows in a) and prone image (arrows in c) at the hepatic flexure. Differential diagnosis: complex haustral fold, normal thickened fold or sessile cancer; b,d corresponding endoluminal 3D images show distorted and thickened haustral fold on supine (arrow in b) as well as prone scan (arrow in d). Conventional colonoscopy showed a flat sessile cancer. Lesson: Endoluminal 3D images are useful to differentiate normal thickened folds from sessile cancers. (compare with Fig. 8.29)

Fig. 8.18a-d. False negative diagnosis - failure to characterise the lesions: sessile cancer: a,c broad based thickened haustral fold on supine (arrows in a) and prone image (arrows in c) at the hepatic flexure. Differential diagnosis: complex haustral fold, normal thickened fold or sessile cancer; b,d corresponding endoluminal 3D images show distorted and thickened haustral fold on supine (arrow in b) as well as prone scan (arrow in d). Conventional colonoscopy showed a flat sessile cancer. Lesson: Endoluminal 3D images are useful to differentiate normal thickened folds from sessile cancers. (compare with Fig. 8.29)

combination with bisacodyl, magnesium carbonate with citric acid (Citramag, Pharmaserve LTD, Manchester, UK), a combination of sodium pico-sulphate with magnesium citrate (Picolax, Ferring Pharmaceuticals Ltd, Berkshire, UK), magnesium citrate combined with bisacodyl tablets and suppository (Losoprep, EZ-EM, Westbury, NY), fleet phosphosoda (Yee 2002; Taylor et al. 2003a, b; Gryspeerdt et al. 2002; Macari et al. 2001).

Compared to standard colon cleansing regimens, each of these reduced preparations showed fewer side effects and disturbances to daily patients life, while inviting improved patient compliance.

The driest preparations Picolax (Taylor et al. 2003a, b), Losoprep (Lefere et al. 2002) and fleet phosphosoda (Macari et al. 2001), however, are associated with more retained residue, with subsequent increased risk of false positive findings. False positive findings are mainly related to small fecal residue: larger residues will shift between prone and supine imaging, while smaller residues stick to the wall.

Therefore, there is the need for fecal tagging: fecal tagging reduces false positive diagnosis (Lefere et al. 2002; Gryspeerdt et al. 2002) (Figs. 8.21-8.23).

Proneimage

Fig. 8.19a-d. False negative diagnosis - failure to characterise the lesions: pedunculated lesions mimicking residual stool: a prone image shows a nodular mass (arrows), with air included in the mass (arrowhead in a); b corresponding endoluminal 3D image shows a nodular lesion (arrow in b); c supine image shows the lesion is highly mobile (large arrows), and suggests the presence of a stalk (small arrows); d the supine-endoluminal image, clearly shows a pedunculated lesion (arrows)

Fig. 8.19a-d. False negative diagnosis - failure to characterise the lesions: pedunculated lesions mimicking residual stool: a prone image shows a nodular mass (arrows), with air included in the mass (arrowhead in a); b corresponding endoluminal 3D image shows a nodular lesion (arrow in b); c supine image shows the lesion is highly mobile (large arrows), and suggests the presence of a stalk (small arrows); d the supine-endoluminal image, clearly shows a pedunculated lesion (arrows)

Technical Artefacts Causing False Positive Findings

Breathing Artefacts

Most published studies using single slice CT have used a collimation of 3-5 mm and a pitch of 1-2, resulting in breath hold times ranging from 35 to 50 s. Such long breath hold periods were prone to breathing artefacts, simulating polyps. The introduction of multislice CT technology now permits thinner collimation (1-2.5 mm), and reduced breath hold time (15-20 s) (Embleton et al. 2003; Taylor et al. 2003c, d). These reduced breath hold times virtually eliminate severe artefacts. If patients are still unable to maintain a breath hold, left decubitus scanning has been shown a valuable alternative to prone scanning, reducing breathing artefacts if used as the second scan (Gryspeerdt et al. 2004) (Fig. 8.24).

Fig. 8.20a-d. False negative diagnosis - failure to characterise the lesions: pedunculated lesions mimicking residual fluid: a supine image shows a thick haustral fold (arrow) and suggests the presence of a fluid level.(arrowheads); b-d corresponding axial (b) and endoluminal 3D image (c) in prone position shows the stalk of a pedunculated lesion (arrows in b-d), proven on conventional colonoscopy (d). Lesson: Pedunculated lesions may mimick fecal residues (they can include air, due to their stalked nature, and are highly mobile) or fluid levels. Identifying the stalk on 3D endoluminal images points to the diagnosis of a pedunculated lesion

Fig. 8.20a-d. False negative diagnosis - failure to characterise the lesions: pedunculated lesions mimicking residual fluid: a supine image shows a thick haustral fold (arrow) and suggests the presence of a fluid level.(arrowheads); b-d corresponding axial (b) and endoluminal 3D image (c) in prone position shows the stalk of a pedunculated lesion (arrows in b-d), proven on conventional colonoscopy (d). Lesson: Pedunculated lesions may mimick fecal residues (they can include air, due to their stalked nature, and are highly mobile) or fluid levels. Identifying the stalk on 3D endoluminal images points to the diagnosis of a pedunculated lesion

Fig. 8.21a,b. False positive diagnosis: adherent fecal residue. Fecal tagging to facilitate differential diagnosis between fecal residue and polyp. Standard colonoscopic cleansing: false positive diagnosis of polyp due to adherent fecal residue: a supine image and; b prone image in a patient with incomplete visualisation of the cecum due to redundancy suggested the presence of a 10-mm polypoied lesion in the transverse colon (arrows in a and b). Since the transverse colon was reached on repeated conventional colonoscopy, and no lesion was detected, this lesion was interpreted as false positive due to adherent fecal residue. Arrowheads in a and b: non-tagged fluid levels, adherent to standard colonoscopic preparation

Fig. 8.21a,b. False positive diagnosis: adherent fecal residue. Fecal tagging to facilitate differential diagnosis between fecal residue and polyp. Standard colonoscopic cleansing: false positive diagnosis of polyp due to adherent fecal residue: a supine image and; b prone image in a patient with incomplete visualisation of the cecum due to redundancy suggested the presence of a 10-mm polypoied lesion in the transverse colon (arrows in a and b). Since the transverse colon was reached on repeated conventional colonoscopy, and no lesion was detected, this lesion was interpreted as false positive due to adherent fecal residue. Arrowheads in a and b: non-tagged fluid levels, adherent to standard colonoscopic preparation mm

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Fig. 8.22a,b. False positive diagnosis: adherent fecal residue. Fecal tagging to facilitate differential diagnosis between fecal residue and polyp.(cont'd). Reduced preparation with fecal tagging using barium as the sole tagging agent. There is a 4-mm lesion on: a supine image (arrow) and; b prone image (arrow). The lesion is hyperdense, pointing towards a tagged fecal residue. Conventional colonoscopy did not show any lesions in this patient

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Fig. 8.22a,b. False positive diagnosis: adherent fecal residue. Fecal tagging to facilitate differential diagnosis between fecal residue and polyp.(cont'd). Reduced preparation with fecal tagging using barium as the sole tagging agent. There is a 4-mm lesion on: a supine image (arrow) and; b prone image (arrow). The lesion is hyperdense, pointing towards a tagged fecal residue. Conventional colonoscopy did not show any lesions in this patient c

Endoluminal

Fig. 8.23a-d. False positive diagnosis: adherent fecal residue. Fecal tagging to facilitate differential diagnosis between fecal residue and polyp (cont'd). Reduced preparation with fecal tagging using barium as the sole tagging agent: a,b axial (a) and endoluminal (b) 3D image in prone position: There is a 6-mm non-tagged lesion at the splenic flexure (arrows). The non-tagged nature suggests the presence of a polyp; c,d correpsonding axial (c) and endoluminal (d) 3D image in supine position. The non-tagged lesion, seen on prone image corresponds to a pedunculated polyp (arrows). Lesson: Fecal tagging reduces false positive findings due to adherent fecal residue, improves conspicuity of polyps and reduces false positives since the tagged or non-tagged nature of the lesions allows easy differentiation between polyps and fecal residues

Fig. 8.23a-d. False positive diagnosis: adherent fecal residue. Fecal tagging to facilitate differential diagnosis between fecal residue and polyp (cont'd). Reduced preparation with fecal tagging using barium as the sole tagging agent: a,b axial (a) and endoluminal (b) 3D image in prone position: There is a 6-mm non-tagged lesion at the splenic flexure (arrows). The non-tagged nature suggests the presence of a polyp; c,d correpsonding axial (c) and endoluminal (d) 3D image in supine position. The non-tagged lesion, seen on prone image corresponds to a pedunculated polyp (arrows). Lesson: Fecal tagging reduces false positive findings due to adherent fecal residue, improves conspicuity of polyps and reduces false positives since the tagged or non-tagged nature of the lesions allows easy differentiation between polyps and fecal residues b a b d c u

Fig. 8.24a,b. False positive diagnosis: pseudopolyp due to breathing artefacts: a axial 2D image obtained in a 66-years-old patient in prone position shows breathing artefacts (arrow); b corresponding endoluminal 3D image shows pseudopolypoied appearance of the colonic wall, caused by breathing artefact (arrow). Lesson: In patients who are extremely short of breath, especially prone scanning can be hampered by breathing artefacts, causing pseudo-polypoid appearance on endoluminal 3D images. Left/decubitus scanning instead of prone scanning can be used as an alternative b a

There are seven different sphincters (concentric rings; valves), distributed throughout the colon: (1) Ring of Busi, (2) Ring of Hirsh, (3) Ring of Cannon, (4) Ring of Payr-Strauss, (5) Ring of Balli, (6) Ring of Moultier, (7) Ring of Rossi (Reeders and Rosenbush 1994).

The sphincters of Rossi, Balli, and Payr-Strauss are involved in nerve reflexes; the sphincters of Hirsch, Moultier, and Busi are a thickening of longitudinal and circular muscle fibers. Cannon's sphincter is an overlap of the superior and inferior mesenteric nerve plexuses.

Persistent spasm of each of these sfincters can produce tumor-like lesions.

Each of these sphincters can cause persistent spasms, mimicking tumoral disease. To reduce those spasm, butylhyoscine (Buscopan) is used as discussed previously.

Besides the routine use of Buscopan, dual position imaging is also useful, as well as additional inflation in case of doubt (Fig. 8.25).

Spasm can be differentiated from tumoral pathology, based upon the smooth contours of the spasms, in contradiction with circumferential tumors. The presence of surrounding lymph nodes also points towards tumoral pathology.

Pitfalls Related to Normal Anatomy and Non-Tumoral Lesions

Ileocecal Valve

The ileocecal valve is located between the large and small bowel, and consists of two segments, the upper and lower lips. The ileocecal valve can appear as a thin slit-like structure, a large intraluminal mass, or is almost invisible. There are three different endo-scopic appearances: the labial type, with a slit-like appearance, the papillary type, with a dome shaped appearance, and lipomatous type, with deposits of fat within the lips.

Most visible valves are of the labial type (78%), 21% is of the papillary type and 3% is lipomatous.

Lipomatous and papillary ileocecal valves can mimick neoplasms, and should be differentiated from polyps on the ileocecal valves (Fig. 8.26).

Prolapsing ileocecal valves appear prominent, irrespective of the labial or papillary morphology, t/

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