Current Issues in Computed Tomography Colonography

Current Issues in Computed Tomography Colonography Lorna Woodbridge, FRCR, and Peter Wylie, MA, MRCS, FRCR Computed tomography colonography has evolved over the past 2 decades to become the primary alternative to optical colonoscopy for detection of colonic neoplasia. With good technique in performance and reporting, accuracy is comparable to optical colonoscopy for cancers and larger polyps. This article discusses the current components of a high-quality examination including contemporary methods of bowel preparation and distension. Also described is the main trial data that have validated the examination. Finally, the use of the technique for nonneoplastic colonic pathology is discussed, and future directions are described including magnetic resonance colonography and wireless capsule colonic imaging. Semin Ultrasound CT MRI 37:331-338 C 2016 Published by Elsevier Inc.

Introduction lthough first described in the early 1990s,1 Computed tomography colonography (CTC) rose to worldwide prominence in 2003 with the seminal article of Pickhardt in the New England Journal of Medicine.2 This suggested a test as accurate as optical colonoscopy (OC), with potential as a colorectal cancer–screening tool, and offering a safer, cheaper, and noninvasive whole colonic imaging alternative. CTC arrived as an imaging tool in the era of evidence-based medicine and has been extensively investigated by both gastroenterologists and radiologists alike. Further trials revealed variable performance results,3,4 but technique, equipment, and radiologist's interpretation have been refined and standardised over the intervening years, and the latest trials reveal again the high accuracy of CTC for detection of colonic polyps and neoplasia.5-7 This article firstly focuses on the underlying components necessary for high-quality CTC. Later, we discuss the accuracy of the test, the nature of the target lesion, extension to other colonic diseases, and future directions in colonic imaging.

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Technique Performing modern high-quality CTC involves a combination of bowel preparation, fecal tagging, air insufflation, and Department of Radiology, Royal Free Hospital, London, UK. Address reprint requests to Peter Wylie, MA, MRCS, FRCR, Department of Radiology, Royal Free Hospital, Pond St, London NW3 2QG, UK. E-mail: [email protected]

http://dx.doi.org/10.1053/j.sult.2016.02.005 0887-2171/& 2016 Published by Elsevier Inc.

scanning the entire large bowel in 2 patient positions. Although there are no standardized protocols, several local and international collaborations have produced guidelines to advise on the optimum strategies to obtain the best quality diagnostic images (International Collaboration for CTC,8 European Society Of Gastrointestinal And Abdominal Radiology,9 and American College of Radiology10) and these are outlined in the following paragraphs.

Bowel Preparation Bowel preparation typically involves both a low-residue diet for up to 3 days before the examination and laxative use to minimize the amount of fecal residue remaining in the colon. The presence of fecal material in the large bowel may not only mask small colonic lesions, but can mimic polyps and tumors and prevent the possibility of same-day OC should this be necessary. The aim of a low-residue diet helps to homogenize the contents of the bowel and aid the fecal tagging process, making it easier to distinguish between a true mucosal lesion and fecal residue. The duration of the low-residue diet varies among institutions and can be from 3 days to 24 hours before the procedure. The laxatives used can be either “dry” laxatives, for example, magnesium citrate or sodium phosphate, or “wet” laxatives such as polyethylene glycol.11 Traditionally, the “dry” preparations are preferred for CTC, as the “wet” laxatives leave significant watery residue in the bowel that may obscure pathology. Ultimately, the preparation used is often dependent on the local protocol and individual patient factors—these may 331

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332 even include such practical considerations as the ability to send purgative and tagging preparations through the postal service. National guidelines in the use of such agents may exist, such as cautions in the use of such materials in patients with impaired renal function or physiological reserve in the United Kingdom.12 Fecal tagging involves adding iodine or barium-based solutions to the bowel preparation regimen allowing ready differentiation of residual fecal material from mucosal soft tissue.13 The 2 most successful multicenter trials of CTC2,5 to date used fecal tagging. A recent European consensus statement9 by multinational members of the CTC working group concluded unanimously that fecal tagging was now mandatory for routine practice. Many centers now use a “minimal prep” or “low prep” regimen where no additional laxatives are used in addition to the low-residue diet and fecal tagging solution (Fig. 1). These tagged solutions themselves can exert a significant cathartic effect because of their hyperosmolar nature. A recent UK study14 showed no statistically significant difference between using laxatives and Gastrografin (Bracco Diagnostics Inc, Princeton NJ) in terms of nondiagnostic examinations. The solution was well tolerated with less diarrhea in the tagged-only regimen. The rate of false-positive lesions (410 mm) was twice as large in the nontagged group although this was not statistically significant in this study (n ¼ 528). The proposed advantages of “laxative-free regimens” include greater patient acceptability, particularly for elderly patients,

and reducing the side effect profile of traditional full purgative colonic cleansing. The potential disadvantage of not using laxatives is suboptimal bowel cleansing if same-day OC is required, although this has not been assessed as yet in any large-scale trial to the authors’ knowledge.

Distension Good distension of all colonic segments is essential for accurate CTC. Insufflation with carbon dioxide using automated devices can improve the degree of colonic distension when compared to room air.15,16 Carbon dioxide is readily absorbed across the colonic mucosa and expired via the lungs leading to quicker desufflation of the colonic distension after imaging. This has been shown to reduce patient discomfort16 compared to room air, reflecting previous trials using carbon dioxide in colonoscopy and barium enema.17,18 Most current guidelines recommend routine use of carbon dioxide insufflation by automated devices.8,9 Mandatory use of intravenous spasmolytics is not recommended in the United States.10 Anti-spasmodic use (eg. Hyoscine Butylbromide) mainly occurs outside the United States, and has been shown to improve both bowel distension19,20 and in 1 study to reduce patient discomfort.21 Polyp detection rates, however, were not improved with hysocine butylbromide (Buscopan) use in a 2003 study by Bruzzi et al.22 The patient must be scanned twice both supine and prone to optimize bowel distension and redistribute fluid and fecal contents to dependent areas of bowel to improve sensitivity. If the patient cannot lie prone, then a decubitus position can be an acceptable alternative. No current recommendations suggest performing CTC only in 1 position, even with the advent of fecal tagging. Dual positioning also has the benefit of improving colonic distension, for example, the rectum is often better distended in the prone position.23 Intravenous contrast has not been shown conclusively to increase detection rates of polyps or colonic neoplasms,24 and is not recommended in UK Bowel Cancer Screening program.25 However, it still has a role in symptomatic patients or in patients where extracolonic review is important as it allows the detection of important extracolonic findings and allows staging in patients where a colorectal malignancy is found. The use of intravenous contrast, as ever needs to be balanced between the benefit to the patient and the negative effects it can entail, such as a possible contrast reaction, induction of contrast nephropathy, increased radiation dose, and additional cost.

Interpretation Figure 1 Coronal reformat from a CTC examination using automated carbon dioxide insufflation, intravenous contrast, and fecal tagging using Gastrografin. No bowel purgative agent is used. Note the excellent colonic distension achieved (eg, the cecum identified by yellow arrow) and the lack of any nontagged facal residue. Such lowresidual volumes of retained fluid are typical of this minimal preparation regimen. (Color version of figure is available online.)

Polyps (Fig. 2) are appreciated as soft tissue densities contrasted against either very low-density gas or high-density fecal tagging solution making them readily identifiable. Polyps generally have a homogenous, soft tissue density with round or lobulated contours when compared with feces that are heterogeneous, may contain gas and can have angulated borders. Unlike feces, polyps should not significantly change position on the prone study whereas feces would move to the

CT colonography

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Figure 2 An axial image from a CTC study. There is a nontagged soft tissue polypoid lesion in the sigmoid colon (arrow) seen on both acquisitions in keeping with a pedunculated polyp, confirmed on subsequent flexible sigmoidoscopy and biopsy.

dependent mucosal surface. In practice, the biggest challenge in a well-prepared, tagged, and well-distended colon is appreciating smaller and sessile polyps from colonic folds, the ileocaecal valve, and hemorrhoidal tissue at the anorectal junction. With less well-prepared colons, fecal matter is often a source of both false-positive lesions and false-negative reports. Lack of colonic distension is a particular issue in areas of colonic diverticulosis (Fig. 3), and differentiating colonic spasm from short pathologic strictures (Fig. 4). Current guidelines recommend CTC reading via dedicated workstations offering both 2-dimensional (2D) and 3-dimensional (3D) interpretation.8-10 Although 2D vs 3D reader paradigms have been the focus of much research in the past, it is clear that a knowledge of both methods is needed for accurate interpretation. Poor quality studies with significant untagged fecal residues would require significant analysis via 2D reading, and equally 3D troubleshooting of known difficult areas such as diverticular segments or the ileocaecal valve on a 2D read is invaluable.26 When using both 2D and 3D reconstructions, the measurements of polyps should be

Figure 3 An axial image from a CTC study. There is a stricture of the mid sigmoid colon (arrows). Although multiple diverticula are present, such lesions present the reporting radiologist with significant difficulties in ruling out colonic neoplasia, particularly in poorly distending strictures.

performed using axial 2D images and avoid a narrow window as this is thought to be more reliable than using reconstructions.27 The largest direct trial of CTC performance against OC5 revealed no significant difference in either 2D or 3D primary reading methods using dedicated interpretation software. With the improvement in CT review software on modern high-speed picture archiving and communication system workstations and the routine use of fecal tagging, the many arguments between reading methods could be historical in the authors' opinion.

Performance and Accuracy CTC performance in published series has changed significantly in the past 15 years. Initial studies in the early 2000s reported poor polyp sensitivity; for example, series by Cotton et al3 and Rockey et al4 in 2004 and 2005 revealed per-patient polyp sensitivities for 410 mm lesions to be 55% and 58%, respectively. However, criticism was directed by commentators because of the use of older scanner technology, manual colonic

Figure 4 An “apple core” type stricture of the proximal sigmoid (arrows) is revealed on a CTC study in which subsequent biopsy revealed to be a moderately differentiated colonic adenocarcinoma (T3N1). Such large pathologies are readily identified on CTC and remain the main target lesion of the investigation in symptomatic individuals.

L. Woodbridge and P. Wylie

334 insufflation, and inexperienced readers. The initial New England Journal publication by Pickhardt in 2003 avoided these issues, using 3D reconstruction software and experienced readers in a large patient cohort (1233) revealing sensitivity to 410 mm polyps per patient of 94% and 89% in 6-9 mm polyps. In total, 2 recent landmark studies in the United States and the United Kingdom have significantly altered the evidence base for CTC performance. The first by Johnson et al5 in 2008 (the “ACRIN” trial) was a large (n ¼ 2531) multicenter trial in a screening population using tested experienced readers with modern large number multidetector-row CT scanners and fecal tagging. Overall, perpatient sensitivity for 410 mm polyps and cancers was 90% using OC as the reference standard. For patients with lesions larger than 6 mm, sensitivity decreased to 78%. These results comparing relatively favorably with the initial experience of Pickhardt but again revealed shortcomings for lesions less than 10 mm in diameter. The second trial performed in the United Kingdom was again multicenter, using experienced readers and modern CT equipment (the “SIGGAR” trial6,7). The patient cohort comprised patients referred to secondary care for investigation of symptoms of colorectal cancer. The trial was designed to avoid the potential inherent biases of within subject comparison of OC to CTC, such as choice of bowel preparation regimen, and the assumption that OC is the reference or “gold” standard. This technique of “segmental unblinding” was used most famously in the Pickhardt New England Journal article of 2003. The technique divides the colonic length into segments with initial colonoscopy of each segment before subsequent review and unblinding of the segment in light of the results of the previously performed CTC. Criticisms of this technique include the accuracy of agreement of the 2 methods in exact colonic position of a lesion. The “SIGGAR” trial attempted to overcome the potential bias of a reference standard by devising 2 parallel arms: OC vs CTC and Barium Enema vs CTC, with both arms randomized in a ratio of 2:1, respectively. In the CTC (n ¼ 533) vs OC (n ¼ 1047), arm detection rates for cancer and 410 mm polyps were the same at 11%. In the other arm, CTC (n ¼ 1277) detected significantly more (7.3% vs 5.6%, P ¼ 0.039) cancers and polyps than barium enema (n ¼ 2527). This “real life” symptomatic trial validated the use of CTC as the primary radiological test in patients with suspected colorectal cancer. In an average-risk screening population, Graser et al28 compared the accuracy of OC, CTC, and other screening tests. CTC compared favorably with OC detecting 96.7% of cancers compared to 100%, and for polyps 45 mm the sensitivity of CTC was 91.3% compared with 97.8%. Overall evidence suggests that with good technique and trained reporting, CTC can have comparable detection rate to OC for cancers and large polyps (Z10 mm) and a more variable performance for smaller (o10 mm) polyps. In the highest performing series, accuracy for smaller polyps is high (and comparable to OC), but criticism remains that results in tightly controlled clinical trails may not reflect “real life” performance. However, OC itself has shortcomings for small

polyp detection. A systematic review of 6 trials involving tandem OC (n ¼ 375) by van Rijn et al29 revealed adenoma miss rates of 2% for lesions Z10 mm and 13% for lesions 6-9 mm. Segmental unblinding in the Pickhardt NEJM article2 allowed CTC to act as the reference standard against OC. Analysis of this data set30 revealed a 12% miss rate for adenomas Z10 mm, which as we have seen are significant lesions, and all current recommendations point to immediate polypectomy. The 2 main controversial areas regarding polyps continue for CTC: the natural history, reporting, and management of small polyps; and the detection and significance of flat polyps.

Polyp Size Colorectal cancer screening relies on the detection of a small precancerous precursor lesion, the colonic polyp that can be safely and completely removed.31 This is based on the longstanding theory of the colonic adenoma to carcinoma sequence, with most colonic carcinomas deriving via small preexisting adenomas undergoing genetic mutation and usually accompanied by an increase in volume.31,32 The difficulty arises on what size of polyp is defined as a target lesion, both for screening and removal. Although this is pertinent for CTC in performance variation for small (o10 mm) polyps and decreased accuracy for diminutive (o6 mm) polyps, it also is highly relevant for OC in both sensitivity and need for polypectomy. Colonic adenomas are common31 and most arise sporadically, increasing in number with age. The target lesion has been defined as the “advanced adenoma”33,34 and this has been defined as either a larger polyp (410 mm diameter) or having either villous or dysplastic components. Without mucosal detail provided by histology or potentially by high-quality in vivo colonoscopy using new techniques of dye spraying and narrow band imaging, CTC therefore has to decide on a size cutoff for both reporting and referral for removal. Polyps above the 10mm size threshold have a significant risk of developing into invasive malignancy- for example 24% over the next 20 years.35 There are no existing gastroenterological, pathologic, or radiological guidelines that these should remain in situ. Fortunately, as we have seen earlier, in modern large series, the sensitivity of CTC for these lesions is 490%. At the other end of the size range, diminutive polyps (o6 mm) have a low risk of representing advanced adenomas. Rex et al33 assessed the rate of cancer and advanced adenoma in o6 mm polyps in 410,000 patients as 0.05% and 0.87%, respectively. Current guidelines suggest that routine reporting of diminutive (o6 mm) polyps is not necessary—both in the United States10 and in Europe, except when confidence is high or with multiple lesions.9 The greatest controversy, therefore, exists for 6-9 mm polyps. They represent a small but significant proportion of lesions detected at screening colonoscopy but have a low rate of sinister histology.36,37 However, the natural history of such

CT colonography small polyps is not fully understood, and several attempts have been made to quantify this issue. Recently, Pickhardt et al38 performed a longitudinal study of 306 polyps of 6-9 mm in 243 patients detected by CTC. A further repeat CTC was performed after a 2-3 years interval. A 20% change in polyp volume was used to categorize stability, regression, or progression; 50% were stable, 28% regressed, and 22% progressed. An absolute polyp volume of 4180 mm3 was shown to predict advanced adenoma (or in 1 case, cancer) status. Hofstad et al39 performed yearly OC on 253 subcentimeter polyps over a 3-year period. No diminutive polyp grew to become a small (45 mm) polyp and only 0.5% of small (6-9 mm) polyps grew to 410 mm diameter. Although the full natural history of the polyp carcinoma sequence is unclear, some guidance is available. There remains a concern regarding some subtype histology (serrated adenoma) irrespective of size and flat polyp morphology that would be covered in the next section. However, most commentators now agree that routine reporting of sub-6 mm polyps is unnecessary and removal of such lesions is associated with the known risks of colonoscopic polypectomy including perforation, bleeding, and very rarely, death. All current guidelines support the routine reporting of 45 mm polyps, but the question of whether to immediately remove small polyps remains unclear, and CTC may provide a noninvasive accurate means of in vivo monitoring of small, usually indolent, lesions.

335 separates such flat, nonpolypoidal lesions into elevated, truly flat, and depressed subcategories. The CRADS radiology working group45 propose general distinction of such lesions from other sessile polyps by a height limit of 3 mm. Both the prevalence and natural history of flat lesions is unclear. Data traditionally suggested a higher incidence in Japan and lower in Europe and in the United States but this may have been the result of nonstandardized terminology and difference in endoscopic technique.46-48 A recent screening cohort of 5107 patients in United States49 underwent CTC revealing an incidence of flat (o3 mm height) polyps of 13.1% (125 of 954 45 mm diameter lesions). In terms of CTC performance, using the definition of flat polyps earlier (height being less than 50% of diameter), Pickhardt revealed sensitivity for detection of flat polyps of 80%. In this series of 1233 screening patients, the sensitivity to polypoid lesions was almost identical at 81%.50 More recently, Kim et al51 showed that contrast coating by tagged fluid in CTC, a very useful sign enabling detection, was associated with larger lesions, serrated histology, and right-sided colonic location. In this study, almost 80% of 160 flat lesions had contrast coating on their surface, greatly aiding detection. Traditionally, Computer-Aided Detection software systems have targeted polyps based on their projection into the colonic lumen. However, development of the mathematical algorithm that underpin such technology may be beneficial in detecting minimally elevated flat lesions as small surface irregularities against a smooth background mucosa.52

Flat Polyps The target precursor lesion of CTC, as discussed previously, has traditionally been the adenomatous polyp which is the lesion associated with the well-established adenoma-carcinoma sequence.32 There is, however, a further group of polyps with malignant potential, the serrated polyp or serrated adenoma,40 so-called because of their toothlike appearance microscopically; these comprise a small subset of hyperplastic polyps, all other hyperplastic polyps thought to have no malignant potential. These serrated lesions undergo genetic alterations involving oncogenes such as DNA mismatch repair, which can lead to microsatellite unstable carcinoma, and these most frequently occur in the proximal colon41 These serrated lesions are divided histologically into sessile serrated adenomas (SSA) and traditional SSA. The SSA lesions are flat against the mucosal surface, located typically in the cecum and ascending colon, and can show submucosal spread.42,43 Thus, they are more difficult to find endoscopically and radiologically, and if they develop into carcinoma they are anatomically predisposed to deeper colonic wall invasion than polypoid luminal polyps. A truly flat colonic mucosal lesion with no elevation or depression from background mucosa would be impossible to visualize in CTC unless it has very different enhancement from surrounding mucosa. However, very few colonic polyps show a completely flat morphology with most having at least some minimally elevated or depressed component. The current most widely used endoscopic terminology is the Paris classification 44 that defines a flat lesion as having height less than the width of closed biopsy forceps (2.5 mm). The Paris classification

CTC Use in Nonneoplastic Conditions To date, most CTC investigative literature regard its use in the diagnosis of polyps and neoplasia. However, it clearly depicts colonic diverticulosis and may also have a limited role in inflammatory bowel disease. Diverticulosis is the most common pathology to affect the colon in the Western world and has a strong correlation with patient's age, with a prevalence of 23%-36% in 30-40–year olds rising to 67.1% in 70-year olds.53 Although many remain asymptomatic, patients can develop long term morbidity from diverticular disease and complications from active flares of diverticulitis. A prior knowledge of background diverticulosis may help reduce the mortality and morbidity associated with this by providing a more timely diagnosis.53,54 Diverticulosis has previously been believed to be a limitation to a good quality CTC study because of suboptimal bowel distension (Fig. 3). However, there is evidence to show that diverticulosis does not impair diagnostic quality55 and does not affect bowel preparation.56 Although the presence of myochosis, which is associated with diverticulosis, can affect sigmoid colon distension, the presence of diverticula alone does not.57 The role of CTC in diverticular disease includes diagnosis, examining the extent of disease, assessing for any complications, and differing between diverticulitis and malignancy. However, CTC is limited in its role of imaging acute

336 diverticulitis because of the increased risk of perforation or complicating already locally perforated segments. OC is currently the reference standard for diagnosing diverticular disease (diverticula being readily differentiated from mucosal disease on luminal views) following an acute episode. However, the presence of diverticular disease is one of the most common causes for a failed colonoscopy,56 and therefore CTC can offer an alternative. CTC has a similar accuracy to OC at diagnosing the presence of diverticula, is better tolerated by the patient, and has fewer failed examinations.58 Imaging extraluminal structures also allows the diagnosis of any complications that may have arisen such as fistulas, abscesses, or strictures. Although not routinely used at present, diverticular disease scoring systems can be used with CTC to help guide surgeons in the management of patients after acute diverticulitis. Flor et al59 devised a diverticular disease severity score based on wall thickness and lumen diameter of the sigmoid colon, which was found to be highly reproducible among different readers and correlates strongly with the probability of subsequent surgery. Traditionally, CT has struggled to differentiate between areas of diverticulitis and colonic malignancy because of the considerable overlap in imaging findings, with both pathologies commonly showing areas of wall thickening and surrounding inflammatory fat stranding. Being able to confidently distinguish between the 2 pathologies using CTC would help reduce the number of unnecessary optical colonoscopies and operations. Lips et al60 have recently shown several features that help distinguish carcinoma from chronic diverticulitis, the most reliable being the combination of lack of diverticula in the affected segment with the presence of shouldering that together provide a high diagnostic certainty of 93%. Although the role of CTC in imaging of inflammatory bowel disease is limited by the risk of perforation in the acute setting and the inability to detect mild, mucosal-only disease, CTC still has a role particularly in those patients where OC has either failed or is contraindicated. Findings in CTC have been shown to be comparable with those from OC in patients with chronic inflammatory bowel disease.61 Although patients require OC to obtain histologic diagnosis, CT has the advantage of diagnosing extraluminal disease such as the extent of wall thickening, fibrofatty proliferation, mesenteric adenopathy, abscess formation, and stricture formation that cannot be assessed with OC alone.62 If CTC is performed rather than conventional CT, the luminal distension allows assessment of the luminal to mucosal interface62 allowing assessment of mucosal abnormalities such as pseudopolyps and the presence of any associated adenocarcinoma or lymphoma.

Future Directions Although CTC is likely to be the prime imaging method for large-bowel disease for some time, there are 2 other colonic imaging methods that may eventually supersede this examination, just as CTC has replaced barium enema. The first is magnetic resonance colonography (MRC) that was originally described in the late 1990s.63 The other is colonic wireless

L. Woodbridge and P. Wylie capsule, an extension of technology used in small-bowel imaging.

Magnetic Resonance Colonography The major attraction of this method is the lack of ionizing radiation that remains a concern in the use of CTC for screening populations and repeated examinations.64 This method, however, uses similar techniques as CTC with purgative bowel preparation, colonic distension, and some commentators recommend dual patient positioning.65 The key challenges for the technique are cost, scanning time, contraindications to magnetic fields (such as cardiac pacemakers), and a greater volume of imaging artifact. The 2 main approaches to the technique have been described. The first is “Dark Lumen” MRC in which a negative contrast agent such as carbon dioxide, room air, or water is used as a distension agent. Intravenous gadolinium contrast is used by investigators to help differentiate enhancing polyps and neoplasia from stool. The second is “Bright Lumen” MRC in which a gadolinium chelate enema is administered and a dual patient position technique is employed. Intravenous gadolinium is not used as the enhancing neoplasia, or colonic mucosa is not differentiated from the “bright” gadolinium-labeled luminal fluid. There are only a small number of studies comparing polyp and cancer detection rates in comparison with CTC. Most recent studies have focused on the “Dark Lumen” technique. Hartmann et al66 published a series of 107 polyps in 72 patients with overall polyp sensitivity of 90%. However, although 100% (22 of 22) of polyps greater than 10 mm were detected, only 78% of 6-10 mm and 9% of smaller than 6 mm polyps were detected. Ajaj et al67 published a series of 122 patients in 2003 with a total number of 50 polyps. Again, MRC had poor sensitivity to smaller polyps: none of the 30 sub-6 mm polyps were detected and 18 of 20 polyps greater than 5 mm in diameter were detected. Interestingly, MRC may have at present a similar timeline to CTC in the 1990s with variable published performance, particularly with diminutive polyps, and little consensus in technique. Improvements in scanning time, receiver coil technology, and refinement of pulse sequences, together with application beyond the research setting, may help to propel MRC to a position as a realistic alternative to CTC, particularly in the screening population.

Colonic Wireless Capsule Wireless capsule endoscopy is the current gold standard imaging technique for visualizing small intestinal mucosal pathology. It is now widely used in the clinical setting, particularly for the detection of early or occult inflammatory bowel disease. Extrapolation of this technology to the colon is now the subject of research and early clinical trials. The potential advantages are lack of radiation exposure, no requirement for colonic distension, and use in the outpatient setting with no OC-related day care procedural cost, intervention, or

CT colonography sedation. The patient requires bowel preparation to allow sufficient colonic mucosal visualization. Criticisms of this technique include the need for greater cleansing bowel preparation regimens than those used for tagged CTC because of optical mucosal visualization. No large-scale trials have been performed to date. A further concern is of the theoretical risk of areas of colonic mucosal nonvisualization because of the “tumbling” of the capsule that is smaller in diameter than the luminal diameter of the colon. This has resulted in innovations such as camera optics attached to both ends of the colonic capsule and the use of accelerometers in which the speed of the capsule through the colon can adjust the frame rate of the camera video output. Feasibility of such a novel technology has been shown by preliminary trials, but full-scale multicenter large-volume comparative trials are required as per the development of CTC.68-70

Summary CTC is the most accurate noninvasive colonic imaging method currently available and is unlikely to be superseded by either MRC or colonic wireless capsule for a considerable time. With meticulous technique and trained interpretation, it has high accuracy in the detection of colonic polyps and neoplasia. As the natural history of the colonic polyps and the processes leading to all types of carcinoma evolve, CTC is well placed to offer a safe and reliable alternative to OC. It may offer an important role in colonic cancer screening either as a primary triage investigation or for completing limited endoscopic examinations. Many of the early challenges of variable performance and lack of published standards have been overcome, and its role is now extending to other nonneoplastic colonic diseases.

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