The Quick Easy Noninvasive Alternative to Colonoscopies Fred Pescatore Md

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Computed tomography colonography (virtual colonoscopy): Review

Richard M Mendelson, Geoffrey M Forbes

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10.1046/j.1440-1673.2001.00988.x

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Review Article  Australasian Radiology (2002) 46, 1–12  Computed tomography colonography (virtual colonoscopy): Review Richard M Mendelson and Geoffrey M Forbes Royal Perth Hospital, Perth, Western Australia, Australia  SUMMARY Computed tomography examination of the colon performed after bowel cleansing and distension of the lumen with gas goes by several different names—CT colonography (CTC) and CT colography perhaps being the most common. Strictly, the term 'virtual colonoscopy' (VC), should be reserved for the process of examining 3-D, simulated endoluminal images with a capability to navigate through the bowel using appropriate software. Computed tomography colonography appears to be the name that has gained favour among radiologists, although it is suspected that 'virtual colonoscopy' will persist as a generic term due to its attractive 'high-tech' connotations for non-radiological medical and lay persons. Whatever the name, the technique has been made possible through the advent of fast helical CT scanners which allow acquisition of a volume of data, and of proprietary software which enables multiplanar reformatting and 3-D endoluminal reconstructions. It is evident that if CTC/VC can be shown to be acceptable to patients, safe, affordable and accurate, it has enormous potential as a diagnostic and screening tool for colorectal neoplasia. Key words: CT colonography; virtual colonoscopy.  INTRODUCTION  METHOD  In 1994 at the Annual Meeting of the Society of Gastrointestinal  The examination may be considered in four stages: patient prep-  Radiologists, David Vining presented a video recording of a  aration, image acquisition, image processing and image reading.  'fly-through' examination of a patient's colon, constructed from images acquired at spiral CT.1 These virtual reality images were  Patient preparation  accompanied by a sound track of the 'Ride of the Valkyries' and  At present, bowel cleansing is required using regimes similar  much 'oohing and aahing' from the audience. One of us (RMM)  to th; ose used for double contrast barium enema (DCBE) or  had the misfortune of having to follow this immediately with  colonoscopy. Our own experience and that of others3 is that a  a presentation on a considerably more mundane subject.  low-volume barium enema preparation has an advantage over  The first clinical publication (by Hara et al.2) relating to virtual  a high-volume colonoscopy preparation as fluid residue at CT  colonoscopy appeared in 1996. Since then there have been  colonography/virtual colonoscopy (CTC/VC) can be a major  many publications, often referring to the 'promise' of this new  problem for polyp detection.  technique. Has the promise become an actuality or does it remain 'virtual'?  The large bowel is insufflated with gas—either room air or CO2—via a rectal catheter, to the level of patient tolerance,  RM Mendelson MRCP FRCR FRANZCR; GM Forbes MD FRACP. Correspondence: Dr RM Mendelson, Department of Diagnostic & Interventional Radiology, Royal Perth Hospital, Box X2213 GPO, Perth, WA 6847, Australia. Email: richard.mendelson@health.wa.gov.au Submitted 15 August 2001; accepted 12 October 2001.  2  RM MENDELSON AND GM FORBES  usually 2–4 L. In our institution we use CO2 as this may cause  rendering, which is associated with semi-transparency of the  less post-procedural discomfort for the patient (as suggested  colon wall, is probably more accurate in detecting small lesions,  4  from barium enema literature ).  the data set is considerably larger than that for surface-shading  Prior to insufflation, an intravenous injection of hyoscine  and the examination takes longer. Hybrid rendering techniques  butylbromide ('Buscopan') is given, unless contraindicated.  are under development. The optimal method of examining  Glucagon can be substituted but there is little evidence that  images has not been fully clarified. The options range from:  5  this is beneficial; indeed, by relaxing the ileo-caecal valve,  (a) examination of 2-D axial images only (scrolling through the  Glucagon may be counterproductive in enabling retrograde  images, examining serially the loops of large bowel from rectum  passage of gas into the small bowel.  to caecum); (b) examination of axial, coronal and sagittal  For a single array scanner in order to allow acquisition of  refomats; (c) examination of 2-D axial, coronal and sagittal  images in a single breath-hold, the patient is pre-oxygenated by  images and routine examination of 3-D endoluminal reconstruc-  face-mask. No sedation is used.  tions; (d) as for (b), but selective use of endoluminal views for problem solving; and (e) examination of endoluminal VC images  Image acquisition  only. Routine complete examination of endoluminal recon-  The degree of bowel distension is assessed on an initial CT  structions is very time-consuming, even with semi-automated  scout view and more gas introduced if necessary. Scans are  'fly-through' techniques. We examine axial images and se-  acquired in both supine and prone positions. Typical scanning  lectively use multiplanar reformats and 3-D reconstructions for  parameters for a single array helical scanner are: 120 Kvp,  problem-solving. This approach has also been advocated  70 mAs, 5 mm collimation; pitch 1.5:1 or 3 mm collimation,  by others.7,8 It is our experience that nearly all lesions larger  pitch 2.0:1.  than 10 mm detected at CT colonography are visible on axial  While some authors advocate standard X-ray dosage scans  images.  in order to detect extra-colonic pathology more accurately, most  Window settings on 2-D images of 1500 (window width) and  use the low-dosage regimes such as that described as the  –200 to –700 (window level) are suitable for polyp detection.  ability to detect colorectal polyps is preserved even with tube  Soft-tissue window settings are reviewed to detect bowel wall  current of 70 mAs. Radiation dosage, of course, is particularly  thickening (Fig. 2), for clarification of intraluminal filling defects  important if the examination is being performed for screening  (heterogeneity) and, routinely, for extra-colonic pathology.  6  purposes in an asymptomatic individual. Even with 70 mAs, it is possible to detect the majority of incidental extra-colonic lesions of importance, such as aortic aneurysms, renal and ovarian tumours. However, if any of these lesions are detected, further imaging will usually be required for clarification. The images are acquired in one breath-hold whenever possible, facilitated by pre-oxygenation of the patient. For a singlearray spiral CT scanner, the breath-hold is about 30–45 s.  Image reconstruction Image reconstruction is at 1–2 mm intervals, giving a total of about 200–400 images in each of the supine and prone scans in an average-sized individual. The practice in our institution is to reconstruct the supine scans at 1 mm and the prone at 2 mm— a compromise to reduce the size of the data set.The images are then transferred to a work-station for reading.  Image reading Various proprietary and third-party software packages are available for examination of the images. The software enables multiplanar reconstructions from the acquired volume data set, as well as 3-D endoluminal reconstructions ('virtual colonoscopy') (Fig. 1). The latter images are formed by surface-shading or volume-rendering techniques and are made possible by perspective projection and real-time rendering. While volume-  Fig. 1. Screen display from 'Navigator' software (GE, Milwaukee, WI, USA) showing (clockwise from top right) axial source images, coronal and sagittal reformats and endoluminal 3-D reconstructions ('virtual colonoscopy').  CT COLONOGRAPHY: A REVIEW  3  Some studies quote the sensitivity of the test on a per patient basis, that is the ability to identify patients with at least one polyp ≥ 10 mm. For dual position scanning, sensitivities for polyps ≥ 10 mm vary from 85–96%, with positive and negative predictive values of 81–96% and 85–97%, respectively.14,16,17 Sensitivity for detection of carcinomas has been the subject of relatively little reporting, and in most published series the numbers of such lesions have been too small for meaningful analysis. The administration of an intravenous contrast medium has been advocated by some authors in order to increase accuracy, as polyps should enhance, whereas faecal material does not. Fig. 2. Axial image, soft tissue windows showing concentric thickening of sigmoid colon due to carcinoma (arrowed).  Morrin et al.19 reported improved reader confidence in assessment of bowel wall conspicuity and increased depiction of medium-sized and large polyps. However, intravenous contrast material increases the costs and risks of the procedure without  PERFORMANCE CHARACTERISTICS OF CT COLONOGRAPHY  any major gain as yet confirmed.  There is a significant 'learning curve' for reading CTC/VC.9  Specificity  The number of examinations required to reach proficiency is  False positive results of CTC/VC remain a potential problem,  unknown but the authors' personal experience suggests that  particularly due to the misinterpretation of faecal residue.  this is likely to be up to 50 examinations.  Specificities, reported on a per patient basis range from 74–96%,11,12,14,16 figures at the higher end of this range occurring  Sensitivity  in studies using dual positioning. The use of 3-D imaging for  Sensitivity on a polyp-by-polyp basis is dependent upon  problem-solving increases specificity.  polyp size. Analysis of the literature presents some difficulties;  Specificity data have been obtained in studies where  several studies report the use of supine only10,11 or prone only12  accuracy of CTC/VC for polyp detection is compared with the  scans, whereas a combination of supine and prone scans  'gold-standard' of colonoscopy. In interpreting such data it is  ('dual position') has been shown to be beneficial in increasing  important to take into account the 'miss rate' of conventional  sensitivity and specificity.13,14 In several studies, the group of  colonoscopy, which may be as high as 6% for polyps ≥ 10 mm,  patients has been preselected to yield a high proportion of  even by expert colonoscopists.20 Some apparent 'false-positive'  positive results;10,11,15,16 some include the operators' learning  CTC/VC examinations may therefore be due to false-negative  curve9,11,15 and some use standard rather than low mA  colonoscopies.  regimes.9,11,12 The published studies that report results of dual position scans suggest a sensitivity for polyp detection (on a polyp-by-  PITFALLS AND SOURCES OF ERROR  polyp basis) of 73–91% for polyps ≥ 10 mm and 22–82% for  Reduced accuracy may be expected to be related to one or  intermediate-sized polyps.14,15,16,17 Sensitivity for polyps ≤ 5 mm  more of the following variables.  is poor in most series. The best results have been published from the Boston group16 which reported sensitivities of 91, 82  Breath-hold misregistration  and 55% for polyps of diameter 10 mm or more, 6–9 mm, and  Artefacts may occur due to mismatching of the data set at  5 mm or less, respectively. The three cancers in this series were  the site of adjacent breath-hold acquisitions (Fig. 3). This can be  detected.  overcome by the use of a single breath-hold acquisition, achieved  Attempts have been made to improve sensitivity of polyp  by pre-oxygenating the patient for a few minutes prior to the scan.  detection by administering an oral contrast agent with the  Scan time, on a single-array helical scanner, for an average-sized  bowel preparation. The rationale for this approach is that the  individual is about 30–45 s. Most patients can breath-hold for this  attenuation of any retained colonic fluid residue is increased,  time after oxygenation. Scans are obtained in a cephalo-caudal  thus increasing the difference between soft-tissue polyps and  direction, and instructions are given to the patient to slowly  surrounding fluid. Results have been mixed. Fletcher et al.14  exhale if necessary.This increases the likelihood that any images  found no advantage for this technique, whereas Vining18  acquired during respiration are from the pelvic colon where  reported increased sensitivity.The technique has not yet gained  respiratory movement causes a lesser problem. The problem  wide acceptance.  is substantially reduced by the use of a multi-array scanner.  4  RM MENDELSON AND GM FORBES  Faecal residue  faecal-tagging techniques and subsequent digital subtraction of  Faecal residue due to inadequate bowel preparation is a  faeces from the image is under investigation.  prominent source of error—usually false-positives for polypoid lesions. Faeces can often be distinguished from polyps when  Fluid residue  the filling defect within the lumen moves to the dependent  As previously stated, this may be a problem with high-volume  wall of the bowel in both supine and prone projections (Fig. 4);  lavage bowel preparation, but this can occur with any prepara-  when there is gas contained within it or heterogeneity of  tion. Subsequent fluid lakes can hide even large lesions, but the  attenuation (Fig. 4); occasionally by an angular configuration or  problem is largely obviated by the use of supine and prone  by high density compared to soft tissue. As previously stated,  scans (Fig. 5).  some authors advocate the administration of intravenous contrast medium to identify contrast-enhancement of polyps19  Poor distension  but most centres do not routinely undertake this. The use of  The degree of distension is dictated by patient tolerance, and this is increased by the use of spasmolytics such as hyoscine butylbromide. However, particularly in the presence of anal sphincter incontinence, ileo-caecal valve incompetence and diverticular disease, distension may be suboptimal, just as may  Fig. 3. Breath-hold misregistration artefact (arrowed) shown on sagittal (below) and endoluminal (above) reformats.  Fig. 4. (a) Prone axial image (photographed inverted [i.e as if patient supine]) showing two small filling defects on dependent (anterior) wall. (b) Supine axial image at soft tissue windows again shows filling defects on dependent surface, and of heterogeneous attenuation, containing gas, consistent with faeces.  CT COLONOGRAPHY: A REVIEW  Fig. 5. (a) Supine image showing fluid in sigmoid loop obscuring much of mucosal surface. (b) Prone image, photographed inverted, at same level shows fluid to have moved away, with resulting good views of colonic surface.  be observed at DCBE. Supine and prone scans may be complementary in rendering dependent collapsed loops on one scan position to be non-dependent and distended on the other (Fig. 6). Carbon dioxide as the distending agent is better tolerated than room air, but this would appear to be due to the reduced post-procedure discomfort experienced with CO24.  SAFETY OF CT COLONOGRAPHY Risks associated with CTC/VC may be considered as those of the bowel preparation (which can be assumed to be the same  Fig. 6. (a) Supine axial image through rectum showing poor distension but suspicion of a mass lesion. (b) Prone image (photographed inverted) showing obvious rectal carcinoma (arrowed). Curved arrow = rectal catheter. (c) 3-D endoluminal view of annular constricting carcinoma.  5  6  RM MENDELSON AND GM FORBES  as for DCBE or colonoscopy preparation), risks of drugs used (such as hyoscine butylbromide and which can be largely avoided by identifying patients at risk), the risk of perforation by the rectal tube or by insufflation, and radiation dosage. We are unaware of any patients who have suffered a bowel perforation as a result of CTC/VC. We use a soft Foley catheter, and although the balloon is distended, we are careful to avoid forming a seal by the tube in the rectum. Nevertheless, in one of our patients some mucosal 'splitting' was identified at colonoscopy immediately following CTC/VC. Although this patient came to no harm, the potential for bowel perforation, albeit small, must be recognized. With regard to radiation dosage, using the 'low-dose' technique described, effective whole body doses of about 5 mSv result with our scanner (GE CTi; GE Medical Systems. Milwaukee, Wisconsin)15 which compares favourably with the dosage that could be expected for an average conventional DCBE. A dosage of 5 mSv gives a lifetime risk of fatal cancer induction of about 1 in 4000–5000.  PATIENT ACCEPTANCE There have been few formal studies of patient acceptance of CTC/VC. In our own series of predominantly symptomatic patients21 who underwent both CTC/VC and colonoscopy, CTC/ VC was highly acceptable, although there was significantly more discomfort from unsedated VC than sedated colonoscopy. However, the median score for pain/discomfort was only 10 on a scale of 0 to 100 (where a score of 0 represented no pain and 100 represented severe pain). No significant differences between CTC/VC and colonoscopy were observed for other parameters—toleration, satisfaction with the procedure, embarrassment. In total CTC/VC was preferred by 27%, colonoscopy was preferred by 23% and in 49% there was no preference. Of those stating a preference for colonoscopy, the majority gave  Fig. 7. (a) Pedunculated polyp in ascending colon (arrowed), approximately 15 mm in diameter, on axial image. (b) Endoluminal 3-D image. Straight arrow = head of polyp; curved arrow = stalk.  sedation as their reason, whereas the commonest reason for preferring CTC/VC was that no sedation was involved! The vast majority would have either examination repeated if necessary.  been performed. It is perhaps unlikely that a true comparative  In an ongoing feasibility study at our institution of CTC/VC  study can be undertaken due to the cumulative radiation  for colorectal cancer screening of asymptomatic average risk  dosage that this would involve. CTC/VC is likely to be better  individuals, the degree of acceptance of the technique was very  tolerated than DCBE, as it is quicker for the patient and requires  good, with the vast majority of subjects willing to repeat the  less change in patient position during the procedure.  examination in the future if required.22  It should be stressed that CTC/VC is not a suitable technique for the detection of mucosal disease such as inflam-  ROLES OF CT COLONOGRAPHY Diagnosis/exclusion of colorectal neoplasia in symptomatic patients  matory bowel disease and angiodysplasia. However, diverticula  Computed tomography colonography may, in future, be con-  colonoscopy. So how are symptomatic patients to be selected  sidered as a replacement for DCBE in symptomatic patients  to undergo either CTC/VC or colonoscopy? Probably in a  suspected of having colorectal cancer (CRC; Figs 7–9). There  similar way that current selection occurs for patients to either  is accumulating evidence that the technique is as accurate as  undergo colonoscopy or DCBE. Often the decision is based on  DCBE for this indication. This is based largely on historical data  availability (especially for GPs who may or may not be able  for DCBE as, to our knowledge, no comparative studies have  to avail themselves of open-access colonoscopy), cost, local  are easily detected (Fig. 10). In certain individuals, CTC/VC may also replace diagnostic  CT COLONOGRAPHY: A REVIEW  7  Fig. 8. (a) Small (approximately 5 mm) polyp in ascending colon (arrowed), seen on axial image; (b) 3-D endoluminal images and (c) colonoscopy.  expertise and local referral patterns. In practice the determina-  Incomplete colonoscopy  tion is also subject to the degree of clinical suspicion for CRC  Most radiologists are familiar with the occasional call from a  and/or the age of the patient23,24—those patients with a higher  gastroenterology colleague who has been unable to complete  pre-test probability of CRC being preferentially referred for  a colonoscopy to the caecum, requesting an immediate DCBE  colonoscopy and those patients with a lower pre-test probability  to avoid the need for a repeat bowel preparation. Most radiol-  being often currently referred for DCBE. It is for these latter  ogists are familiar with the problems that can ensue (Fig 11).  patients that CTC/VC is likely to replace DCBE, with the added  The bowel is often distended with air, making it difficult to  bonus that CTC/VC may be able to identify some extra-colonic  opacify the right side of the colon. The advantage of CTC/VC in  pathologies. It is also possible that in such patients the use of  this situation is that this air is an aid rather than a hindrance.  intravenous contrast material may be beneficial, although this  Patients tolerate CTC/VC better than DCBE in this clinical  needs to be determined. In addition, in those patients in whom  context and the examination is complete in the vast majority of  colonoscopy is likely to prove difficult or undesirable (either  cases.25,26  on the basis of a prior difficult examination or due to contra-  A further indication for CTC/VC is in the patient with a  indications to colonoscopy or sedation), CTC/VC can provide  stricturing lesion, such as a carcinoma, which the endoscopist  an alternative to diagnostic colonoscopy.  cannot traverse. As, in most cases, gas can be passed proximal  8  RM MENDELSON AND GM FORBES to such a lesion, CTC/VC can be utilised to exclude significant neoplastic lesions proximal to the known lesion.27  Colorectal cancer screening As most authorities would agree that above-average risk subjects should ideally undergo colonoscopy for screening,28 this discussion will be limited to asymptomatic individuals with an average risk of CRC (Figs 12,13). Colorectal cancer is a disease that ideally lends itself to screening: it is common; mortality is high if it is detected late but the prognosis is excellent if early disease is treated; and there is a premalignant phase (the adenoma) which has a relatively long dwell time during which it can be detected and treated relatively safely.  Fig. 9. (a) Axial and (b) 3-D endoluminal views of malignant lesion in transverse colon (arrowed).  Fig. 10. Diverticulum (arrowed) on endoluminal view of sigmoid colon. Axial image is inset bottom right corner of image.  Fig. 11. Patient with iron deficiency anaemia. Colonoscopy was incomplete. (a) Axial and (b) endoluminal 3-D images show polypoid cancer in caecum (arrowed; confirmed surgically).  CT COLONOGRAPHY: A REVIEW  9  Fig. 12. Screening examination in an asymptomatic patient shows (a) an approximately 2 cm lobulated lesion in the right colon on axial image (arrowed), confirmed on (b) endoluminal 3-D image and (c) on colonoscopy. Biopsies showed a dysplastic adenoma.  Unfortunately, the tools at our disposal for screening  DCBE and colonoscopy have drawbacks that make it unlikely  are imperfect. Faecal occult blood testing (FOBT) is most  they will gain widespread acceptance as tools for mass  widely advocated as the only test to have been shown to reduce  screening. Double contrast barium enema has its advocates—  mortality from CRC (by 15–33%) when applied to mass popula-  principally in the USA—but there have been no formal studies  29  tion screening. It is also cheap, safe and can be administered  of DCBE in this context. Setting aside considerations of com-  by the general practitioner. Accuracy depends on the type of  pliance, DCBE is probably not of adequate accuracy without  FOBT used and the frequency of testing, but on an individual  concomitant FS33 (which would increase costs and almost  basis FOBT misses between 21 and 63% of cancers and most  certainly decrease compliance) and there is a significant radia-  adenomas, and has a false positive rate of 2–13%.30 Flexible  tion dose. Colonoscopy has been vigorously supported by  sigmoidoscopy (FS) is under trial in Australia31 and overseas as  some authorities, but a certain level of competence is required  a tool for population screening. The rationale for FS is that most  to achieve adequate rates of caecal intubation and, although  neoplasms occur within reach of FS and that distal adenomas  it is difficult to determine the exact figure, the completion rate  may be predictors of proximal lesions. However, 25% of sub-  outside specialist centres may be 80–90%34,35 or less. Taking  jects have only proximal adenomas (proximal to the splenic  into account the need for sedation, consequent bed fees  flexure) and are therefore beyond reach of FS.32  and cost of time off work, colonoscopy is relatively expensive.  These factors have prompted calls for 'whole colon' screening. Such methods include DCBE, colonoscopy and  In addition, there are insufficient colonoscopists to support a population screening programme.  CTC/VC. While accuracy is important, other important issues  For these reasons VC has been advocated as a potential  must be considered with regard to a screening test, such  contender for a screening test. Currently it is more accurate  as acceptability, compliance, availability, safety and cost. Both  than FOBT and can compete with FS with regard to accuracy  10  RM MENDELSON AND GM FORBES fore having no malignant potential. Although most cancers develop from adenomatous polyps, the 'dwell time' for a polyp to turn to cancer is estimated at about 10 years for a polyp < 1 cm in size.36 If screening takes place, say, every 5–10 years the dwell time for such a small lesion allows an enlarging lesion to be picked up on subsequent examinations. There is also evidence that some small polyps may undergo spontaneous regression.37  DETECTION OF EXTRA-COLONIC PATHOLOGY There is the potential to detect non-colonic disease at CTC, which may be incidental or related to symptoms. While this may be an advantage if the abnormality explains the patient's symptoms or is an incidental finding (such as an aortic aneurysm) which has significant potential benefit for the patient, there are often considerable costs to pay for detection of incidental lesions, both in terms of further investigation and of morbidity incurred by that investigation or treatment of the abnormality.This 'double-edged sword' needs to be factored into the costs of any screening programme using CTC/VC for CRC detection. Initial reports of incidental pathology detected during CTC suggest that the incidence of such findings ranges from 15–41%.38,39 While many of these lesions are trivial and of no clinical significance, monetary costs and anxiety may be engendered by their discovery.  ACCREDITATION AND TRAINING ISSUES There is a significant 'learning curve' for interpretation of CTC/VC. Our personal experience suggests that about 50 Fig. 13. Screening examination in an asymptomatic patient shows an annular constricting rectal carcinoma on (a) axial and (b) 3-D images.  examinations are required to become reasonably proficient. It would be a tragedy both for patients and for the future of the technique if the use of CTC/VC became widespread without  for detecting medium and large-sized polyps, but importantly  sufficient safeguards to ensure that mistakes in interpretation  it examines the whole large bowel. It also has several potential  are minimized. It is up to the profession to determine how these  advantages as a screening tool: it is minimally invasive and quick  safeguards are achieved.  for the patient. The scan takes only a few minutes; no sedation is  There is also the question of who is best suited to interpret  required, and initial studies have shown it is highly acceptable  the images. Radiologists may think it obvious that they are most  to asymptomatic subjects in a screening context.22 Its 'high-tech',  qualified for this task. Others, who are adept at 'real' colonos-  virtual-reality profile makes it potentially attractive to the lay  copy, are likely to think otherwise. Should we gird our loins for  public. While employing ionizing radiation, dosages are con-  yet another 'turf battle'? Not only would accreditation guidelines,  siderably less than conventional DCBE when low dosage  applicable to radiologists and non-radiologists alike, aid in  protocols are used. Last, there is the potential to detect inci-  avoiding such an eventuality, they would be to the undoubted  dental extracolonic pathology, such as asymptomatic aortic  benefit of patients. It is incumbent upon appropriate profes-  aneurysms and renal carcinoma although, using the low-dosage  sional bodies to define these guidelines.  regimes, images of extracolonic pathology are not optimal. The lack of sensitivity of CTC/VC for detection of polyps of  THE FUTURE OF VIRTUAL COLONOSCOPY  5 mm or less has been cited as a limitation to its applicability.  Computed tomography colonography/virtual colonoscopy is a  Does this matter in the context of a screening programme?  technique in evolution. There is much research and develop-  Probably not—the chances of a 5 mm lesion being malignant at  ment to be undertaken both at a clinical and technical level in  discovery are negligible; indeed, only about 50% of diminutive  order to define its various potential roles. Technical develop-  polyps are adenomatous, the rest being hyperplastic and there-  ments that require further assessment include:  CT COLONOGRAPHY: A REVIEW  11  1. The effect on accuracy for polyp detection of multi-slice  abdominal CT dose parameters and the increased radiation  helical scanners. Large clinical studies are yet to be published.  dosage. While there is an argument for using standard dosage,  There are theoretical benefits of multi-detector helical CT  thus enabling detection of extra-colonic causes of symptoms in  scanners, in relation to the shortening of acquisition time for  symptomatic patients, the use of low-dose parameters is likely  the images (and thus reduction of respiratory artefact), and  to remain prevalent in the screening context.  in the detection of smaller polyps. Initial clinical reports are encouraging.40 However, the question of whether it is necessary  CONCLUSIONS  or desirable to detect diminutive polyps in the context of a  Computed tomography colonography/virtual colonoscopy is a  screening programme needs to be resolved.  technique in evolution that has great potential as a diagnostic  2. The feasibility of examinations after minimal or no bowel  tool for colorectal neoplasia, both in symptomatic patients and  preparation, using faecal 'tagging' agents and subsequent digital  as a screening tool in average-risk subjects. As hardware and  subtraction techniques of faecal material (so-called 'virtual  software develop and as operators' experience increases there  preparation'), as patients undergoing CTC/VC, colonoscopy or  will undoubtedly be advances in the performance characteris-  DCBE usually find the bowel preparation the most uncomfortable  tics of the test. Many issues, technical, clinical and related  aspect of the examination. Initial reports are encouraging.41,42  to training and accreditation, remain unresolved and are the  3. Computer-aided polyp detection software allowing semi-  subject of continuing assessment.  automated reading of examinations, thus reducing radiologists' time commitment—important if the technique is adopted for screening of large numbers of individuals for CRC. Early clinical  ACKNOWLEDGEMENTS We are grateful to John Edwards, Dianne Murray, Noellene  43  reports show that this is feasible.  4. Determination of the optimal rendering process for 3-D reconstructions—surface-shaded, volume-rendering or some  Foster, Chris Wood, Melanie Rosenberg, Barbara Taylor and the Medical Illustration Department, Royal Perth Hospital. An abbreviated version of this article has previously  hybrid technique. A recent study has suggested that volume44  rendering is superior to surface-rendering, but whether there  appeared in Hospital Medicine.  is a difference in clinical practice remains to be determined. 5. The role of innovative projections that allow the maximum mucosal surface to be examined in the shortest time, for  REFERENCES 1.  example 'virtual pathology', Mercator projections, 'panoramic' views.45,46 6. Magnetic resonance imaging-based colonography/virtual  2.  colonoscopy. This has been shown to be feasible and has the advantage of lack of ionizing radiation.47,48 However, availability of scanners, particularly those able to scan the abdomen in a  3.  breath-hold, and the need for intracolonic contrast are problems that need to be overcome. A preliminary report of the use of  4.  CO2 as a contrast agent using single-shot fast-spin-echo MRI is of interest.49 Clinical aspects of CTC/VC that require clarification include:  5.  1. The role in screening of average-risk asymptomatic subjects for colorectal neoplasia. Studies are currently underway  6.  into this role. 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