virtualcolonoscopy

Virtual colonoscopy
H.M.Fenlon
Department of Radiology,Mater Misericordiae Hospital,Eccles Street,Dublin 7,Ireland (e-mail:********************)
Virtual colonoscopy is a powerful new method of imaging the colon.It involves thin-section helical computed tomography (CT)of the clean,air-distended colon,with images viewed in a variety of two-and three-dimensional formats (Figs 1and 2).Since its ®rst description in 1994the method has been studied extensively and its performance has steadily improved thanks to the development of ultra-fast helical CT scanners (multidetector CT)and advances in computer software for image reconstruction 1±4.The appeal of virtual colonoscopy is that it is a simple,safe and minimally invasive way of evaluating the entire colon,with reported polyp detection rates surpassing those published for double-contrast barium enema and approaching those of conventional colonoscopy.
Although a variety of techniques has been described,the same basic principles apply to all:full bowel cleansing,air distension of the colon using a rectal enema tube,acquisition of both supine and prone thin-section images of the colon,and image interpretation using a combination of axial and multiplanar or endoluminal reconstructions.Optimizing patient preparation is an important considera-tion when performing virtual colonoscopy as both
retained
Fig.1a Axial computed tomogram demonstrating a normal,clean and well distended colon.b Endoluminal view at virtual
colonoscopy of a normal sigmoid colon.c Same segment of normal colon as in b ,seen at conventional
colonoscopy
Fig.2a Virtual colonoscopy image demonstrating a 12-mm polyp (arrow)in the transverse colon.b Same polyp (arrow)as in a ,seen at
conventional
colonoscopy
Leading article
ã2002Blackwell Science Ltd
British Journal of Surgery 2002,89,1±31
intraluminal¯uid and stool may mimic or disguise colonic pathology.Currently,full bowel cleansing using either a phosphosoda(Fleetâ,E.C.De Witt and Co.,Runcorn, UK)or a polyethylene glycol(Klean-Prepâ,Helsin Birex, Dublin,Ireland)preparation is required;phosphosoda preparations are preferred as they result in a drier mucosal surface3.All investigators now agree on the need for maximal air distension of the colon and routine acquisition of both supine and prone scans to help overcome problems with collapsed segments of colon and to differentiate residual stool from pathology.The routine use of spasmo-lytics(glucagon or hyoscine butylbromide)has been abandoned because they have not been shown to improve colonic distension,patient comfort or test accuracy5.For the patient,a complete virtual colonoscopy examination should require no more than10±15min in the scanning department.For the radiologist,although a substantial learning curve exists,a full virtual colonoscopy examination can be reviewed using axial two-dimensional images and limited three-dimensional reconstructions in as little as 10min.
Published results indicate that the sensitivity of virtual colonoscopy for colorectal cancer exceeds95per cent;for polyps measuring1cm or more in size it is80±95per cent and for those of6±9mm it is65±80per cent,falling to35±65per cent for polyps measuring5mm or less1±5.Similarly encouraging data exist for speci®city for polyps and cancer, with an overall speci®city exceeding87per cent for lesions of1cm or more.Virtual colonoscopy provides a total colonic examination in over90per cent of patients,even in those who have distal occlusive colorectal cancer that cannot be passed endoscopically,in the frail and the elderly, and in circumstances of failed or incomplete colono-scopy6,7.Indeed,both published results and personal experience suggest that virtual colonoscopy should be the investigation of choice in patients following failed colono-scopy,and local clinical practice has changed accordingly. In theory,virtual colonoscopy has several advantages over existing colorectal cancer screening techniques8,9.As a full structural colonic examination,its diagnostic potential should be much greater than faecal occult blood testing and, for the same reason,it should yield substantially more neoplasms than¯exible sigmoidoscopy.As a total colonic examination it competes directly with barium enema and colonoscopy.To date,all studies have involved direct comparisons of virtual and conventional colonoscopy,with no formal comparison with barium enema yet published. Performance data so far suggest that virtual colonoscopy may surpass the published performance of barium enema for polyp detection,but this is still to be proven.Inherent advantages of the new method over barium enema include no requirement for colon®lling with barium,a lack of complex overlapping radiographic lines,no need for a second bowel catharsis before colonoscopy if a polyp is detected,greater patient comfort and acceptance,and the ability to evaluate extracolonic structures and to provide a `one stop'staging examination of the abdomen and pelvis if cancer is identi®pared with standard colonoscopy, virtual colonoscopy is associated with a substantially lower risk,has no need for sedation,analgesia or recovery time, has a better patient acceptance,allows the entire colon to be visualized in the vast majority of patients,and eliminates `blind spots'proximal to colonic folds as the anatomy can be viewed from any perspective.However,an obvious dis-advantage of the new method is that tissue cannot be retrieved and all patients with an abnormal test result will require colonoscopy.
Although virtual colonoscopy seems a potentially attrac-tive method of screening for colorectal cancer,many issues must be addressed before it can challenge the existing tests. While initial published results are promising(the®ndings of several large series are currently in press),they have been achieved in a handful of academic centres and in highly selected patient rge-scale multicentre studies in patients of average risk are planned,and some are currently under way in the USA and Europe.The results of these studies will be crucial to the future of virtual colonoscopy as a screening tool.Considerable effort has been focused on optimizing and standardizing the technique of virtual colonoscopy before commencing these trials,but as both CT and software technologies are advancing rapidly the results of these efforts may be obsolete even before they reach publication.Present research efforts are focused on developing oral barium preparations that opacify residual stool,thereby limiting or even eliminating the need for bowel cleansing.The ability to evaluate the colon without bowel catharsis is likely to have a major impact on the public's acceptance of a screening technique.
It is clear that there is a threshold polyp size(5mm)below which the performance of virtual colonoscopy drops precipitously,but no clear consensus exists as to the importance of identifying and removing such diminutive polyps.Many of these are hyperplastic with no malignant potential,and for tiny adenomas the risk of malignancy is extremely low.One could argue in favour of a test such as virtual colonoscopy that selectively identi®es only medium and large polyps rather than promoting a policy of universal polypectomy.It is accepted that¯at adenomas and¯at carcinomas exist,but these are dif®cult to detect with any technique(including virtual colonoscopy). Ultimately,procedural cost will be a major factor in determining the success of virtual colonoscopy as a screen-ing procedure10.As it is a`diagnosis only'test,it must be provided at a cost not much greater than that of barium
2Leading article·H.M.Fenlon
British Journal of Surgery2002,89,1±3ã2002Blackwell Science Ltd
enema and less than that of ing published performance data it has been reported that virtual colonoscopy could be as cost-effective a screening option as colonoscopy if its initial compliance rates were better than that of colonoscopy by15±20per cent.For virtual colonoscopy to be successful in population screening an ef®cient and coordinated collaboration is required between radiologists and endoscopists,whereby patients with normal virtual colonoscopy®ndings(approximately80per cent)are discharged immediately and those with a positive result(approximately20per cent)are referred directly for same-day colonoscopy and polypectomy or biopsy,without the need for a second bowel preparation.
In summary,virtual colonoscopy is a promising new technique for imaging the colon.While there appears to be suf®cient evidence to support its routine use in patients following failed or incomplete colonoscopy,further evalua-tion in the form of multicentre trials is required before it can compete on a widespread basis as a colonic cancer screening tool.Current research interest is focused on developing preparations that allow reduced colonic cleansing,thereby improving patient compliance and optimizing the electro-nic manipulation of data sets to facilitate ease of review and enhance operator performance.Virtual colonoscopy is still a technique in evolution,but the potential for improvement is great.
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H.M.Fenlon·Leading article3
ã2002Blackwell Science Ltd British Journal of Surgery2002,89,1±3。