Virtual colonoscopy is in the early stages of its development. Its feasibility has clearly been demonstrated, and larger, multicentre studies are needed to determine its sensitivity and specificity outside specialist units. Multiple technical parameters can be altered during image acquisition, and a coherent technique has yet to emerge. Multi-slice spiral computed tomography has just been introduced and will reduce examination time even further. Furthermore, the principles underlying virtual colonoscopy by spiral computed tomography can be just as easily applied to magnetic resonance imaging, which carries no radiation penalty.14
Presently, most limitations apply to the processing and analysis of images after their acquisition—analysis of a single study takes about 30-60 minutes. However, the current constraints imposed by available processing power and digital storage are likely to diminish. Also, it seems that diagnosis can usually be made from the two dimensional images, and generating a three dimensional image, the most demanding application in terms of computer power, may be necessary only for problem solving, such as differentiating between a polyp and a haustral fold.15
Other rendering techniques are also being developed. It is possible to generate three dimensional images that resemble conventional fluoroscopic double contrast enema studies or to artificially straighten the colon, open it, and view the rendered, flattened surface en face, imitating the pathologist's view. Navigation software is also progressing; luminal “fly through” navigation can be automated and collision avoidance techniques used, with simultaneous antegrade and retrograde viewing.
At present, full colonic cleansing is mandatory for an acceptable examination, but workers are already investigating the possibility of using oral contrast medium with a view to tagging faecal residue so that it can be differentiated from pathological features and possibly digitally excluded from the image. Furthermore, because virtual colonoscopy generates images of the colonic wall, it is possible to apply algorithms that automatically detect and label regions where it is thickened, alerting the radiologist to the possible presence of polyps. Perhaps flat adenomas can be detected this way.
The development and refinement of all of these techniques are likely to have tremendous impact on the speed and accuracy of virtual colonoscopy and its interpretation in the near future. The performance characteristics of virtual colonoscopy are certain to improve with improvements in hardware and software innovation and radiologists' experience. With regard to its potential as a screening tool, its performance in real populations and its cost, patient acceptability, and availability will need to be determined.