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Abstract
The term virtual endoscopy (VE) refers to using either spiral computed tomography (CT colonography) or magnetic resonance scanning (MR colonography) combined with advanced computer technology to produce high resolution two- and three dimensional imaging of the large bowel.
In detecting colonic lesions, researches have proved that VE achieved diagnostic accuracy that exceeds that of barium enema and approaches that of conventional colonoscopy. However, VE is minimally invasive, with no need for sedation or analgesics during the procedure. There is no risk of perforation, and all colon segments can be examined. Intramural, extraintestinal and any additional lesions can also be evaluated easily.
Like all 3D reconstructions, it does not add any new information to the axial source images but their main role is in permitting optional visualization of complex anatomic structures like differentiation between polyps and haustral folds.
VE requires a meticoulsly cleansed and dry bowel for optional results, because retained fecal material or fluid can lead to diagnostic errors. Also, adequate colonic distension is crucial for high quality VE.
CTC uses air as the distending media, while MRC uses water (dark lumen MRC) or saline spiked with gadolinium (bright lumen MRC) for opacification of the large bowel lumen.
Both CTC and MRC have a role in diagnosis and staging of colorectal cancers, in postoperative surveillance, after incomplete colonoscopy and for evaluation of inflammatory bowel diseases.
Almost simultaneous with the birth of VE were suggestions that it could be used for screening for colorectal cancer as it proved to be able to reliably detect clinically relevant polyps (> 1cm) with high accuracy.
The advantage of MRC over CTC include lack of ionizing radiation, superior soft tissue contrast and higher sensitivity to IV contrast.
The disadvantages of MRC include lower spatial resolution, high susceptibility to motion artifacts, longer examination time, and currently limited availability.
VE is not competitive to conventional colonoscpoy or double contrast barium enema, all of these techniques are complementary and each possess unique capabilities.
For the future, it is hoped that VE would be used as the first line screening test and conventional colonoscopy can be reserved only for patients with positive results from screening or those with a higher than average risk.
It is to be concluded that, in order for a new method to be substituted for an established one, it should first be shown to be as safe, acceptable, available, and cost effective as the method it is replacing.