Spiral CT assists development of virtual colonoscopic techniques

GE develops auto-navigation algorithms

Spiral CT and virtual reality techniques are being employed toprovide new diagnostic capabilities as an alternative to video-basedcolonoscopy screening. For example, Dr. David Vining of the BowmanGray School of Medicine in Winston-Salem, NC, has used spiralCT data and advanced graphics computers to render images of theinside of the colon. A joystick allows him to navigate up anddown the colon, looking for abnormalities. Vining has performedthese virtual colonoscopies on 20 patients, successfully identifyingfour patients who were later found to have cancer.

GE Medical Systems of Milwaukee is working on a similar concept,using CT data for the colon and MRI data for other anatomicalstructures, such as the vessels of the brain. They have developedauto-navigation algorithms that facilitate moving through thetwists and turns of the large intestine. The company is planningclinical trials of the technique soon.

Dr. Roger Shifrin, one of Vining's colleagues, believes thathelical CT's fast acquisition of data is what has enabled virtualcolonoscopy.

"Helical CT data can be obtained in 30 seconds, whereasMRI or conventional CT takes minutes. Patient movement duringthis time can lead to discontinuities in the rendered images,"Shifrin said. "In addition, MRI can produce artifacts inthe imagery, and the spatial resolution is not as good as CT."

While virtual colonoscopy has the advantage of being noninvasive,it may not be able to provide the same visualization that video-basedcolonoscopes can, according to gastroenterologist Dr. CharlesAdelmann.

"Observing the contour, lining, and color of tissues,as well as blood vessels, is an important part of a video colonexamination," Adelmann said. "CT colonoscopy must showthat the resolution is as good as video imaging systems."

Another challenge facing the development of CT-based virtualcolonoscopy includes methods of differentiating fecal residuefrom polyps. Researchers are working on new contrast media andadvanced image rendering techniques to help with this differentiation.

Virtual colonoscopy systems also require enormous computerhorsepower, such as high-end workstations from Silicon Graphics.Reducing the complexity of algorithms will help lower the computationalpower needed. But simultaneously, computer hardware advances arealso dramatically lowering the cost of these systems. Many expectthat in three to five years, today's high-end workstations willbe cost-effective enough for use in many medical settings.

Virtual techniques are not expected to replace colonoscopes,which will still be needed to perform biopsies or remove canceroustissue, but they could replace the use of colonoscopy as a screeningmethod. In addition, virtual techniques allow physicians to navigateoutside the colon to examine cancers in surrounding tissues, somethinga video colonoscope can't do. Use of CT data can also pinpointthe exact location of abnormalities for later surgical treatment.

If the virtual techniques prove to be clinically effective,Vining believes it could significantly reduce the cost of screeningnow performed with video colonoscopy methods. For example, currentexaminations cost $900 to $1800, whereas virtual examinationsshould be about $450 to $650, he said.