NEW ORLEANSVirtual endoscopy of the gastrointestinal tract is a
rapidly advancing technology, Bradford J. Wood, MD, said in an
interview with Oncology News International. Radiologists should
become familiar with the virtual endoscopic appearance of a variety
of GI pathologies so that they can speak a common language with
surgical and gastrointestinal endoscopists, he said.
Dr. Wood and his colleagues from Massachusetts General Hospital
reviewed the clinical applications of virtual endoscopy (also known
as endoluminal imaging) of the GI system in a poster presentation at
the American Roentgen Ray Society annual meeting. Dr. Wood is
currently assistant professor of radiology at Georgetown University
and an NIH staff clinician in the Department of Inter-ventional Radiology.
Virtual endoscopy has been studied mostly in the tracheobronchial
tree and colon (see Figure 1).
Colon cancer screening is likely to be the first and most
important medical application of this technology, Dr. Wood
said, since patients have been very reluctant to accept colonoscopy.
Helical CT scanningthe methodology for obtaining the data used
in virtual endoscopyis widely available, Dr. Wood said. The
scan is very brief, taking less than a minute in the prone and supine
positions. As a result, the radiation dose is much lower than with
conventional CT scans.
The innovative aspect of virtual endoscopy is the conversion of the
helical CT data into three-dimensional volumetric images by
sophisticated computer software, creating a virtual
fly-through of the conduit being studied. The virtual
images allow the clinician to visualize the whole inner
surface, rather than piecemeal evaluation of slices, and to see
beyond strictures and obstructions, Dr. Wood said.
Helical CT postprocessing allows study of GI applications once
thought to be non-CT clinical issues, he said, such as depiction of
mucosal processes and the relationship of abnormalities to the bowel
itself. Dr. Wood said that this information is recorded on axial CT
images, but is not normally processed and absorbed by most viewing systems.
Currently, the special conversion software is expensive and not
widely available, he noted. In addition, he cautioned, interpretation
of virtual endoscopy images takes more time and requires training to
discriminate between pathology and artifact, as well as normal anatomy.
Shaded surface display methods are most commonly used because they
are cheaper and faster than true volume rendering. Unfortunately, he
noted, these methods are prone to thresholding error and loss of
data. Until cheaper and faster methods of true volume rendering
become available, he said, one solution might be for hospitals to
transmit helical CT data to special centers for conversion and interpretation.
Like any technologic innovation, virtual endoscopy has its
strengths and weaknesses, Dr. Wood commented. For example, the
sensitivity and specificity are not equal to colonoscopy, but with
colonoscopy, the gastroenterologist often cannot see beyond
obstructions and strictures, and must estimate the precise location
Virtual endoscopy allows for multiple views, including areas not
accessible by endoscopy, and can establish the exact location of
lesions, he said. Although mass contour and lumen shape may be better
depicted by virtual endoscopy, certain anatomic information, such as
mucosal detail, is better seen with real endoscopy, Dr. Wood noted.
On the downside, he added, the procedure does not allow for tissue sampling.
Virtual endoscopy can assist in the characterization and depiction of
a number of common GI pathologies, including gastric masses (see Figures
2 and 3), gall bladder disorders, and inflammatory bowel disease.
The small bowel is difficult to image by endoscopy, contrast
barium fluoroscopy, or CT imaging, Dr. Wood said. It is often
the last place to look for GI pathology, which is generally a
diagnosis of exclusion. The virtual endoscope can be placed in the
small bowel where fiberoptic scopes cannot pass, he said. And while
the thickened folds of inflammatory bowel disease may not be seen on
axial CT images, they can easily be depicted with endoscopy.
Overall, Dr. Wood said, virtual endoscopy has a number of potential
uses, including a role in planning surgery or real
endoscopy. It can also be used as a teaching tool.