Research News: 2002 - 2003 - 2004

Virtual Colonoscopy Prepares to Enter the Mainstream
Author: Judy Yee, MD

Although colon cancer is the third most common cancer in the United States, less than 30% of individuals eligible for screening actually undergo evaluation. Virtual colonoscopy, also referred to as CT colonography, is a minimally invasive imaging technique that combines volumetrically acquired helical CT scan data with advanced graphical software to create two and three-dimensional views of the colon. It is a faster, safer and potentially more easily tolerated technique than standard colonoscopy. Patients do not require sedation and can return to normal activities immediately following the CT study. If virtual colonoscopy is added to the algorithm for colorectal cancer screening, it is hoped that asymptomatic or average risk individuals age 50 and above will more likely seek screening.

Figure 1a. Axial view of the colon demonstrates a pedunculated polyp.

Figure 1b. A three-dimensional endoluminal view of the same lesion showing the polyp on a stalk.

Technique and Performance Studies
Currently, patients undergo bowel cleansing the day before the CT examination. Research efforts are underway to develop a “minimal preparation” version of virtual colonoscopy using ingested barium or iodinated contrast to tag residual material in the colon obviating the need for laxative cleansing. Colonic distention must also be optimal in order to obtain a maximally diagnostic test. We have currently converted to electronic carbon dioxide insufflation to achieve reliable distention, which is also more comfortable for patients.

Patients are scanned in two positions, typically: supine and prone. Multidetector CT scanners can scan the entire colon in less than 20 seconds in each position. Low-radiation dose CT protocols are used with thin slice thickness. Axial, multiplanar reformatted images, and three-dimensional views are used in an integrated approach to detect polyps and cancer. The three-dimensional images of the inside of the colon may be viewed as a movie loop with an appearance similar to what is seen on an actual colonoscopy. Studies have found that CT colonography has a sensitivity range of 83 to 100% and a specificity range of 93 to 100% for the detection of 10 mm or larger polyps when using a per patient matching scheme. When each polyp found on virtual colonoscopy is matched with each polyp found on colonoscopy, the per polyp sensitivity range is 75 to 100% for the detection of 10 mm or larger polyps. Large polyps measuring 10 mm or larger are considered clinically significant because they have greater potential to develop into cancer. In the largest published study to date of 300 subjects performed at the San Francisco VA Medical Center, virtual colonoscopy identified 90% of the large polyps that were found on colonoscopy using a per polyp matching scheme. Virtual colonoscopy identified every patient with clinically significant polyps in this study. Confirmatory multicenter performance studies are under way in asymptomatic or screening patients with results pending late 2003.

Figure 2. Images of a pedunculated polyp from colonoscopy and CT colonography showing excellent correlation.

Current Indications
Virtual colonoscopy has value in evaluating the proximal colon when fiberoptic colonoscopy is unsuccessful due to a distal obstructing lesion or colonic tortuosity. CT colonography is an excellent alternative test in patients who have contraindications for conventional colonoscopy or in patients on anticoagulant therapy. Elderly and frail patients will also find that virtual colonoscopy can provide an alternate test for evaluation of the colon.

In addition to its effectiveness at identifying colon cancer, virtual colonoscopy can also detect diseases outside of the colon. This can lead to the discovery of problems such as renal cancers, aortic aneurysms, and even lung cancers. Data analysis of extracolonic findings is being performed in 500 patients as part of an ongoing study at the San Francisco Veteran Affairs Medical Center.

Figure 3. Three-dimensional endoluminal view of the colon showing the appearance of normal haustral folds and a small rounded polyp.

Figure 4. Three-dimensional endoluminal view is helpful in confirming an ovoid polyp initially detected on axial images.

Future Areas Of Development
Areas of research include the evaluation of alternative displays that allow the radiologist to view larger areas of the colonic surface at one time. A “virtual dissection” view splits the colon along its longitudinal axis, opening the colon so that it may be inspected like a surgical pathologic specimen. Other map-like projections of the colon are also under investigation. The accuracy of these novel visualization methods needs to be determined.

Computer aided detection (CAD) can enable radiologists to read more exams in less time, while detecting more potential areas of concern. Research is underway to develop CAD systems for use in combination with virtual colonoscopy with a focus on improving sensitivity and specificity. CAD will save not only time but also money, since a single radiologist could read more studies more effectively. Before that happens, however, CAD for virtual colonoscopy has to be merged effectively with existing platforms so it can efficiently interact with available software.

Figure 5. Virtual dissection view showing the colon split along its longitudinal axis for display. Note the small polyp indicated by the arrow.

Radiologists typically need additional training to interpret virtual colonoscopy studies. Although the axial images are familiar to many, performing segmental tracking is a concept specific to colonic evaluation. The endoscopic view is a new visualization technique that requires training. In response to the need for radiologist education, UCSF has developed an intensive Virtual Colonoscopy Workshop specifically to train radiologists how to perform and interpret these exams.

Mainstream Usage
Virtual colonoscopy is becoming more widely available around the country at community hospitals and screening centers as well as at academic hospitals. It is currently an option for patients who can afford to pay out of pocket for this less invasive test; the cost can range anywhere between $500- $1,500. Efforts are under way to develop a method for reimbursement for CT colonography. However, a standardized cost needs to be established for CT colonography that must be significantly lower than colonoscopy in order for it to be available to the most number of patients for colorectal cancer screening.

Figure 6. Three-dimensional overview of the colon obtained non-invasively.

Virtual colonoscopy is here to stay. It is a chance for radiologists to regain some of the territory lost with the double contrast barium enema, which has seen a decline due to decreased referrals from our gastroenterology and medicine colleagues. Gastroenterologists, like radiologists, are faced with a manpower shortage and can see the benefits of using CT to screen for colorectal cancer. Patients referred for colonoscopy would then be more likely to have a significant lesion for biopsy. Virtual colonoscopy must be a cooperative effort between radiologists, gastroenterologists and colorectal surgeons to ensure that there is an easy and effective referral system for patients who are found to have a lesion.