Better and faster computed tomography (CT) equipment and more sophisticated software now make the concept of CT colonography (CTC), also known as "virtual colonoscopy," a reality.

Within the last 6 months, two new products have helped push this technology to the leading edge of clinical practice. At RSNA 2001, Voxar Limited (Edinburgh, Scotland) unveiled Colonscreen, their CTC workflow and reporting solution. This product uses three-dimensional (3D) imaging software to provide two-dimensional (2D) and 3D images of a patient's colon. The system also provides real-time image data manipulation and a reporting system.

E-Z-EM, Inc. (Westbury, NY), in conjunction with Vital Images, Inc. (Minneapolis, MN), introduced their dedicated CTC system, InnerviewGI, on March 11, 2002. InnerviewGI uses Vital Images' Vitrea 2 technology to render 3D images in seconds, and provides multiple 3D view options as well as synchronized 2D and 3D views of the colon.

Paving the way for CTC

"The thought of using CT for detecting lesions in the colon has probably been around since CT first came into view in the 1980s," said Matthew Barish, MD, vice chairman of radiology, director of the Virtual Colonoscopy Center at Boston Medical Center, and Voxar's clinical director, "but the length of time it would take to do the procedure with the older CT scanners would have precluded any real study." He credits two technologic breakthroughs with making CTC clinically feasible. "One was the helical CT that allowed scans to be done in a much more rapid fashion and as one continuous volume of data," he said. "The newer, multislice CT scanners led this to become a truly viable alternative."

The second advance was increased computer power. This has allowed the radiologist to view these images in various forms, in both a 3D methodology as well as on computer screens (rather than on film). With this technology, notes Barish, "you can cine through them in a movie mode that allows you to pick up lesions that wouldn't be visible on film."

Jay D. Miller, president and chief executive officer of Vital Images, agrees. "With the multislice spiral CT scanners, you can generate the higher resolution images faster than you used to be able to," he said. "Clearly, on the CT side, not only can you get better images, but you can also get them faster."

On the software side, Miller remarked, "The ability to do volume rendering very quickly, very easily, and cost effectively is probably the biggest change. Even 6 or 7 years ago, it took hours to generate simple volume rendering of the colon. Now you can review a colon data set, interactively, in both 2D and 3D in a matter of minutes."

Miller also cited a third factor: Improved patient preparation. "We are a lot smarter about patient preparation before the exam," he said, "including insufflation of the colon and all of the things that have to happen outside of doing just the CT exam and the volume rendering. All those things are coming together right now."

Conventional colonoscopy vs. CTC

Barish cited several advantages for patients choosing CTC over conventional colonoscopy. "I think the main reason has to do with both a sense of comfort for the patient, as well as a safety profile," he said. "Conventional colonoscopy carries a certain risk for perforation or injury to the bowel and that can lead to patient harm."

The second reason, he noted, is time and convenience. "Conventional colonoscopy is done with sedation, so the patient has to have someone bring them to the procedure," he explained. "There is a period of pre-anesthesia time, the actual procedure, 3 to 6 hours of recovery time, and the need to have someone drive the patient home. So there is a fairly long period of time during which patients are removed from their daily life or work."

Using CTC, on the other hand, patient exam time can be reduced to as little as 10 minutes. "The set-up time for the CT is minutes," he noted. "The actual scan time on the new scanners is about 20 to 30 seconds for each of the two scans. Then after the scan is done, the patient is free to go. Since there is no sedation, they can go back to their routine activities."

Miller also sees advantages of CTC in increased visualization. "If there is an occlusion of the colon, for example," he said, "using conventional colonoscopy, sometimes you can't get the scope past the occlusion. Clearly you can do that with CTC. Also, we certainly can interrogate every square millimeter of the surface of that colon virtually, whereas with the scope, sometimes you can't see the complete surface."

Sensitivity and specificity

Several studies have looked at the sensitivity and specificity of this technique with varying results. A study by Mendelson et al ¹ looked at the accuracy of CTC in detecting colorectal polyps and cancer in 100 patients with either colonic symptoms or a family history of bowel cancer. Use of conventional colonoscopy found 121 polyps in 47 patients, while CTC found only 28 polyps in 19 patients. Using supine plus prone scans, the sensitivity of CTC was 73% for polyps with a diameter 10 mm or greater and 19% for smaller polyps, with an 8% false-positive rate. With supine-only views, the sensitivity dropped to 57% for the larger polyps and 11% for the smaller ones. Positive and negative predictive values for CTC were 88% and 89%, respectively, for supine plus prone scans.

A study by Fenlon et al, ² however, found substantially higher sensitivities. A total of 100 patients at high risk for colorectal cancer underwent CTC immediately before conventional colonoscopy. Prior to CTC, each patient had a rectal tube inserted and the colon insufflated with air to the maximum tolerable level. Each patient also received 1 mg of glucagon intravenously to minimize smooth-muscle spasm and peristalsis and improve patient comfort. The entire colon was visible in 87 patients using CTC and in 89 patients with conventional colonoscopy. Forty-nine patients had abnormal findings, with a total of 115 polyps and 3 carcinomas. CTC identified all 3 cancers and 20 of the 22 polyps that were >= 10 mm in diameter (91%). For the smaller polyps, CTC identified 33 of 40 measuring 6 to 9 mm (82%) and 29 of 53 that were ¾ 5 mm (55%). There were 19 false-positive findings of polyps, but no false-positive findings of cancer. For adenomatous polyps, CTC correctly identified 46 of the 51 (90%) that were >= 6 mm in diameter and 12 of 18 (67%) of those that were smaller.

Miller expects these rates to increase as the technology improves. "Clearly, with improvements in the quality of the images, in patient prep, and in our ability to visualize the data sets, we will continue to see the sensitivity and specificity of this tool climb," he said.

Patient preparation

"Although the goal of patient prep is the same for both conventional colonoscopy and CTC, the dynamics of the process can be slightly different," according to Allyson Mortati, global product manager of virtual colonoscopy at E-Z-EM. Conventional colonoscopy typically uses polyethylene glycol preparation, which can leave too much fluid in the bowel for CTC. To address this, E-Z-EM offers a saline osmotic preparation, LoSo Prep, which can be used in conjunction with their low-residue diet, Nutra Prep, and Tagitol, a stool-tagging agent. (For additional information on these products, see RSNA Roundup. Appl Radiol . 2002;31(1):6-7.)

One area of patient preparation that is specific to CTC is insufflation of the colon. "Insufflation is an absolute requirement for CTC," said Mortati. "You have to have the colon completely distended to be able to see all of the walls, to be sure that you have given a complete exam." Typically this has been achieved using manual inflation with room air via a hand-squeezed bulb. In early March 2002, E-Z-EM began marketing an automated CO ₂ insufflation kit, ProtoCO2l. This kit offers several patient and physician advantages, according to Mortati. "The biggest problem with (insufflation with room air) is that there is a lot of discomfort in recovery for the patient because they have to expel the air. Room air is 70% nitrogen and there is no way for it to get out except the way it came in. In contrast, CO ₂ for insufflation gets absorbed by the bowel and gets exhaled out."

"In addition to that," she continued, "there is some further patient comfort benefits in that this is constant-pressure insufflation, rather than the manual insufflation that can create pressure spikes. Also there is a productivity improvement with automated insufflation; the doctor or the technician isn't standing there squeezing the bulb, so he or she is free to do other things."

Radiation dose

One area of concern with CTC is radiation dose. "Clearly, there is concern about radiation dose," noted Miller. "The idea of CTC is one in which radiation dose needs to be looked at very carefully," agreed Barish. "As more and more people learn this technique, one of the fears is that the radiation dose will creep higher since people don't know that it must be done in a method that reduces the radiation dose. The dose can be limited to very low levels by reducing the milliamperage, the power output, of the CT scanner."

This concern was echoed by Riccardo Iannoccone, MD, of the University of Rome, speaking at the European Congress of Radiology Meeting in Vienna this year. ³ He noted that in order for CTC to become useful as a widespread screening tool, there must be a reduction in cost and in radiation dose. "For us, the main issue is dose exposure," he said.

According to Iannoccone, there are three main reasons why standard CT doses are high. First, patients typically undergo two scans to maximize lesion detection. Second, the multislice scanners used in CTC deliver higher radiation doses than single-slice machines. Third, he noted that few attempts have been made to develop and test low-dose protocols. ³

Iannoccone presented results of a study he conducted using a low-dose multislice CT protocol. ³ In this study, the images were obtained from 27 patients with suspected colorectal lesions using 2.5-mm slice collimation, 3-mm slice thickness, 1-mm reconstruction interval, 17.5-mm/sec table speed, 140 kVp, and 10 mAs on a Somatom Plus 4 Volume Zoom scanner (Siemens Medical Solutions, Erlangen, Germany).

Using this protocol, the CT dose index (CTDI) dose exposure did not exceed 1.37 mGy for each scan, with a total mean CTDI dose exposure of 1.7 mSv for men and 2.3 mSv for in women ³ . According to Iannoccone, previous studies using a standard regimen reported a the mean dose exposure ranging from 4.8 to 5.0 mSv for male patients and from 7.0 to 7.8 mSv for female patients.

Following image acquisition and volume rendering, two experienced radiologists read 2D axial multiplanar reformations and 3D endoluminal views and rated the image quality on a scale of 1 (nondiagnostic) to 4 (excellent). Use of this protocol resulted in a mean image quality score of 3.1 for images of the colon and 1.8 for structures outside the colon, such as the lymph nodes and liver.

In terms of sensitivity, all 9 colorectal cancers were identified accurately using the low-dose CT protocol and 12 of 14 polyps were found, for a sensitivity of nearly 86%.

"Ultra­low-dose CTC provides a substantial radiation dose reduction," concluded Iannoccone. "It allows colonic assessment with sensitivity that is comparable to protocols that produce higher radiation doses, although our data is certainly subject to confirmation from larger series." ³

Patient selection

"I think for the average- to low-risk patients, CTC would be an ideal test," concluded Barish. "It's very good for those patients who need to be screened but the likelihood of them having a significant polyp is low to moderate. This is because if they do have a polyp, they will have to go on to conventional colonoscopy to have it removed. So therefore, in the high-risk population, it is probably not the ideal test since those people would then need to undergo two tests, one for diagnosis and one for treatment. It is also very good for any patient who would not normally undergo a screening test, either due to pain or if there are other conditions that preclude their ability to undergo conventional colonoscopy, such as heart disease, lung disease, etc. where anesthesia for the procedure or the pain of the procedure would put them at higher risk."

Miller also views CTC as another option for those who may be unwilling to undergo conventional colonoscopy. "One of the big drivers behind this is that we want to increase the compliance rate," he said. "The problem is that people don't have conventional colonoscopies done for a whole host of reasons. We anticipate that because CTC is less invasive, people will be more likely to have the procedure done. Colon cancer is very slow growing and very predictable, for the most part," he concluded. "If you can find the polyps early and remove them, you can save a lot of lives."