CT vs. MR: Virtual Colonography Making A Mark on Diagnosis & Care

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Virtual colonography (VC) stands poised on the brink of acceptance as a screening tool for colon cancer, tempered by the realities of reimbursement issues, while a whirlwind of research activity on numerous fronts excels. While optical colonoscopy remains the gold standard, advances in the virtual model show promise for future screening activities. Currently CT colonography is used in certain specific instances, such as when optical colonoscopy has failed or with patients for which anesthesia poses a risk.

In the United States, most of the activity focuses on the use of multidetector CT scanners for colonography exams, while in Europe, researchers pursue feasibility studies and technique refinement for using magnetic resonance imaging for the same purpose. CT research encompasses perfecting preparation and imaging techniques to obtain reliable results, development of computer assisted detection capabilities, and advances in software-based computer algorithms to improve analysis of 2D and produce 3D image data sets.

The objective of MR and CT colonography is the same - finding colorectal cancer early as well as adenomatous polyps that can lead to cancer over a decade or more. Colorectal cancer strikes 130,000 Americans each year and kills about 56,000. And interestingly, about 75 percent of all new cases of colorectal cancer occur in people who have no known risk factors other than age - with research indicating that the risk for colorectal cancer increases substantially after age 50. Further, the American Cancer Society estimates that only one-third of at-risk Americans are being screened each year for colon cancer via colonoscopy, whether invasive or virtual - so more patient-friendly testing is needed to increase the numbers of Americans being screened.

MR colonography

MR colonography is in the earliest stages of development, with the prospect of several years before large scale clinical trials will be conducted to study diagnostic efficacy, according to D.J. Lomas, MD, professor of clinical MRI at the University of Cambridge and Addenbrookes Hospital in Cambridge, U.K., who has been involved in a pilot study to determine the feasibility of using MR imaging for VC.

A 1.5T MRI scanner is a prerequisite to MR colonography, Lomas has found in his research using a GE 1.5T Signa Excite HD scanner with multichannel array coils that cover the abdomen and pelvis.

One area of his research involves the use of MR fluoroscopy with very rapid MR imaging with frame rates varying from very slow to one per second to as many as 10 to 15 frames per second. As these techniques become refined, he sees promise in future applications.

"To use an analogy: Contrast-enhanced MRA [magnetic resonance angiography] when it was being developed 10 years ago was considered a clumsy and lengthy procedure," says Lomas. "As technique applications and technology were integrated and improved, it has now become a routine examination that is performed simply."

Judy Yee, MD, associate professor and vice chair of radiology for the University of California at San Francisco and chief of radiology at the San Francisco Veterans Administration Hospital, explains an obvious advantage to MR in that there is no ionizing radiation. This means that for certain patients, such as young women, the use of MR would be preferable.

However, Yee notes, CT is more readily available and less costly than MR. "The other advantage for CT is that the procedural time is less than MR - on the order of less than half the time for MR colonoscopy," she says. "For CT, the scan time is so fast that you can scan the entire abdomen and pelvis in both supine and prone positions, and the actual scan time in each position is about 10 seconds. With MRI, there are additional sequences required, and each of the sequences takes longer."

Multislice CT may not be essential for colonography, but it is certainly preferred, Lee says. "The advantage is that you are able to scan using thinner collimation, which improves your resolution on the axial and 3D images."

Finally, Yee explains that although clinical studies suggest that MR may be comparable to CT in performance when looking for large polyps (10mm or greater), there would need to be other advantages of MR to propel it into wide acceptance for colonography screening. There are fewer research centers in the U.S. studying the benefits of this approach, and CT scanners are more widely available here.

Over the past few years in the U.S., research