Combined reading of 3D ‘filet’ and 2D axial views may prove optimal for CT colonography

Researchers have found that a 360° virtual dissection CT colonography postprocessing technique equals the diagnostic accuracy of 2D axial imaging for colon cancer screening. The 3D "filet" presentation of that data can boost reading speed by 28%.

Acceptance of CT colonography among radiologists and referring physicians grows by the day. A decade's worth of research shows technical advances and improvements in patient acceptance and test performance that may lead to increased detection of colorectal polyps and cancer.

Controversy remains, however, concerning the best CT scanning protocols and display methods for accurate interpretation. The virtual dissection, or filet view, reconstruction is a technique that unfolds the colon wall from cecum to rectum. It allows quick inspection of its mucosal surface. New CT scanners, on the other hand, can provide higher spatial resolution through thinner slice collimation.

An investigative team led by Dr. C. Daniel Johnson, a professor of radiology at the Mayo Clinic in Rochester, MN, examined what the combination means for the diagnostic accuracy and reading efficiency of colon cancer screening exams.

Johnson and colleagues prospectively enrolled 452 asymptomatic patients previously scheduled for screening colonoscopy to also undergo CT colonography. The investigators used a pool of three experienced radiologists to review images from each patient using a randomization algorithm.

In one half of the study group, one radiologist was assigned to interpret on a 2D display two images with slice thicknesses of 1.25 mm and 2.5 mm. Another radiologist also read two images with slice thickness of 1.25 mm and 2.5 mm each but used a 3D filet view display. In the other half of the study group, one radiologist was assigned to interpret two images with the same slice thickness (1.25 mm) using 2D and 3D displays. The other radiologist read two images obtained with a slice thickness of 2.5 mm on 2D and 3D displays. A total of 1808 interpretations were performed.

Researchers confirmed the presence of 64 adenomas equal or larger than 6 mm. Of this total, 26 were 1 cm or larger. They found no advantage in using 1.25-mm slices instead of 2.5-mm slices for diagnosis. Interpretation was equally good for both image reconstruction techniques, but double reading using the filet view as the primary reconstruction scheme and axial images for correlation and problem-solving reduced interobserver variability.

Results were published in the September issue of the American Journal of Roentgenology.

Double review using both primary 3D and 2D search yielded sensitivities of 84% and 95%, respectively, for neoplasms 1 cm or larger with either 2.5-mm or 1.25-mm slice data sets. CT colonography identified 100% of adenocarcinomas. Colonoscopy recorded sensitivities of 77% and 20% for large neoplasms and adenocarcinomas, respectively.

"Decisions regarding slice thickness should include consideration of patient dose, image noise, and radiologist preference. Combining primary 2D and 3D helps reduce interobserver variability and can result in accuracy rates comparable to colonoscopy," the researchers wrote.

The filet view display allows inspection of the entire colorectal mucosa and each anatomic segment on a single image. Filet view still requires seamless interaction with 2D and endoluminal 3D displays, but it provides interpretation times 28% shorter compared with a conventional 2D approach (an average of 10.4 versus 14.5 minutes). Double review using both conventional and virtual dissection, on the other hand, compensates for poorer performing reviewers, the researchers wrote.

The results confirm that filet view reconstruction, also called virtual dissection, is not yet ready for clinical use as a primary mode of polyp detection, according to Dr. Perry J. Pickhardt, an associate professor of radiology at the University of Wisconsin Medical School in Madison. In the study, filet view interpretations were compared to 2D reconstructions known to be suboptimal. On the other hand, 3D polyp detection using the undistorted endoluminal, or fly-through, view offers a significant advantage over 2D.

"This study is relatively small in terms of asymptomatic screening, making it difficult to draw any firm conclusions," Pickhardt said.

Radiologist Dr. William Glenn, an image manipulation and postprocessing expert, disagrees. He argues for colon flattening and other aids, such as computer-assisted diagnosis and speed reading. They would help increase the acceptance of CT colonography among nonacademic radiologists and lower public resistance to colon screening, he said.

"(This) could be a huge benefit to more patients getting screened," he said.

For more information from the Diagnostic Imaging archives:

Use of filet view software simplifies CT colonography

GI docs find value in virtual colonoscopy

3D CT colonography overcomes diverticulosis

3D postprocessing tools shed light on hidden polyps