Computed tomographic colonography: current and future status for colorectal cancer screening

CT colonography using 360-degree virtual dissection: a feasibility study

OBJECTIVE

Using a 3D rendering technique called "virtual dissection," we sought to evaluate polyp and fold distortion using a colon phantom, estimate the polyp detection performance in humans, and estimate the added benefit of double interpretation and computer-aided diagnosis.

MATERIALS AND METHODS

A colon phantom containing 144 polyps of varying sizes (5-12 mm) and shapes (flat, sessile, pedunculated) was scanned. Polyp shape and distortion at virtual dissection were categorized as flame, club, pea, or bizarre. Haustral fold distortion was graded. The CT colonography examinations in 20 consecutive patients (colonoscopically proven normal findings, n = 5; polyps > or = 1 cm, n = 17 in 15 patients) were blindly reviewed by three radiologists using the virtual dissection technique. The added benefits of double interpretation and computer-aided diagnosis were tabulated.

RESULTS

Sessile polyps appeared flame (35/48 [73%]) or pea (11/48 [23%]) in shape. Flat polyps appeared flame-shaped (31/47 [66%]) or pea-shaped (16/47 [34%]). Pedunculated polyps were flame (15/45 [33%]), club (20/45 [44%]), or pea (6/45 [13%]) in shape. Axial distortion occurred along the longitudinal axis. The sensitivities of the three observers for polyps of 1 cm or more were 16/17 (94%), 14/17 (82%), and 15/17 (88%). The specificities were 5/5 (100%), 5/5 (100%), and 4/5 (80%). Sensitivities after double interpretation and computer-aided diagnosis improved but did not reach statistical significance.

CONCLUSION

Although distortion of colonic structures exists at virtual dissection, it does so in recognizable patterns, so that sensitivity for polyp detection is not compromised.

Computed Tomographic Colonography (CTC): Possibilities and Limitations of Clinical Application in Colorectal Polyps and Cancer

Colorectal cancer is the second leading cause of cancer-related deaths in Europe and the United States. Most colorectal cancers develop from adenomatous polyps over a number of years. Early detection of polyps eliminates the risk of subsequent carcinomas.

Computed tomographic (CT) colonography is a diagnostic technique detecting colorectal neoplasms. With the introduction of multidetector-row computed tomography (MD-CT), CT colonography (CTC) has gained influence as a new diagnostic tool in early detection of colonic pathologies by acquiring volumetric CT data sets of the abdomen. This volumetric data is analyzed using CTC workstations, which provide an interactive display of 2D and 3D images of the colon. In several studies, CTC revealed a high accuracy (sensitivity/patient: 83–100% and specificity/patient: 93–100%) in detecting pathological colonic changes. Furthermore, CTC is an excellent diagnostic technique for the evaluation of patients with incomplete conventional colonoscopy and allows the assessment of extracolonic abdominal and pelvic organs.

In this article, the status of CT colonography as a method of detecting colonic polyps and colorectal carcinomas using single- and multidetector-row CT will be reviewed.

Three-dimensional display modes for CT colonography: conventional 3D virtual colonoscopy versus unfolded cube projection

The authors compared a conventional two-directional three-dimensional (3D) display for computed tomography (CT) colonography with an alternative method they developed on the basis of time efficiency and surface visibility. With the conventional technique, 3D ante- and retrograde cine loops were obtained (hereafter, conventional 3D). With the alternative method, six projections were obtained at 90 degrees viewing angles (unfolded cube display). Mean evaluation time per patient with the conventional 3D display was significantly longer than that with the unfolded cube display. With the conventional 3D method, 93.8% of the colon surface came into view; with the unfolded cube method, 99.5% of the colon surface came into view. Sensitivity and specificity were not significantly different between the two methods. Agreements between observers were kappa = 0.605 for conventional 3D display and kappa = 0.692 for unfolded cube display. Consequently, the latter method enhances the 3D endoluminal display with improved time efficiency and higher surface visibility.

CT colonography at different radiation dose levels: feasibility of dose reduction

PURPOSE

To investigate the sensitivity and specificity of polyp detection and the image quality of computed tomographic (CT) colonography at different radiation dose levels and to study effective doses reported in literature on CT colonography.

MATERIALS AND METHODS

CT colonography and colonoscopy were performed with 100 mAs in 50 consecutive patients at high risk for colorectal cancer; 50- and 30-mAs CT colonographic examinations were simulated with controlled addition of noise to raw transmission measurements. One radiologist randomly evaluated all original and simulated images for the presence of polyps and scored image quality. Differences in image quality were assessed with the Wilcoxon rank test. Scan protocols from the literature and recent (unpublished) updates were collected.

RESULTS

In nine of 10 patients with polyps 5 mm in diameter or larger (sensitivity, 90%) and in seven of 17 patients with polyps smaller than 5 mm, polyps were correctly identified with CT colonography at all dose levels. Specificity for patients without polyps 5 mm or larger was 53%-60% at all dose levels and for patients without any polyps was 26% (at 100 and 50 mAs) and 48% (at 30 mAs). Image quality decreased significantly as the dose level decreased. The median effective doses (supine and prone positions) calculated from protocols reported in the literature and updates were 7.8 and 8.8 mSv, respectively.

CONCLUSION

Although image quality decreases significantly at 30 mAs (3.6 mSv), polyp detection remains unimpaired. The median dose for CT colonography at institutions that perform CT colonographic research is currently 8.8 mSv.