Report from RSNA: Colon screening benefits from low-dose CT and new fly-through tool

December 1, 2006

German researchers have found that low-dose 64-slice CT colonography accurately detects colorectal polyps in a screening population. They also tested a new 3D visualization tool that enables simultaneous antegrade and retrograde views, significantly decreasing interpretation time.

German researchers have found that low-dose 64-slice CT colonography accurately detects colorectal polyps in a screening population. They also tested a new 3D visualization tool that enables simultaneous antegrade and retrograde views, significantly decreasing interpretation time.

Dr. Anno Graser, a radiologist at the University of Munich, presented the study on Tuesday during a scientific session at the RSNA meeting. The study is part of the Munich Colorectal Cancer Prevention Trial.

Graser and colleagues evaluated 300 asymptomatic subjects (154 women) who underwent same-day CT colonography and optical colonoscopy after complete bowel cleansing. Scans were acquired at 120 kV, 100 reference mAs (supine), and 40 reference mAs (prone). Online dose modulation was employed, and effective patient dose was calculated for all subjects. Radiologists used a primary 3D reading approach with 2D correlation.

Researchers divided the colon into six segments: cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum. Presence of polyps was rated according to size groups: small (5 mm or less), medium, 6 to 9 mm, or large (10 mm or greater). The median effective patient radiation dose was 4.2 mSv.

Overall, 138 adenomatous and 88 hyperplastic polyps in 112 patients were detected: 170 small, 38 intermediate, and 28 large. Sensitivity and specificity of CTC for adenomas were, respectively:

  • 96% and 100% for large polyps

  • 92% and 96.7% for intermediate

  • 78.9 and 89.5% for small

Per-patient sensitivities were 100%, 98%, and 80% for large, intermediate, and small polyps, respectively. Optical colonoscopy missed one large, two intermediate, and eight small polyps (sensitivities 96%, 95%, and 89.5%).

While CT colonography is accurate in detecting colorectal polyps, researchers cautioned that the detection of flat lesions remains problematic. And though video colonoscopy performed equally well, Graser said it is an imperfect standard of truth.

In another study, Graser and colleagues compared 3D CT colonography reading times using conventional 3D fly-through visualization (standard view) and a new 3D visualization tool (panoramic view) that enables simultaneous antegrade and retrograde views, unfolding the colon as the fly-through is being performed. Graser reported that reading times in the literature range from five to 53 minutes.

Researchers reviewed 200 CT colonography data sets from 100 patients from the ongoing single-institution trial comparing CT colonography and optical colonoscopy. Reading times for standard 3D interpretation with 2D for problem solving were documented. The same cases were interpreted again with the panoramic view without using the results from the standard view interpretation. Reading times for this technique were measured and documented.

The area of colonic mucosa that was visualized on a unidirectional fly-through from rectum to cecum was recorded for standard view and panoramic view. Differences in percentage of visualized colonic mucosa and reading times were tested for statistical significance using the t test.

Mean CTC interpretation time decreased from 18.5 minutes with 3D standard view software to 10 minutes using 3D panoramic view. The difference was significant (p

After unidirectional fly-through, the panoramic view had visualized an average of 95.5% of the colonic mucosa, whereas the standard view visualized only 79.4%, requiring an additional flight from rectum to cecum.

Researchers concluded that the new visualization technique significantly decreased interpretation time in 3D CT colonography. It visualizes the complete colonic mucosa in a unidirectional fly-through, whereas standard 3D tools require bidirectional fly-through to achieve complete visualization.

A participant questioned the potential learning curve for being able to look peripherally as well as straight ahead. Graser said that the overall fly-through speed will decrease because of the need to look peripherally. But the time savings are greater by not having to to fly-through bidirectionally.

"No one has the time to do two fly-throughs. So far, we've determined that this is a pretty nice tool to go through once," he said.

Studies presented at the RSNA meeting suggest that the promise of dual-source CT will be fulfilled by a big boost in temporal resolution and a corresponding increase in its sensitivity to coronary artery disease.

DSCT accurately identified significant coronary artery disease, defined as more than 50% stenosis, without the use of beta blockers in a study involving 35 patients, reported Dr. Kostantin Nikolaou, CT section chief at Ludwig-Maximilians University in Munich. The patients were evaluated using both DSCT and conventional angiography.

All 35 studies were deemed diagnostic. Significant disease was found in 17 patients with 32 disease segments. On a per-patient level, no false negatives and two false-positive cases were identified.

"At least in this 35-patient population, we reached a sensitivity of 100%. We lost a little specificity, but we were able to exclude disease correctly in all cases," Nikolaou said.

The per-patient evaluation was particularly important because coronary CT will ultimately be used to determine if invasive diagnosis is justified.

"We have to find occlusive disease and exclude it correctly," Nikolaou said. "This was successfully achieved with this patient cohort."

On a per-segment basis, eight false-positive findings and four false-negative segments were reported. In detection of significant stenoses, DSCT showed a sensitivity of 88%, specificity of 98%, positive predictive value of 89%, and negative predictive value of 100%.

For more online information, visit Diagnostic Imaging's RSNA Webcast.

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