With colon masses, it's what's on the inside that counts. Using intravenous contrast, imagers can better distinguish masses from stubborn stool or benign polyps from malignancies.While contrast enhancement is effective, it may be best reserved for symptomatic patients rather than the screening population, researchers reporting at the RSNA meeting said Thursday. Radiation dose and cost issues both figure in this assessment.
"There is still some controversy about whether IV contrast has value in CT colonography," said Turkish researcher Dr. Aytek Oto. "We found that the enhancement may help distinguish and characterize masses in the colon."
Carcinomas show significantly higher density than benign polyps, according to research by Oto and colleagues. Residue in the colon, of course, doesn't enhance at all, while both benign and malignant masses more than double in brightness 70 seconds after administration of 150 mL of IV contrast.
Among 28 symptomatic patients, researchers found 18 benign polyps and 21carcinomas, including two smaler than 5 mm. They were able to measure distinct values for each type: With contrast, polups enhanced to 82.1 Hounsfeld units, while carcinomas measured 89.1 HU.
Researchers in Frankfurt, Germany, are taking that distinction and using it as the basis for a preliminary CAD system, in which masses are color-coded according to their intensity and, therefore, the chance of malignancy.
Distinction between more subtle findings may not always be clear, said London researcher Dr. Alice Gillams. In her own investigation of 80 symptomatic patients, she found that while enhancement was similar, cancers enhance heterogeneously.
"It could be very useful it there is significant residual stool, or if the diagnosis is in doubt," she said. "It would be most useful in the 6 to 20-mm range -- larger masses are usually fairly obvious."
Gillams' protocol used 100 to 150 mL of IV contrast for prone and supine acquisitions before and after contrast. She used multislice CT at 1-mm collimation with 1.25-mm effective slice width and a pitch of 1.2. The scan produces 300 to 400 slices and delivers a dose of 8 to 10 mGy. The information obtained outweighs the high dose in an elderly, symptomatic population, she said.
