AJR: Higher contrast CT lowers dose, maintains image quality

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The use of high contrast enhancement may compensate for image degradation in CT scans with high noise indexes, allowing substantially lower radiation doses without compromising imaging quality, a study published in the October edition of the American Journal of Roentgenology found.

The study explained that while the use of low tube voltage for CT reduces radiation dose without degrading image quality, increased image noise does accompany the lower voltage, which "can hamper lesion detectability in the abdomen and pelvis, where tissue contrast is intrinsically low," the study's lead author, Haruo Watanabe, MD, from Gifu University Hospital in Gifu, Japan, and colleagues wrote.

"The purpose of our study was to prospectively evaluate the effect of varying noise index and iodine mass on radiation dose, contrast enhancement, image noise and image quality in contrast-enhanced abdominal CT," the authors wrote.

The study included 195 patients with suspected abdominal disease referred for abdominal CT for diagnostic workup or vascular structure evaluation. Patients were randomly divided into three groups, protocols A through C, each with 65 individuals. All patients underwent abdominal contrast-enhanced CT using a 16-slice scanner with 120 kVp automatic tube current modulation.

Protocol A was scanned using a noise index of 12 HU and iodine contrast mass of 521 mg I/kg. Protocols B and C underwent scanning with noise indexes of 15 HU and contrast masses of 521 and 600 mg I/kg, respectively. The higher noise indexes (15 HU) used in protocols B and C resulted in radiation doses 31 to 32 percent lower than the 12 HU noise index used in protocol A, the authors observed.

The researchers found no significant difference in diagnostic acceptability, depiction of vessels and subjective image noise between protocols A and C, while diagnostic acceptability was significantly lower in protocol B. "This finding suggests that an increase in iodine mass in the contrast material may compensate for image degradation due to a lower tube current output (higher noise index). Therefore, use of greater contrast enhancement may be an approach to reduction of radiation dose without compromising diagnostic image quality," according to Watanabe and colleagues.

No difference in depiction of arterial trunks and most arterial branches was observed between protocols, although spleric and left gastric arterial branch depictions were inferior in protocol B relative to protocol A. Depictions of inferior phrenic artery and inferior mesenteric arterial branches in protocol B were also inferior to protocols A and C. The findings were statistically significant.

Watanabe and colleagues also found that despite significantly greater contrast enhancement in protocol C, objective image noise was unaffected from the preset noise index. "This finding indicates that the administration of increased iodine mass does not compensate for increased quantitative image noise due to low tube current output."

The authors cited weaknesses to the study, including that they were unable to evaluate tumor detection, tumor conspicuity or tumor-to-liver contrast-to-noise ratio because of the low incidence of tumors within the sample. They also wrote that the sample body mass range, 29 to 88 kg, was narrower than a sample population range in Western countries, where more overweight patients may cause greater image degradation.

The authors concluded that the "study showed that in terms of subjective image quality and vessel depiction with CT angiography, CT scans acquired with higher contrast enhancement at 30 percent reduction in radiation dose are comparable to CT scans obtained with a full radiation dose."