Seesaw balancing radiation dose and i.v. contrast dose: evaluation of a new abdominal CT protocol for reducing...

The purpose of this study was to evaluate an abdominal CT protocol in which radiation dose was reduced and i.v. contrast dose increased for young patients and radiation dose was increased and i.v. medium dose decreased for elderly patients. The hypothesis was that these adjustments would result in constant image quality and a reduction in age-specific risk.

Patients were divided into four age groups of 25 patients each: group 1, 16-25 years; group 2, 26-50 years; group 3, 51-75 years; and group 4, older than 75 years. The quality reference tube load ranged from 100 to 300 mAs, and the i.v. contrast dose ranged from 600 to 350 mg I/kg. Group 3 was the reference group. Signal-to-noise and contrast-to-noise ratios for a hypothetical hypovascular liver metastatic lesion were calculated. Subjective image quality was evaluated by visual grading characteristic analysis in which four readers assessed the reproduction of seven image-quality criteria.

Radiation dose was reduced 57% in the youngest group, and the i.v. contrast dose was reduced 18% in elderly patients. There were no statistically significant differences between the groups with respect to signal-to-noise and contrast-to-noise ratios. Subjective image quality was graded significantly lower for four criteria in group 1 compared with group 3. No significant difference was found in comparisons of groups 2 (except for one criterion) and 4 with group 3.

It is possible to balance radiation dose and contrast dose against each other and maintain signal-to-noise and contrast-to-noise ratios. Subjective image quality was affected by increased noise level on the images but was judged acceptable in all groups except the one with the lowest radiation dose.