Radiation Dose and Contrast Agent Reduction In Abdominal CT Examination with Low Tube Voltage

A Phantom Study

Thinh Xuan Tran, MD.

Objectives

The purpose of study was to investigate image quality and radiation dose of multi-detector computed tomography (MDCT) examination on an abdominal phantom using a standard protocol and a low tube voltage (kV) protocol with the low contrast agent dose at different tube current-time products (mAs).

Materials and Methods

The cylindrical phantom of a 10 cm thick and 30 cm diameter was done from the minced pork roll and used as the abdominal CT scan. The center of the phantom was a contrast enhanced injection syringe of the 40 mm diameter as a target object. The inside of the phantom contained a smaller cylinder module with the 8 cm diameter of low HU tissue equivalent background material.

The contrast-enhanced phantom CT examination was performed by using with different tube voltage protocols of 120 kV and 80 kV at different tube currents 250-650 mAs. The volume CT dose index (CTDIvol), CT number, image noise, contrast to noise ratio (CNR) and dose length product (DLP) were recorded. Image quality is semiquantitatively assessed by two blinded radiologists using a five-point scale. The differences between mean values of CNR, image noise and scores of LCD (low contrast detectability) obtained at 120 kV with 300 mAs and 80 kV with 250-650 mAs were compared respectively. The SPSS version 18 software was used to analyze all statistical data. A statistically significant difference when P value < 0.05 was considered.

Results

The radiation dose was reduced by 38% at 80 kV and 650 mAs. The values obtained at 80 kV and 250-650 mAs, compared to the values obtained with 120 kV at 300 mAs. The iodine CT number with 80 kV at 650 mAs was significantly higher (P=0.006) while the noise value obtained at 80 kV with 650 mAs was higher than that obtained at 120 kV with 300 mAs but no significantly difference (P=0.052). For quality image, there were no significantly differences in mean values of CNR and mean scores of LCD. The CNRs obtained between two protocols 120 kV at 300 mAs and 80 kV at 650 mAs were 69.37 and 65.13 (P=0.091). The mean scores of LCD assigned at 120 kV with 300 mAs and 80kV with 450-650 mAs (P>0.05).

Conclusions

The radiation dose can be significantly reduced by up to 38% at low tube voltage protocol of 80kV and with reduced contrast agent dose of 16% while maintaining compatible image quality without degradation of CNR and LCD. The study has shown the greatest benefits in children, small sized adults and contrast CT examinations.

Keywords: computed tomography, radiation dose, low tube voltage, low contrast agent dose and image quality.