Automatic Tube Current Modulation versus Fixed Tube Current in Multi-detector Row Computed Tomography of Liver: Comparison of Image Quality and Radiation Dose

Abstract

The purpose of this study was to compare the image quality, diagnostic acceptability, and radiation exposure between z-axis automatic tube current modulation (ATCM) and fixed tube current techniques in multi-detector row computed tomography (CT) of liver. One hundred and eighty-two patients referred for performing dual-phase contrast-enhanced CT examination to assess liver tumors were divided, according to the study periods, into four groups based on different scanning parameters: (1) 100 kVp, fixed current (350 mA); (2) 100 kVp, ATCM; (3) 120 kVp, fixed current (350 mA); (4) 120 kVp, ATCM. Patient medical records of body height, body weight and body mass index (BMI) were obtained. We used signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) to quantitatively evaluate the quality of hepatic CT images and CT angiography. Radiation dose measurements were generated automatically by the CT unit with calculation of dose-length products (DLP). The axial CT images and post-processing 3D CT angiographic images were reviewed by two subspecialist radiologists who were blinded to the CT parameters. They ranked subjective image quality using a four-point quality rating independently. Data were analyzed with statistical tests. The results showed that the averaged tube current of un-enhanced images in the ATCM groups was 22.7% lower than in the fixed tube current groups. As compared with fixed tube current groups, the ATCM groups decreased 8.6% and 8.7% at 100 kVp, and 44.2% at 120 kVp on arterial and venous phases respectively. The mean effective dose of the ATCM group was lower than in the fixed tube current group by 6.2% and 35.9% at 100 kVp and 120 kVp respectively. There was basically no significant difference in the quantitatively assessment of CT and CTA image quality between the ATCM and fixed tube current groups. Both 100 kVp and 120 kVp ATCM groups showed positive correlation coefficients between the tube current and BMI or body weight (100 kVp, r=0.663, 0.724; and 120 kVp, r=0.792, 0.789 respectively). In the image quality indices of hepatic CT and CTA, no significant difference existed among the four groups or between the two radiologists. In conclusion, by using ATCM, the radiation exposure and effective radiation dose of dynamic contrast-enhanced multi-detector row CT of the liver could be effectively reduced with maintenance of the image quality.