Optimal Setting of Automatic Exposure Control Based on Image Noise and Contrast on Iodine-enhanced CT

Abstract

The aim of this study was to investigate variations in image noise and contrast using automatic exposure control (AEC) and different tube voltages on nonenhanced and iodine-enhanced hepatic computed tomography. Nonenhanced and iodine-enhanced simulated liver phantoms and AEC were used. Tube current was automatically adjusted with the noise index. Two types of assessments were performed: at a fixed noise index of 10 Hounsfield units and at different noise indexes, keeping the same contrast-to-noise ratio at different tube voltages (100, 120, and 130 kV). Image noise was measured, and contrast between the computed tomographic number of the simulated liver and nodule was computed. At a fixed noise index, image noise on iodine-enhanced images was 10% to 13% higher than on nonenhanced images at the same tube voltage. At 130 and 100 kV, contrast was 33.86 and 46.90 Hounsfield units, respectively, and image noise was almost the same. Contrast-to-noise ratios at 100, 120, and 130 kV were 3.31, 3.22, and 3.37, respectively, and volume computed tomographic dose index fell from 22.94 to 12.49 mGy with decreasing tube voltage. With AEC, image noise on iodine-enhanced images was higher than on nonenhanced images despite identical noise index settings. As tube voltage decreased, contrast on iodine-enhanced images increased. Considering noise index and contrast variations at different tube voltages, the optimal use of AEC on iodine-enhanced computed tomography facilitates a reduction in x-ray tube output while maintaining contrast-to-noise ratio.