The purpose of our study was to investigate the relationship between image quality and radiation dose for the filtered backprojection (FBP) with smooth kernel and the iterative reconstruction (iDose) based on image noise, image resolution, CT number, and low-contrast detectability. We used the Catphan phantom and scanned at 65, 45, 32, and 20 mAs on a 64-detector CT. Image reconstruction algorithm and kernel were employed FBP with standard (C type) and FBP with smooth (A type) kernel as images obtained at 20-65 mAs. Regarding to 20-45 mAs, we additionally reconstructed it using the iDose. After scanning, we measured image noise, full width at half maximum (FWHM), and CT number and assessed low-contrast detectability. Image noise acquired at iDose was 10.9, 11.1, and 11.2 HU corresponding to 45, 32, and 20 mAs, respectively. Compared to FBP with standard kernel, FBP with smooth kernel increased the image noise range from 6.7 HU at 65 mAs to 12.3 HU at 20 mAs with decreasing tube current-time product. Unlike iDose and FBP with standard kernel, there was a statistically significant difference between FBP with standard and smooth kernel with respect to image resolution (P=0.002). Reconstruction algorithm of the iDose resulted in the same or better image quality improvements despite a reduction in the radiation dose compared to the FBP with standard or with smooth kernel. From our findings, iDose facilitates the reduction in radiation dose while maintaining image quality.