Objective:To comprehensively and accurately analyze the out-performance of low-dose chest CT(LDCT)vs.standard-dose CT(SDCT).Methods:The image quality,size measurements and radiation exposure for LDCT and SDCT protocols were evaluated.A total of 117 patients with extra-thoracic malignancies were prospectively enrolled for non-enhanced CT scanning using LDCT and SDCT protocols.Three experienced radiologists evaluated subjective image quality independently using a 5-point score system.Nodule detection efficiency was compared between LDCT and SDCT based on nodule characteristics(size and volume).Radiation metrics and organ doses were analyzed using Radimetrics.Results:The images acquired with the LDCT protocol yielded comparable quality to those acquired with the SDCT protocol.The sensitivity of LDCT for the detection of pulmonary nodules(n=650)was lower than that of SDCT(n=660).There was no significant difference in the diameter and volume of pulmonary nodules between LDCT and SDCT(for BMI＜22 kg/m2,4.37 vs.4.46 mm,and 43.66 vs.46.36 mm3;for BMI≥22 kg/m2,4.3 vs.4.41 mm,and 41.66 vs.44.86 mm3)(P＞0.05).The individualized volume CT dose index(CTDIvol),the size specific dose estimate and effective dose were significantly reduced in the LDCT group compared with the SDCT group(all P＜0.000l).This was especially true for dose-sensitive organs such as the lung(for BMI＜22 kg/m2,2.62 vs.12.54 mSV,and for BMI≥22 kg/m2,1.62 vs.9.79 mSV)and the breast(for BMI＜22 kg/m2,2.52 vs.10.93 mSV,and for BMI≥22 kg/m2,1.53 vs.9.01 mSV)(P＜0.0001).Conclusion:These results suggest that with the increases in image noise,LDCT and SDCT exhibited a comparable image quality and sensitivity.The LDCT protocol for chest scans may reduce radiation exposure by about 80％compared to the SDCT protocol.