[en] Objective: To investigate the feasibility of reduced radiation dose for CT pulmonary angiography (CTPA) and the possible lowest radiation threshold by a phantom study. Methods: The CT value difference between air within the trachea and the extracorporeal background region was measured in 132 consecutive patients. A noise-measurement phantom and a pulmonary embolism (PE) phantom were made of phenol-formaldehyde, and both phantoms and a water phantom were scanned with standard and lower radiation doses as follow: 280, 200, 160, 100, 90, 80, 70, 60, 50, 40, 30, 20, 15, and 10 mA respectively, at a fixed voltage of 120 kVp. Standard and soft tissue algorithms were used to reconstruct the images. Three experienced doctors independently evaluate the image quality and the efficiency of detecting PE of the images with various doses. The Pearson correlation analysis, two-tailed paired t test, ANOVA, and Kappa test were employed for the statistical analysis. Results: The CT value difference between air within the trachea and the extracorporeal background region in 132 consecutive patients ranged from 20. 00 to 55.00 HU, which had a positive correlation with weight [(64.99±11.86) kg], weight-height ratio [(38.71±6.13)kg/m], and BMI[(23.11±3.38) kg/m²] (r=0.228,0.374,0.449 respectively; P< 0.01). The image noise level with soft-tissue reconstruction algorithm [(16.55±9.08), (16.42± 9.40) HU] was significantly lower than that of the image with standard reconstruction algorithm [(22.43± 11.25), (21.99±11.67) HU] (F=4.316, P<0.05). The image noise level with soft-tissue reconstmction algorithm at 100 mA was similar to that of the images with standard reconstruction algorithm at 280 mA, and the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of the image of PE phantom was 23.05 and 20.52 respectively, without any impairment in detectability of embolus. The image noise level with soft-tissue reconstruction algorithm at 60 mA was similar to that of the image with standard reconstruction algorithm at 160 mA, while the SNR and CNR was 18.01 and 15.97 respectively, also with acceptable detectability of embolus. When the tube current was reduced below 30 mA, the image quality decreased significantly, with the SNR and CNR was lower than 12.36 and 10.95 respectively, and the detectability of embolus was degraded. The consistency of the image quality grading by 3 observers was excellent (K=0.807,0.712,0.904, respectively; P<0.01). Conclusions: The 100 mA may potentially be the ideal low dose tube current setting, with radiation dose only equal to 36% of 280 mA (standard dose). The 30 mA may possibly be a minimum radiation dose for detecting PE. The soft-tissue reconstruction algorithm was favorable in preserving the SNR when the radiation dose was reduced. (authors)