OBJECTIVE: To investigate the differences in imaging quality and radiation dose in CT pulmonary angiography (CTPA) by using fast-kV switching dual energy CT imaging and 3D Smart mA modulation at different body mass indices (BMIs) and at different noise index (NI) values with an adaptive statistical iterative reconstruction (ASIR) algorithm. METHODS: Four hundred patients who underwent CTPA were equally divided into two groups: A (18.5 kg/m(2) <= BMI < 24.9 kg/m(2)) and B (24.9 kg/m(2) <= BMI <= 4.9 kg/m(2)). The groups were randomly subdivided into four subgroups (n = 50): A1-A4 and B1-B4. The patients in subgroups A1 and B1 underwent fast-kV switching dual energy CT imaging. The other patients underwent 3D Smart mA modulation with the ASIR algorithm at NI values 26, 36, and 46 for A2/B2, A3/B3, and A4/B4, respectively. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of all images were calculated after CTPA. Images were then subjectively evaluated using a 5-point scale. The volume CT dose index and dose-length product (DLP) were recorded and their means calculated. The DLP was converted to the effective dose (ED). RESULTS: In group A, the SNR, CNR, and subjective image scores showed no statistical differences (P > 0.05). The ED in subgroup A4 was 67.12% and 31.53% lower than that in A1 and A2, respectively. In group B, the variables showed no significant differences between the subgroups B3, B1, and B2 (P > 0.05). The ED in subgroup B3 was 50.12% and 35.95% lower than that in B1 and B2, respectively. CONCLUSIONS: Setting different NI values according to BMIs and applying the ASIR algorithm can more effectively reduce the radiation dose in CTPA than in fast-kV switching dual energy CT, while maintaining image quality. Imaging may be performed at NI = 46 in patients with lower BMI (group A) and at NI = 36 in patients with higher BMI (group B).