This prospective, randomized single-center study evaluated vascular attenuation, volumetric lung iodine density, and overall image quality in dual-layer spectral CT pulmonary angiography (CTPA) performed using three different iodinated contrast medium protocols. A total of 150 patients undergoing CTPA for suspected pulmonary embolism were equally assigned to one of three protocols: 40 mL at 4 mL/s (protocol A), 30 mL at 3 mL/s (protocol B), and 20 mL diluted with 20 mL saline at 4 mL/s (protocol C). On conventional images, pulmonary artery attenuation exceeded 200 HU in over 90% of cases studied with protocols A and B, compared with 70% of those imaged using protocol C. Nonetheless, low-energy virtual monoenergetic reconstructions significantly increased vascular attenuation across all groups, achieving a minimum value of 269 HU. Despite this improvement, protocol C showed significantly lower pulmonary artery enhancement compared with the higher-dose protocols. Aortic enhancement remained diagnostically adequate at low-energy reconstructions in all groups. Although protocol C yielded the poorest iodine map quality and lowest volumetric lung iodine density, perfusion defects were successfully detected in all cases of pulmonary embolism. Overall, dual-energy spectral CT with low monoenergetic imaging ensured diagnostic vascular attenuation across all protocols. Notably, even the lowest contrast dose maintained diagnostically usable iodine maps, albeit with reduced iodine density and image quality.