A CT scan of pulmonary embolism simply shows a filling defect in the pulmonary artery, that is, by injecting contrast into a peripheral vein, the contrast will flow back through the vein into the heart and will be distributed into the artery. Therefore, during the CT of pulmonary embolism, we will scan the pulmonary artery at the time when the contrast agent is expected to flow back into the heart and into the pulmonary artery, and pay special attention to this time period. In this process, where the contrast is developed, simply put, where it is seen to be bright, these conditions will be seen. However, there are some places that are not bright and are missing a piece, and this filling defect is usually considered to be a sign of pulmonary embolism in this area. Under normal circumstances, there should be a complete and adequate contrast, and if there is no contrast, it is usually considered to be a lesion of pulmonary embolism.