Abnormal low-density shadow in the portal vein usually refers to portal vein thrombosis, and the possible causes are venous thrombosis or cancer embolism. 1. Portal vein thrombosis: usually refers to complete or partial obstruction of blood flow in the portal vein and branch veins due to blood clotting or embolus dislodgment in the blood vessel, which is common in patients with cirrhosis and portal hypertension. 2. Cancer embolism: Cancer embolism is a malignant lesion in which cancer cells have progressed to the blood vessels, leading to embolism in the blood vessels. It is a malignant lesion. It is considered that the disease has entered the terminal stage and needs timely treatment, which is commonly found in hepatocellular carcinoma. If abnormal low-density shadow in portal vein is found on examination, the cause of the disease should be further investigated, the diagnosis should be clarified and the treatment should be standardized.