In recent years, domestic and international studies have confirmed that the two lesions, pulmonary embolism (PTE) and deep vein thrombosis (DVT), are different manifestations of one pathological process. Some data show that 60% of DVT patients can lead to PTE, and 90% of PTE is from DVT. this shows that the incidence of DVT and PTE is high, and also shows that the two are inseparable pathological processes. Therefore, early diagnosis and early treatment are crucial. Direct lower extremity venography has long been the gold standard, but this angiography often shows the femoral vein and inferior vena cava poorly and has misdiagnosis, and now this standard has been challenged by other diagnostic imaging methods, such as CTV and MRV, which are non-invasive for the diagnosis of DVT and are very meaningful. In this paper, we especially recommend and discuss the diagnostic value of indirect venography for DVT. I. CT venography includes two methods of direct CT venography and indirect CT venography. 1, direct CT venography: direct venography refers to the imaging method of CT scan with iodine contrast agent injected from the dorsal foot vein in order to show the inferior vena cava and the whole abdominal vein well. The contrast agent is non-ionic type such as iodophoresis (300mgI/ml) 40ml, diluted with 200ml saline, injection speed 2ml/s, 35 seconds delay after injection to start the scan, scan range from ankle to inferior vena cava, single helix Pitch can be 2, MSCT available 1:6, 120KV, 250mA, layer thickness 5mm. The thrombus can be seen as an intraluminal filling defect. Computerized post-processing of raw scan data, such as MRP, CPR, SVR, etc., is also available. Compared with indirect CT venography, this method is invasive and the injected vein is prone to thrombosis. If deep vein thrombosis occurs, it can be injected from superficial vein, and only superficial vein is easy to be visualized. The superficial vein filling compresses the deep vein, and it is not easy to visualize, so it is not used now. 2.Indirect CT venography: It is a new diagnostic method of deep vein thrombosis in recent years, which is a continuation of spiral CT pulmonary arteriography (CTPA) after intravenous injection of contrast agent, without re-injection of contrast agent, and can simultaneously examine the pulmonary artery, abdominopelvic and lower limb deep veins to clarify the presence of pulmonary embolism and lower limb deep vein thrombosis. It has been shown that 50% of patients with deep vein thrombosis can lead to PTE, and it is considered that these two diseases are different manifestations of one pathological process, thus it is advocated that when examining the pulmonary artery, the presence of DVT must be examined at the same time. The technique is as follows: the patient is placed supine on the examination table, and after completion of single- or multi-layer spiral CT pulmonary angiography, instead of injecting contrast, a transverse scan of the abdominopelvic and lower extremity veins is performed 2.5-3 minutes after the original contrast injection. The scan is performed from the ankle to the lower edge of the diaphragm, with a layer thickness of 5 mm and an interval of 30 mm, usually 25-30 layers. The heel is padded with a blanket or pillow before the scan to avoid poor visualization due to compression of the calf veins. Second, lower extremity vein anatomy Statistics show that 50%-90% of DVT can lead to PTE, DVT formation in femoral vein or N vein accounts for 50%, while calf vein is less than 5%, rare superior vena cava system and right atrium. The pelvic veins and lower extremity veins are the greatest risk for acute venous thrombosis. The veins of the lower extremities are divided into a deep vein group and a superficial vein group. The deep vein group is accompanied by arteries and includes the common femoral vein trunk, deep femoral vein, superficial femoral vein, N vein, anterior tibial vein, posterior tibial vein, and peroneal vein. The greater saphenous vein, the largest superficial vein, is injected into the femoral vein on the medial side of the thigh, and the lesser saphenous vein is injected into the N vein on the lateral side of the leg. A venous valve blocks the return of blood, allowing blood to flow in one direction from the superficial veins to the deep veins. When deep vein embolism occurs, the vein thickens and the valve is damaged, making the valve close incompletely and blood flow from the deep vein to the superficial vein, resulting in superficial varicose veins. III.CT signs of DVT The filling defect due to DVT is a partial or complete intraluminal filling defect that is surrounded by a ring of enhanced blood flow with higher CT values, usually seen at two consecutive levels. Sometimes blockage of venous branches at the embolus is seen, and collateral circulation formation is seen; the deep venous group is obstructed and superficial venous dilatation is seen. It may also show luminal narrowing. This method was first proposed by Loud in 1998 as a diagnostic method to obtain information on both PTE and DVT. CTPA contrast is applied without additional intravenous contrast to rapidly visualize the lower extremity veins, pelvic veins and inferior vena cava to detect the presence of thrombosis. This imaging procedure considers thromboembolism as an integral disease and uses a rapid single test to identify the cause (vein) and target organ (pulmonary artery) of thromboembolism.