Lung perfusion imaging can be harmful to the human body, including obstructive retention during the examination, as well as the use of contrast agents with a certain amount of radiation, which can cause some radiation damage to the human body, but is relatively mild. In addition, there are some people who have contraindications to the test, and also need to avoid the test, otherwise it will cause significant harm to the body. 1, obstructive retention: lung perfusion refers to the use of radionuclide-labeled radionuclide tracer larger than the diameter of the pulmonary capillaries (8μm), less than the diameter of the precapillary artery (100μm) from the vein injection, the particles with the blood flow through the right heart after mixing to reach the lung The particles are mixed with the right heart and reach the pulmonary capillary network, where they are embedded in the capillaries because their diameter is larger than the diameter of the pulmonary capillaries. The number of particles distributed in the pulmonary capillary network is proportional to the blood flow in this area, and thus can be used to reflect pulmonary blood flow, such as the diagnosis of pulmonary artery infarction and pulmonary hypertension. However, obstructive retention occurs when there is embolism in the pulmonary capillaries. 2. Radiation injury: The contrast agent used to perform lung perfusion is a substance with some radiation, which usually causes slight radiation injury to the body, such as causing a small amount of normal cell death. It is usually necessary to follow medical advice and choose the appropriate contrast agent according to the subject’s condition. And to drink more warm water to speed up its excretion, can reduce the possible harm to the human body; 3, other cases: there are some people can not be lung perfusion imaging, such as pregnant women, and other patients with serious heart and lung diseases, such as severe heart and lung failure, serious heart valve disease, arrhythmia, etc., otherwise it may lead to aggravation of the disease, or can not be well developed.