Mediastinal diseases include mediastinal tumors (benign and malignant), cysts, acute and chronic mediastinitis, mediastinal hernia, and mediastinal emphysema. The human thoracic cavity is divided into two pleural cavities, left and right, and the middle part of the two pleural cavities is called the mediastinum. The mediastinum contains the heart, large intra-thoracic blood vessels, trachea, esophagus, nerves and lymphatic tissues. The mediastinum can be divided into several areas, from the sternal angle (i.e., where the sternal stalk meets the sternal body, which can be felt on the body surface as a distinct transverse ridge) to the lower edge of the fourth thoracic vertebral body by drawing a horizontal line backward, above this line is called the upper mediastinum, below the line is called the lower mediastinum. The upper mediastinum is bounded by the trachea, the anterior part is the anterior superior mediastinum, and the posterior part is the posterior superior mediastinum. The lower mediastinum is divided into three parts: the anterior mediastinum before the pericardium, the middle mediastinum where the pericardium is located, and the posterior mediastinum between the pericardium and the spine. The anterior superior mediastinum mainly contains the thymus and the intrathoracic thyroid gland, while the posterior superior mediastinum contains the trachea, esophagus, aortic arch and its three cephalic-arm vascular branches, thoracic duct, vagus, and nerves. The lower anterior mediastinum has the lower thymus, lymph nodes, fat and connective tissues, etc. The lower posterior mediastinum has the esophagus, thoracic duct, descending aorta and its branches, the odd vein, the hemi-odd vein, the vagus and sympathetic nerves, etc. The following tests are available for mediastinal lesions: 1. Chest fluoroscopy Fluoroscopy is a direct examination of the patient placed between the X-ray tube and the fluorescent screen. It can do comprehensive dynamic direct observation, such as heartbeat, transverse septal activity, gastrointestinal motility, joint activity, etc. Fluoroscopy is also used in X-ray imaging for positioning observation, gastrointestinal imaging, and for indicating diagnostic operations such as cardiac catheterization, fracture repositioning, foreign body removal, etc. X-ray fluoroscopy is most commonly used in the chest to examine the lungs, pleura, mediastinum, heart, and large blood vessel lesions. It can also be used for the examination of limb bone, soft tissue, foreign body in body cavity and gas, stone, contraceptive ring, etc. 2.Lung perfusion imaging The lung perfusion imaging agent is 99mTc-labeled large polymerized human serum protein particles (99mTc-MAA) or 99mTc-labeled microspheres with a diameter of about 10-30μm and a dosage of 0.5-1.5mg, which is about 100-300,000 particles. When the reagent is injected into the vein, it enters the right heart with the blood, mixes well with the blood in the right heart, and then is perfused into the vascular bed in the lung via the pulmonary artery with the blood flow, and the larger particles can temporarily embolize the pre-capillary vessels of the lung, while the smaller ones embolize the pulmonary capillaries. （The amount of (99mTc-MAA) embolization in the vascular bed of each part of the lung is proportional to the local blood perfusion, so the image of radioactive distribution of (99mTc-MAA) embolization in the lung taken by scanner or γ camera is the image of blood perfusion in the lung, and the higher radioactivity indicates better local blood perfusion, and the lower indicates poorer blood perfusion. If the radioactivity is absent in a certain area, it means that there is no blood perfusion in that area, which means that the blood vessels perfusing that area are already blocked or occluded. The distribution of radioactivity in normal human lungs is basically uniform. Due to the influence of gravity on blood flow and (99mTc-MAA), the radioactivity in the lung tip is slightly lower when injected in sitting position, and slightly higher in the dorsal lung when injected in supine position. The number of vessels blocked in one imaging session is only a few parts per million and a few parts per million of the total number of pulmonary precapillary vessels and capillaries, so it does not cause significant hemodynamic changes and changes in lung function. 3.Chest plain film Chest plain film, also known as chest radiography. The X-ray images obtained from radiography are much clearer than those from fluoroscopy, with the following advantages: (1) the images of the examined area are permanently retained on the film for analysis, discussion and review; (2) they can be kept as scientific research data; (3) radiography can show microstructures, such as early sources of disease above 2 mm, which are clearer than fluoroscopy. (4) radiography can examine thicker parts of the body and enable patients to receive less X-ray. 4.CT examination of the chest CT examination of the chest is a method of examining the chest through X-ray computed tomography (CT).