Veterinary Ultrasound examination can confirm the presence of pleural effusion in animals and indicate its characteristics; identify potential lesions, such as anterior mediastinal masses; and guide interventional or therapeutic thoracentesis. Pleural effusions appear as echogenic or anechoic material that separates the chest wall from the diaphragm and lung surface. The echogenicity of the fluid is affected by the concentration of free cells. Transudates, modified transudates, and chyles are usually hypoechoic or anechoic. Exudates, malignant pleural effusions, or hemorrhages are usually echogenic. Changing body position can help veterinary ultrasound distinguish between free fluid and cystic fluid. Free fluid usually accumulates on the gravity side of the chest cavity, while cystic fluid is not affected by gravity and always remains in the same position in the chest cavity.

In the quantitative assessment of pleural effusion, veterinary ultrasound measurement methods are preferred to veterinary X-ray

measurement methods. In the field of research, veterinary CT can accurately and quantitatively assess the amount of pleural effusion. Although it is widely believed in human medicine at home and abroad that ultrasound examination is more sensitive than X-ray examination for the detection of pleural effusion.

For medium-sized dogs, when the amount of pleural effusion reaches 100ml, the interlobar fissure will widen and can be visualized on X-ray films. By gradually injecting normal saline into the left pleural cavity (non-gravity side) of the experimental dog lying on its right side, observing the changes in the right pleural cavity (gravity side), try to find the lowest limit of veterinary B-ultrasound detection of pleural effusion. As the amount of fluid injected into the pleural cavity increases, the lung lobe gradually separates from the chest wall, and a small amount of echo-free liquid dark area can be seen in the gap. During exhalation, the liquid level is deeper than during inhalation. This change is obvious at the apex of the lung.