Spine sign: visualization of the vertebral bodies in the thoracic cavity above the diaphragm – indicative of pleural effusion.

In the absence of pleural effusion, the spine is obscured by air in the lung, and is cut off at the diaphragm. Because fluid is a good transmitter of sound waves, spine is seen when there is fluid around the lung.

Increase the depth of the image to visualize the sign better.

While moderate to large effusions are obvious, spine sign particularly helps to detect small effusions with debris or clotted blood. Following is an example.

Short video on spine sign: