Thoracic spine sign (ultrasound)
The thoracic spine sign, or spine sign, on lung ultrasound is an indirect indicator of the presence of a pleural effusion or hemothorax. It represents the visualization of the vertebral bodies in the thoracic cavity above the diaphragm which are usually not seen unless there is a fluid collection.
A curvilinear or low-frequency probe can be used and is placed longitudinally in the lowest or dependent area of the lung where fluid tends to collect. This corresponds to the right or left upper quadrant views similar to that of a FAST scan. The diaphragm is identified, below which vertebral bodies can be seen as hyperechoic or bright white structures .
When no pleural fluid is present, there is a sharp cut-off of the vertebral bodies at the level of the diaphragm, above which they cease to be seen due to air in the lungs impeding the transmission of ultrasound waves; hence deeper structures (namely the vertebral bodies) cannot be visualized .
Thoracic spine sign
In the presence of a pleural fluid collection, the transmission of ultrasound waves is enhanced, allowing us to visualize the presence of the vertebral bodies; which are seen as a continuous hyperechoic line extending both above and below the diaphragm .