Sinusoid sign (ultrasound)

The sinusoid sign is a dynamic sonographic sign, present when respiratory variation decreases the distance between the parietal and visceral pleura, when separated by a pleural effusion. Classically demonstrated in M-mode, the appearance of which the moniker is derived, it is specific for the identification of a pleural effusion, although insensitive, as it may be absent with dense or heavily septated collections.

A subtype of the sinusoid sign is the "jellyfish sign," which alludes to the supposed appearance of this respiratory dynamic on B-mode.

The dynamic counterpart to the quad sign, it demonstrates the fluid dynamic of the collection observed, thus distinguishing a pleural effusion from other lung pathology, such as consolidation, or a hiatal hernia, both of which may appear anechoic and mimic the sonomorphology of a pleural effusion.