Bat sign | Radiology Signs

Answer:

The bat sign is the normal anatomical landmark of a longitudinal intercostal scan: two adjacent ribs with their acoustic shadows form the wings and the bright pleural line between and slightly deeper than them forms the body of the bat. Identifying it is the mandatory first step of any lung ultrasound examination, because it proves that the operator has located the pleural line correctly; every subsequent finding, lung sliding, A-lines, B-lines, consolidation and effusion, is defined by its relationship to that line. Once the bat sign is obtained, the pleural line is assessed for sliding, thickness, irregularity and small subpleural consolidations, and the field below it for the horizontal reverberation A-lines of normal aerated lung and the vertical B-lines of interstitial syndrome, where three or more B-lines in one intercostal space indicate an increased extravascular lung water or interstitial process. Standardised protocols such as BLUE for acute respiratory failure and the eFAST examination in trauma depend on this landmark, and comparing zones between hemithoraces is how focal disease is separated from diffuse. Errors follow directly from misidentifying the pleural line: the fascial planes of the chest wall can mimic it, in which case sliding is falsely absent, and scanning in the sagittal plane between ribs loses the reference points altogether.

Why is it called so?

The two rib shadows and the pleural line between them outline the wings and body of a bat.

Pathophysiology

Ribs are strongly reflective and cast complete acoustic shadows, so their superficial cortex appears as a bright convex line with darkness beneath. Between them, the beam passes through soft tissue to the pleura, where the large impedance mismatch between soft tissue and air produces a bright specular reflection, the pleural line, lying about half a centimetre deeper than the rib cortex. This fixed anatomical relationship makes the appearance reproducible anywhere on the chest wall and independent of pathology, which is why it is used as the reference landmark. All the artefacts of lung ultrasound arise at this air-tissue interface: A-lines are repeated reverberations between the pleura and the transducer, and B-lines arise when fluid-thickened interlobular septa allow the beam to ring down into the lung.

Alternative names: Bat wing sign of the chest wall (not to be confused with bat wing pulmonary oedema)

Other associated named signs: Seashore sign, A-lines and B-lines, and the lung point sign

References

1. Beshara M, Bittner EA, Goffi A, Berra L, Chang MG. Nuts and bolts of lung ultrasound: utility, scanning techniques, protocols, and findings in common pathologies. Crit Care. 2024;28(1):328. PMID: 39375782.