Pneumonia and dynamic air bronchograms
In general, evaluation of an air-rich organ is a limitation of ultrasound because air scatters the ultrasound beam and does not allow proper visualization of the underlying structures. Therefore, normal lung tissue cannot be visualized unless it is consolidated. To give a rough idea, the estimated subpleural air content in pneumothorax, normal lung, interstitial syndrome, alveolar syndrome, atelectasis and pleural effusion is 100%, 98%, 95%, 10%, 5% and 0% respectively. Consolidations are highly fluid-filled, and over 95% reach the pleura, so ultrasound can image the pathology directly and lung appears like liver (= hepatization).
As we know, air bronchogram refers to the phenomenon of air-filled bronchi being made visible by the opacification of surrounding alveoli. It is almost always caused by an airspace disease, in which something other than air fills the alveoli. On a sonogram, air bronchograms appear as white structures (air is white on ultrasound). They are punctiform if transverse to the beam and linear if longitudinal. An air bronchogram which moves with respiration (= dynamic air bronchogram) excludes bronchial obstruction and helps distinguish between consolidation and atelectasis. In a hepatized lung, dynamic air bronchograms make pneumonia more likely, while static or no air bronchograms make atelectasis more likely. In a study, the dynamic air bronchogram had a specificity of 94% and a positive predictive value of 97% for pneumonia. However, the absence of dynamic air bronchograms does NOT rule out pneumonia. That’s when the overall clinical presentation becomes important to make a diagnosis.
The following images show atelectasis with static air bronchograms. Pleural effusions are often associated with some degree of lung collapse at the bases.