Pitfalls

Disease states other than pneumonia can also result in appearance of consolidation on ultrasound—just as in X-ray. Consolidation is not specific to pneumonia and must be clinically correlated. Any disease state that results in loss of aeration of alveoli either through fluid accumulation or collapse, including pulmonary contusions, hemorrhage, and atelectasis, can appear as areas of sonographic consolidation.

While there are some findings that can help differentiate alveolar disease from atelectasis, these are not always reliable. In pneumonia, gas moving within the bronchioles of consolidated lung can sometimes be seen and will appear as bright dots moving in a liner fashion through an area of consolidation. These are known as dynamic air bronchograms and are generally thought to rule out atelectasis. Atelectasis can also be differentiated from pneumonia in that surrounding b-lines are unlikely to be present. Color doppler can be applied to the area of hepatization to assess flow to the area, which will be decreased or absent in cases of atelectasis but normal or increased in pneumonia.

Video 2: Dynamic air bronchogram differentiating consolidation from atelectasis.

 

In children the thymus can sometimes be visualized and confused for hepatized lung. The thymus appears as hypoechoic structure with bright echogenic foci scattered throughout and is often described as a “stary sky” appearance. Apart from its anatomic location on the chest, the thymus also has clearly demarcated margins due to a fibrous capsule and no surrounding b-lines or shred sign. Additionally, if color doppler is applied there is hypo vascularity compared to an area of consolidated lung.

Video 3: Thymus, well demarcated hypodense area with a “starry sky” appearance.

If the probe is not directly over the lung but is over the liver or spleen, the diaphragm can at times act as a reflective surface giving the mirror image of the liver or spleen appearing above the diaphragm. This should not be mistaken with hepatized lung—hepatized lung can be imaged directly through the pleura and will not disappear with fanning of the probe. A mirror image will disappear as the transducer is fanned or moved cranially above the diaphragm.

Ultimately clinical context is most useful in determining the etiology of positive ultrasound findings.

Also remember:

• Some areas of lung are difficult to access and image (supraclavicular, scapular, axillary)
• Consolidation that does not reach the pleural line cannot be seen with LUS
• Early pneumonia without consolidation will mimic other aetiologies