Lung Ultrasound: Pneumonia

Pneumonia is an inflammatory, most commonly infectious process involving the lungs. Typically the alveoli in intensely inflamed areas fill with inflammatory fluid or pus, and this is known as consolidation. The changes may be widespread, patchy or lobar.

Ultrasound can detect the pulmonary changes associated with pneumonia as long as the process involves some of the outer (non-mediastinal) pleural surface – as it almost always does.

Pneumonia progresses though stages, and the ultrasound changes vary depending on the degree and extent of consolidation. Pathologists have long described the mascroscopic changes associated with consolidation as “hepatization” of the lung. The sonographic appearance of frank consolidation looks remarkably liver-like and is also termed hepatization.

Typical ultrasound features of pneumonia

Ultrasound appearance of pneumonia explained

The ultrasound appearance of pneumonia

Early pneumonia – B-lines and tiny areas of sub pleural consolidation

• In early pneumonia fluid fills only some of the alveoli.
• Where fluid filled alveoli are surrounded by air filled lung, B-lines, a form of short path reverberation artefact result.
• In the appropriate clinical setting a localised patch of numerous B-lines, often with tiny areas of sub pleural consolidation, suggests early pneumonia.

Solid appearing consolidated lung – hepatization

• Inflammatory and purulent fluid fills the alveoli and the lung appears solid, with a homogenous relatively fine echotexture similar to liver.
• Atelectasis also results in solid, non-aerated lung and differentiating the two conditions can be difficult. In pneumonia the volume of lung remains unchanged or actually increases, with airspaces filled with inflammatory liquid.
• In atelectasis the alveoli are collapsed rather than fluid filled, and lung volume reduced. Ultrasound may help to identify the cause of atelectasis – like a large pleural effusion. More subtle findings can be include scalloped pleural borders as the pleura are pulled in toward the collapsed lung, and crowded bronchi no longer separated by the same volume of aerated lung.
• Clinical features in both history and examination may also help differentiate pneumonic consolidation from atelectasis and its numerous causes.

Irregular consolidation / air interface – the shred sign

• If there is lobar consolidation the borders of the consolidated areas will be linear and well defined – as consolidated and aerated lung lie adjacent on opposite sides of the linear pleural fissures.
• More commonly there are smaller areas of consolidation. Where these abut the pleural surface they are linear, but their deeper borders usually demonstrate an irregular interface with underlying aerated lung. This irregular junction between consolidated and aerated lung is known as the “shred sign”.

Aerated bronchi – air bronchograms and dynamic air bronchograms

• Air within the consolidated area may remain in small aerated patches of lung, or more commonly air remains within small bronchi. The echogenic appearance of small air bubbles all lined up within a bronchus is known as the the sonographic air bronchogram.
• When the small bubbles in an air bronchogram can be seen to bubble in and out with each breath the term “dynamic air bronchogram” is used. This means that there is no complete bronchial obstruction and infers the solid lung is more likely due to true consolidation rather than collapse associated with reabsorption atelectasis.
• With time the air within air bronchograms is replaced with fluid. Fluid filled bronchi have a branched hypoechoic appearance with relatively echogenic walls. Colour Doppler fails to demonstrate flow within the lumen.

Colour Doppler interrogation – flow remains

• The pulmonary arterial and venous vasculature is well demonstrated in areas of consolidation.

Associated pleural effusion or empyema

• A small, hypoechoic parapneumonic effusion is frequently demonstrated. Echogenic debris within the effusion can suggest empyema.

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