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Lung

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Artefacts

THE PLEURA

US cannot visualise pathology that does not extend to the pleura and so most lung signs begin at the pleura. A good analysis of the pleura is essential to help narrow the differential of non specific signs like B lines and A lines. 

Normal pleura is thin (<3mm), regular (ie straight line) and bright white (1). As the visceral pleura slides on the parietal pleura, it creates a twinkling artefact. This is best visualised with the pleura in the middle of the screen, in B mode. 

Normal lung: A lines and pleural sliding

 Lung signs begin at the pleura

PLEURAL THICKENING

A thickened pleura is easily visualised on US and doesn't need to be formally measured. The straight line of the pleura also becomes shredded and irregular. 

There are several causes of pleural thickening seen on US:

  • infective
  • inflammatory
  • malignancy (mesothelioma, lymphoma, mets)
  • post exudate (eg empyema, haemothorax)
  • pleurodesis

To determine the cause of pleural thickening and irregularity: the distribution of disease should be considered. 

LOCALISED and UNILATERAL

  • infective
  • trauma
  • malignancy
  • plaques

DIFFUSE and UNILATERAL

  • pleurodesis
  • post exudate
  • malignancy

LOCALISED and BILATERAL

  • pulmonary fibrosis
  • infective (eg viral pneumonitis, PCP, COVID19)
  • ARDs

Malignant pleural thickening is more likely to be unilateral, hypoechoic, show vascularity and >10mm thick (1). Pleural effusions may also be hypo echoic. However, these will change dimensions with respiration due to expansion of the lung into the pleural space with inspiration. 

Hypoechoic pleural thickening in a patient with mesothelioma.

To determine the cause of pleural pathology consider the distribution of disease

Bilateral pleural disease is more likely to be benign. 

Infective and inflammatory pleural thickening is more likely to be hyperechoic. There may be small hypoechoic areas (<10mm) within the pleura. These are considered to be sub pleural consolidations (3). 

Thickened pleura with sub pleural consolidations

Irregular pleura in a patient with pulmonary contusions

PLEURAL SLIDING

Visceral pleura sliding on the parietal pleura creates the twinkling artefact which has been likened to ants marching back and forth across the screen (see clip above). A B mode assessment of pleural sliding is the most accurate. However, the presence of pleural sliding can be further confirmed with M mode. 

Place the M mode cursor over the pleura and look for the seashore sign. M mode is a single line of US sight represented over time. The line of sight passes through the horizontal lines of subcutaneous tissue and this is presented as straight lines running parallel horizontally across the screen. It then encounters the pleural line which is shown as a bright white straight line deep to the subcutaneous tissue (sea). Due to reflection of most of the US at the pleura, and the constant pleural movement, everything deep to the pleura has a grainy appearance similar to sand (shore)

Seashore sign: M mode of pleural sliding with the linear transducer 

Presence of pleural sliding excludes a pneumothorax in that specific area of lung

Absence of sliding can be caused by several pathologies, including pneumothorax

ABSENT PLEURAL SLIDING

An absence of sliding (like pleural thickening) has a wide DDx. Assessment of the integrity of the pleura may enable narrowing of the list of differentials. 

THIN PLEURA

  • pneumothorax
  • absent/inadequate ventilation

Absent pleural sliding: thin pleura and lack of b lines/ comet tails point to pneumothorax


THICK PLEURA
  • adhesions
  • pleurodesis
  • infiltrating malignancy

Thickened pleura, commet tails and absent pleural sliding: post infectious adhesions

M MODE OF ABSENT PLEURAL SLIDING

M mode of absent pleural sliding will create parallel horizontal lines like a barcode above and below the pleural line. In a pneumothorax, there will be no disturbance to the horizontal lines. 

BARCODE SIGN

  • pneumothorax

Barcode sign: pneumothorax

LUNG PULSE

Absent pleural sliding due to adhesions, absent ventilation etc will show a lung pulse. This is the heart beat transmitted by the lung to the transducer. It is a vertical deflection originating at the pleural line and repeating at regular intervals (in sync with the QRS on the patient monitor/patient pulse). The presence of lung pulse excludes pneumothorax.

Lung pulse: continuous parallel horizontal lines above and below the pleural line occur at regular intervals (indicates lung present; excludes pneumothorax.)

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Pneumothorax

REFERENCES

1. Rachel M. Mercer, Ioannis Psallidas & Najib M. Rahman (2017) Ultrasound in the management of pleural disease, Expert Review of Respiratory Medicine, 11:4, 323-331.

2.Kamel, Khaled & Elhinnawy, Yasmine. (2018). Role of transthoracic ultrasound in differentiation of the causes of pleural thickening. 260-265.

3. Dietrich CF, Mathis G, Cui XW, Ignee A, Hocke M, Hirche TO. Ultrasound of the pleurae and lungs. Ultrasound Med Biol. 2015;41(2):351-365.