FUSIC Lung

FUSIC Lung Views

The lungs lie within thorax either side of mediastinum. The position of the heart on the left of the mediastinum affects the shape of the lung on the left compared to the right side.

The right lung is made up of three lobes (upper, middle, lower) and the left lung of two lobes (upper and lower). Surrounding the lungs is the parietal pleura and the rib cage. Beneath the lungs is the diaphragm which separates the intra-abdominal and thoracic cavity.

Scanning points

The lung ultrasound is a 6 point scan – 3 points on either side of the thorax are scanned.

The 6 points are the upper zone anterior chest wall, lower zone anterior chest wall and the posterolateral zone of each side.

Lung ultrasound scanning points:
1&2 performed with probe positioned longitudinally
3 is in the postero-lateral axillary position with the probe positioned coronally

FUSIC lung overview

Photo by Robina Weermeijer on Unsplash

1. Normal thoracic structures on lung ultrasound

It is important to initially identify normal thoracic structures on lung ultrasound to help orientate yourself.

Place the US probe with the longitudinal marker pointed in the head direction on the anterior chest wall starting on the first scanning point.

It is important to be able to identify the:

Ribs
Subcutaneous tissues
Pleura
Diaphragm
Heart
Upper abdominal organs

It is also important to realise the views of the lungs are similar on the 2 scanning points on the anterior chest wall. The view in the postero-lateral axilla is different and brings the diaphragm and upper abdominal organs into view.

Normal thoracic structures

Ribs are high in calcium, so they will be seen as a very bright line on the anterior surface (reflect back most of the US beam) with a posterior acoustic shadow cast below (dark shadow behind the rib).

If you move the transducer towards the cartilaginous part of the ribs, you will then be able to look through the ribs (‘lung window’) and appreciate the intercostal muscles and pleura behind. The pleural line is seen as a bright white line just below the rib margin.

It is important to understand that in many instances, the image you see below the pleural line is all artefact and is not the lung parenchyma that is being visualised.

This helps in understanding why the appearance of both normal lung and pneumothorax is identical below the pleural line – because they both generate the same artefact on B-mode ultrasound imaging.

Anterior chest wall

Labelled ultrasound image of the anterior chest wall anatomy using a linear probe

Posterolateral axillary view

The posterior-lateral axillary view is where the border between the intra-abdominal cavity and the thoracic cavity is viewed. These are separated by the diaphragm, however, this is not always seen as normal aerated lung can obscure the view. When there is basal lung pathology such as consolidation or pleural effusion the diaphragm is more easily visualised.

With respiration the lung will be visualised coming in and out of this window. The probe can be moved superiorly and posteriorly to bring into view different areas of the lung.

Right posterolateral axillary view showing the liver and kidney

Left posterolateral axillary view showing the spleen and kidney

2. Normal aerated lung

The next step is to be able to identify normal aerated lung on ultrasound. It is important to realise that the appearance of normal lung on ultrasound is as a result of artefact as the air in the lung does not reflect the ultrasound beams back towards the probe.

Batwing sign

The first lung ultrasound finding to confirm you are in the correct position is to look for the two rib shadows or the “Batwing Sign.” This ensures that your probe is in between two ribs.

Batwing sign on lung ultrasound - denoted by the yellow line

A-lines

A lines are generated when an ultrasound wave hits the pleural line, gets reflected back to the probe and then instead of being absorbed by the probe it reflects back down towards the lung. As this wave hits the pleura and reflects back to the probe it will have travelled from probe to pleura twice and so the machine will ‘paint’ a line that appears within the lung itself, twice the distance of the probe to pleura.

This process repeats and so multiple A line artefacts may be generated each equidistant from the previous. As some of the ultrasound wave is absorbed at each step the amplitude of the signal reduces sequentially and so the A-lines get less pronounced the deeper they are shown.

↔︎ A-lines demonstrated on lung ultrasound - note how the probe, plural line and A-lines are all equidistant

Diaphragmatic movement - lung curtain

In a patient with no pathology within the lung, the interface between solid abdominal organs and lung appears like a curtain coming in and out of the picture.

This is because the aerated lung comes into the field of view as the patient inspires and out as they expire. The aerated lung obscures the view of the abdominal organs and appears like a black curtain is moving across the screen.

Lung curtain sign coming into and out of view from the left of the screen obscuring the view of the liver

Lung sliding - 2D

In health, the visceral and parietal pleura are closely opposed, separated only by a minute layer of fluid which is not appreciable on ultrasound. The pleural line is seen as a bright white line just below the rib margin. During ventilation this fluid layer allows the pleura to slide across each other and this is appreciated on ultrasound as a shimmering appearance, this is usually described as ‘lung sliding.’ It is sometimes also referred to as like ‘marching of ants.’

Lung sliding is an important concept to grasp. Its presence means the parietal and visceral pleura are directly opposed and the lung is ventilating, and so rules out pleural effusion, pneumothorax and a host of conditions which dramatically reduce lung ventilation (e.g. very low tidal volume ventilation, ARDS, severe pneumonia, fibrosis).

The pleural line can be seen to give the appearance of a line of 'marching ants'

Lung sliding - M-mode: Seashore sign

It can be difficult sometimes to clearly determine if lung sliding is present or not. Another option is to use M-mode across the pleural line.

In a patient with lung sliding, the pattern generated will be like ‘waves on a beach’, where the waves represent the intercostal tissues, the pleural line the edge of the sea and the beach is the scattered artefact from the lung below. M-Mode can be used to confirm or rule out lung sliding.

M-mode demonstrating normal lung sliding with the seashore sign

3. Abnormal findings on lung ultrasound

Once you are able to identify normal aerated lung on ultrasound it is important to be able to distinguish this from abnormal findings.

Absent lung sliding

Lung sliding may be reduced or absent in a variety of circumstances. There is no lung sliding present when the parietal and visceral pleura become separated by air (pneumothorax) or fluid (pleural effusion). Furthermore, severe COPD or anything that hyperinflates the lungs can markedly reduce pleural sliding.

Absence of lung sliding can be seen using B-mode or using M-mode (stratosphere sign/ barcode sign). The most clinically relevant fact about lung sliding is that if lung sliding is present, you can rule out pneumothorax with 100% sensitivity. On the flip side, the absence of lung sliding can have many causes with pneumothorax being one of them.

2D clip of absent lung sliding - the appearance of 'marching ants' has disappeared - however this can be hard to discern

Absent lung sliding - M-mode: Stratosphere sign

Absent lung sliding on M-mode has the appearance of a stratosphere with horizontal lines the whole way down the image, with the brightest usually being the pleural line. This is also referred to as the barcode sign.

The reason for this appearance is that there is no scatter of the ultrasound waves by the moving visceral pleura because it does not oppose the parietal pleura. Instead, all you see below the pleural line is reverberation artefact from the pleural line and tissues above.

M-mode of absent lung sliding - this is known as the 'stratosphere' or 'barcode' sign

Lung point

To confirm the presence of a pneumothorax, you should look for the lung point sign.

The lung point is when you can see the transition between normal lung sliding and the absence of lung sliding. This is the transition point between the collapsed lung and normal lung. It can be visualised in both 2D and M-mode.

If a lung point is present, you can rule in pneumothorax with 100% accuracy.

The lung point can be sliding in and out of view from the left side of the screen

Lung pulse

Another sign that may be elicited using M-mode is called the lung pulse. This is created by the cardiac impulse being transmitted through the lungs to be picked up by the ultrasound probe, as occurs both in health and in many pathological conditions. It may therefore be seen in combination with both the ‘seashore’ and ‘stratosphere’ appearances. It is viewed on ultrasound as regular, subtle vertical lines (T lines) across an M mode trace.

↔︎ M-mode demonstrating lung pulse - subtle vertical lines at a regular rate

It is important to understand that for the lung pulse to be visible on M mode, the parietal and visceral pleura must be opposed.

If there is air between the pleura then this would prevent transmission of the lung pulse from the lung to the probe (and you would see the stratosphere sign only). If there is fluid between the pleura it usually acts as a ‘shock absorber’ and so similarly prevents the lung pulse from being transmitted to the probe.

Pathology

Seashore/
Stratosphere

Lung Pulse

Normal

Seashore

Present

Pneumothorax

Stratosphere

Absent

One lung intubation, breath holding, pleurodesis

Stratosphere

Present

Low tidal volume ventilation, ARDS, severe pneumonia/inflammation/fibrosis

Seashore/
Stratosphere

Present

Difference in M mode appearances of various lung pathologies

B-lines

B lines are seen if there is increased amount of interstitial fluid (as in pulmonary oedema, contusion, infection or inflammation), or interstitial thickening (as in fibrosis).

They arise from a marked difference in acoustic impedance between an object and its surroundings – in this case thought to be the aerated lung and the adjacent interlobular septae – creating a phenomenon of resonance. B lines are sometimes referred to as ‘comet tails’, ‘sprays’ or ‘rockets’ by some authors.

In normal lung, especially at lung bases, up to 2 B-lines between two adjacent ribs can be considered normal. 3 or more B-lines between ribs spaces (or close together in a transverse image) are pathological. They can be localised, disseminated, homogenous or non-homogenous depending on underlying pathology. They are present in any disease process affecting the lung interstitium.

The commonest cause of B-lines is pulmonary oedema. Very severe lung oedema / inflammation causes B-lines to fuse with a hyperechoic confluent pattern that fills the space between two ribs (white lung).

B-lines start at the pleural surface and travel at least to a depth of 18cm. They will move with respiration. They will appear as a single line, however with more interstitial fluid they can coalesce and finally the lung will look bright with individual B lines difficult to separate. A feature of B lines is that they will obliterate A lines.

B-lines rule out a pneumothorax: As they commonly indicate alveolar/interstitial pathology which means there is no air between the visceral and parietal pleura (this sign would be obliterated otherwise).

Multiple B-lines can be seen extending down from the pleural line

When there is multiple B-lines they can sometimes fuse to form a waterfall sign or a white lung.

4. How to distinguish between different pathologies

So far we have gone through how to identify normal and abnormal signs on lung ultrasound. Using these findings we can help distinguish different lung pathologies.

Alveolar syndromes

Alveolar syndrome encompasses the ultrasound appearance of atelectasis and consolidation. Lung ultrasound is very useful at identifying consolidation because most lung consolidation is in contact with the pleura.

Consolidation

Consolidation can be translobar or non-translobar and the appearaces are slightly different:

• Translobar – when most of the lung is inflamed and fluid filled due to the consolidation it gives an US appearance similar to the liver. This is known as hepatisation or tissue-like sign.

• Non-translobar – in less extensive consolidations there is more air within the lung which doesn’t easily transmit US beams. So it gives an irregular border of the lung tissue – this is known as the shred sign. B-lines often project from the irregular border of the consolidated lung.

Air/fluid bronchograms can be seen in both consolidation or atelectasis, however if there is evidence of dynamic air/fluid bronchograms then this is a sign of consolidation. This is where fluid is seen moving within the bronchogram during the respiratory cycle. This is not seen in atelectasis as the atelectatic lung is collapsed.

Air/fluid bronchograms are either circular or linear depending on how the US beam passes through them.

Atelectasis

Atelectasis is a complete or partial lung collapse or closure (loss of volume) of the entire lobe of the lung. The cause is often multifactorial and occurs when alveoli become deflated or possibly filled with secretions/fluid.

This means that ultrasound appearance is similar to consolidation and it is difficult to distinguish between the two.

Atelectatic lung will often increase in size during recruitment manoeuvres / physiotherapy / CPAP and will lose its tissular pattern on US as aeration increases, often taking on a normal lung appearance but with multiple B-lines.

Translobar consolidation - seen as hepatisation.
Dynamic air fluid bronchograms can also be seen - the black lines are the air filled bronchograms and the fluid which is white can be seen moving along them (dynamic).

Non-translobar consolidation - seen as the shred sign
The lung has an irregular border and the appearance of B-lines extending from the irregular border.

Pneumothorax

Lung ultrasound is useful for ruling out pneumothorax. It is important to remember A-line can be seen in normal aerated lung and with a pneumothorax. The most common place for a pneumothorax on lung US is the anterior chest wall.

If all of these are met then you should look for the lung point where the two pleural layers re-join. With respiration this will be seen moving in and out of the view. If a lung pulse is found with the other above signs then a pneumothorax is highly likely.

Pleural Effusions

The vast majority of simple pleural effusions will be visible in the posterolateral region of the supine patients. Simple pleural fluid will usually be anechoic in appearance and will allow the passage of ultrasound waves beyond the pleural line so you will start to see definition of the deep organs.

The appearance of pleural effusions on ultrasound are:

Echo-free (black) below pleural line
It may contain hyperechoic densities, be loculated or contain septations
It can change shape with respiration and diaphragm movement

The pleural effusion can be seen surrounding the lung and above the diaphragm - there is underlying hepatisation and this most likely represents consolidation

Jellyfish Sign

In moderate to large pleural effusions, it’s not uncommon to see lung atelectasis on lung US. This collapsed lung within the effusion moves with respiration and appears like a jellyfish. Also known as the ‘wizard hat’ sign.

The jellyfish sign tends to favour atelectasis over pneumonia as the collapsed lung can change its shape with respiration whereas with pneumonia / consolidation, the lung is much firmer. Jellyfish sign also occurs more readily with transudates as the viscosity of exudate effusions tends to hamper mobility of lung tissue.

The pleural effusion has caused the underlying lung to collapse giving the appearance of a 'wizards hat' or 'jellyfish' sign - this is most likely underlying atelectasis

Interstitial syndromes

Interstitial syndromes are an ultrasound appearance that helps detect the evidence of interstitial oedema. These artifacts are formed from the contrasting acoustic impedance of alveolar air and interstitial thickening from fluid or fibrosis. Presence of B-lines are the common appearance that links all these conditions!

Cardiogenic pulmonary oedema

The features of cardiogenic pulmonary oedema on lung ultrasound are:

Bilateral and symmetrical
B-lines – increased number of B-lines with increasing severity
Pleural effusions in severe pulmonary oedema
Lung sliding is present

Combined with a cardiac echo which may show associated LV impairment

Non-cardiogenic pulmonary oedema

Acute respiratory distress syndrome (ARDS)

ARDS is a non-cariogenic pulmonary oedema and diffuse lung inflammation syndrome

The profile of ARDS on lung ultrasound is:

Bilateral or unilateral B-lines
Lung sliding usually present – may be diminished due to pleural inflammation

No evidence of LV failure on echo

ARDS

CPO

Bilateral B-lines

Bilateral
B-lines

Yes

Homogenous B-lines

Yes

Subpleural irregularities

Yes

Reduced B-line in response to diuresis

Yes

Higher denstiy posteriorly

Variable

Always

Pleural sliding

Diminished

Greatly diminishedy

Lung pulse

Present but diminished

With absent pleural sliding highly likely

Pleural Effusion

Evidence of septation

Free flowing effusion

Other - dynamic effect of recruitment

Little change with recruitment

Increased size and aeration with recruitment

Difference in M mode appearances of various lung pathologies

COVID-19

Early / Mild COVID-19 Infection

COVID-19 has a particular pattern in early and mild disease. This involves development of lung ultrasound B lines. The thin pleural line thickens and develops a lumpy texture to it. Areas with lots of B lines are next to areas with none, causing ‘skip lesions’ or spared areas of normal lung. The pleural line will also thicken causing discrete ‘subpleural consolidations’. These features initially appear anteriorly in COVID-19.

Progressing / Moderate COVID-19 Infection

As COVID-19 disease develops, the B line numbers increase in areas affected, from mainly basal to other areas of the lungs. The B lines also increase in number to become coalescent.

Severe COVID-19 Infection

The lung will be progressively de- aerated. Alveoli will be full of fluid, secretions or there may be a secondary infection. Lung consolidation can now be easily seen on ultrasound and this can appear tissue-like. The changes are mainly postero- basal.

C-lines 'light-beam artefact' and COVID-19

B-lines are frequently referred to in patients with COVID-19 on lung ultrasound. By definition B lines must originate from the pleura and erase A lines.

In COVID-19 vertical lines often originate from sub-pleural consolidations and not from the pleural line itself. These lines, although similar to B lines, are not strictly B lines as they do not originate from the pleura and do not erase A lines. These lines are termed C lines.

The ‘light-beam artefact’ seen in COVID-19 patients, are thought to be confluent C lines. C lines are artefacts caused by viral-induced irregularities of the pleural surface and not caused by interstitial oedema, which gives rise to B lines.

The clinical implications of the difference between B lines and alveolar oedema and between C lines and pleural based defects are still unclear regarding diagnosis and treatment.