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Lung ultrasound, sonoanatomy, and standard views 

Lung ultrasound, sonoanatomy, and standard views
Chapter:
Lung ultrasound, sonoanatomy, and standard views
Author(s):

Ashley Miller

DOI:
10.1093/med/9780198749080.003.0014
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date: 26 October 2021

Introduction

This chapter outlines the technique for performing a systematic assessment of the lung using US that ensures that significant pathology can be identified. A 3-point scan is described, and the appearances of normal lung are reviewed.

Limitations of clinical assessment and the role of ultrasound

The clinical assessment of a patient with acute respiratory failure using a stethoscope has a poor sensitivity and specificity in identifying the correct diagnosis. In experienced hands, point-of-care LUS outperforms conventional chest radiography and can have an accuracy approaching that of CT scanning. LUS can be an extremely powerful tool in the management of critically ill patients with acute respiratory failure, including assessing the underlying diagnosis, identifying the cause of an acute deterioration in gas exchange during ventilatory support, and investigating failure to wean from ventilatory support. LUS provides a rapid bedside test that does not involve exposure to ionizing radiation, has minimal revenue costs, and can be repeated, whenever indicated, without any significant risks to the patient.

LUS has a steep learning curve and can largely obviate the need for the majority of chest X-rays that are undertaken in ventilated patients. However, correct use and interpretation only come with appropriate knowledge, training, and experience.

Anatomy

The lungs are cone-shaped structures situated in the thoracic cage on either side of the mediastinum, to which they are connected by their roots. The shape and positioning of the heart mean that the left lung is different in shape to the right lung, thus differing in their surface projections. The right lung comprises an upper, middle, and lower lobe, while the left lung has an upper and lower lobe. Each is suspended in its pleural cavities and surrounded by visceral and parietal pleura. The pleura are invaginated sacs and so are continuous around the hilum. The serous pleural membranes are closely opposed (except at their recesses), with a tiny amount of fluid between them, allowing them to slide over one another with respiration. The parietal pleura lines the thoracic wall, diaphragm, and mediastinum. The ribcage encases the pleural cavities. Below the lungs, the diaphragm separates the thoracic and abdominal cavities. It is dome-shaped, with its highest point (deep in the thorax) extending as high as the fourth intercostal space anteriorly. The liver on the right and the spleen and stomach on the left are located immediately below the diaphragm in the peritoneal cavity.

Surface anatomy

The surface anatomy of the lungs is shown in Figure 14.1. Anteriorly, the lungs extend to the sixth costal cartilage on the right and the fourth on the left (because of the heart). Because the ribs run in a caudad direction as they extend around the thorax from the spine, posteriorly, both lung bases can be identified by the level of the tenth thoracic vertebrae. The fact that the lungs can only be visualized down to the horizontal level of the anterior fifth intercostal space catches many novices out—most are surprised by how cephalad the lung bases are situated. The parietal pleural extends significantly lower than the visceral pleura, so pleural effusions may be seen extending caudad to this.

On the right, the border of the upper and middle lobes is located at the third intercostal space anteriorly. The oblique fissure runs roughly in the fifth intercostal space between the posterior and anterior axillary lines, and the pleura of the middle lobe can be visualized with US anteriorly in the fourth and fifth intercostal spaces. The lower lobe can be located in the fifth to eighth intercostal spaces in the posterior axillary line. Posteriorly, the upper and lower lobes meet under the fifth intercostal space.

On the left, the interlobular fissure is steeper than the right. The surface anatomy of the upper lobe is located anteriorly, while in the axilla, the lower lobe is located under the seventh and eighth intercostal spaces. Posteriorly, the upper and lower lobes join in the fourth intercostal space.


Figure 14.1 Surface anatomy of the lung.

Figure 14.1 Surface anatomy of the lung.

Technique

Probe selection

Scans can be performed with a linear (high-frequency vascular access), curvilinear (low-frequency abdominal), or phased array (low-frequency echocardiography) probe. Each has advantages and disadvantages that are outlined in Table 14.1. The curvilinear probe is the best single probe to use, as it can be used to perform a comprehensive scan and elicit all the signs of LUS with the best overall resolution. The high-frequency linear array probe will provide the highest-resolution images of the pleura and can be used if there is difficulty with identifying pleural sliding.

Table 14.1 Ultrasound probes and their merits

Advantages

Disadvantages

Linear

High frequency allows the superficial pleura to be easily visualized with good appreciation of lung sliding

High frequency and narrow sector width mean lung bases cannot be visualized

Curvilinear

Can be used to perform a comprehensive exam and demonstrate all signs

Lower frequency means lung sliding is slightly harder to appreciate than with a linear probe

Large footprint means some angulation is needed to eliminate rib shadows

Phased array

Can be used to perform a comprehensive exam and demonstrate all signs

Its small footprint is useful for getting in between the ribs

Resolution is less good than the other two probes

Probe positioning

By convention, the left side of the US display as you look at it must be either cephalad or the right-hand side of the patient. It is therefore essential for the probe’s marker dot to be in the correct orientation. A number of different approaches to undertaking a systematic LUS examination have been described. Each lung may be scanned in six regions—upper and lower anterior, mid-axillary, and posterior positions (see figure 23.1). A simpler approach is to use only three points on each hemi-thorax that appears to have a diagnostic accuracy that is as high for most pathologies as using more examination points. Scanning at 28 points in the anterior and axillary areas has also been described to quantify a pulmonary oedema ‘score’. For most situations, the simple 3-point approach is appropriate. It is imperative that the operator has a consistent standardized method for each scan performed.

In the 3-point scan, the probe is placed in the second intercostal space, mid-clavicular line for the upper anterior point. It lies over the upper lobe. The lower anterior point is located at the fourth intercostal space at the outer edge of the nipple (in a man). This point will miss the heart on the left. It lies over the middle or lingular lobe (Figure 14.2). The postero-lateral point is located by moving laterally and posteriorly, from the lower anterior point, until limited by the mattress of the bed and then going caudad by one or two rib spaces, so the diaphragm is located. It lies over the lower lobe.


Figure 14.2 Surface anatomy for the 3-point scan. The upper and lower anterior points are highlighted. To identify the correct positions, apply two hands side by side (without your thumbs) over the anterior chest, with your wrists in the anterior axillary line and your upper little finger resting along the clavicle. Your lower little finger will be aligned with the lower border of the lung (the phrenic line). The upper anterior point corresponds to the base of the middle and ring fingers on the upper hand. The lower anterior point lies under the middle of the palm on the lower hand. The postero-lateral point is located laterally and posteriorly from the lower anterior point as far as possible behind the posterior axillary line (limited by the bed).

Figure 14.2 Surface anatomy for the 3-point scan. The upper and lower anterior points are highlighted. To identify the correct positions, apply two hands side by side (without your thumbs) over the anterior chest, with your wrists in the anterior axillary line and your upper little finger resting along the clavicle. Your lower little finger will be aligned with the lower border of the lung (the phrenic line). The upper anterior point corresponds to the base of the middle and ring fingers on the upper hand. The lower anterior point lies under the middle of the palm on the lower hand. The postero-lateral point is located laterally and posteriorly from the lower anterior point as far as possible behind the posterior axillary line (limited by the bed).

Reproduced from James Thomas and Tanya Monaghan, Oxford Handbook of Clinical Examination and Practical Skills 2Ed., © Oxford University Press 2014, Figure 6.1, p. 141. By permission of Oxford University Press. https://global.oup.com/academic.

The 28-point scanning technique examines the second to fifth intercostal spaces at the parasternal, mid-clavicular, anterior axillary, and mid-axillary lines (without the fifth intercostal space exam on the left due to the position of the heart). As this was designed to evaluate pulmonary oedema specifically, it is necessary to add postero-lateral points inferior to the posterior axillary line in each rib space down to the level of the diaphragm, in order to examine for consolidation and pleural effusions. It should be noted that a comprehensive exam can be performed on critically ill supine patients without needing to move them. It is possible to get posterior to the posterior axillary line by pushing the heel of the probe into the bed.

Probe orientation

When scanning anteriorly, the probe should be placed in a longitudinal (cephalad–caudad) orientation. This ensures that the rib and pleural lines are both captured in a single image and clearly differentiated. If the probe is placed transversely, the immobile rib line may be mistaken for a non-sliding pleural line by a novice sonographer, resulting in an erroneous diagnosis of pneumothorax. However, temporarily siting the probe transversely can help in examining a larger area of the pleura and in finding the lung point in a pneumothorax (Chapter 15). When scanning postero-laterally, it is advised that while a cephalad–caudad orientation should be maintained, it is desirable to rotate the probe approximately 30° posteriorly in order to get the probe’s footprint in the intercostal space and eliminate the rib shadows from the image. Be aware here that the cephalad portion of the image will then often be slicing through soft tissue posterior to the lung, particularly the more posterior you are.

Machine settings

Whatever you are looking at should fill your screen. Having too great a depth reduces the frame rate and thus the image quality. For most subjects, a depth setting of around 10 cm is appropriate when using the curvilinear probe. The frequency should be set to its highest setting with curvilinear and phased array probes when assessing anterior lung sliding. The default setting should be used when imaging the lung bases. If the focus point(s) are not preset, these should be at the level of interest. The gain setting may need to be changed during the examination. Anterior lung sliding may be best seen with the gain setting low, which is then increased to improve the image of a consolidated lung base or pleural effusion. Contemporary machines often have a number of image processing settings to minimize artefact generation. As artefacts are a core component of LUS, it may be necessary to disable these options in order to obtain optimal images. However, A-lines and B-lines (see p. 129) are usually visible (if present) in all settings.

Normal sonoanatomy

Anterior

Immediately in the near field, the soft tissue above the ribs will be seen. Deep to this will be the rib line and then approximately 0.5 cm below this, the thin, bright white pleural line will be visible. Centre the pleural line between two ribs in the image, and the ‘bat sign’ is demonstrated with the rib shadows looking like the wings of a bat (Figure 14.3). In a normal aerated lung, only shadow and artefact will be visible deep to the ribs and pleural line.


Figure 14.3 Normal ‘bats wing’ appearance. The bright pleural line is visible between two ribs. A lines are seen as deeper replications of the pleural line.

Figure 14.3 Normal ‘bats wing’ appearance. The bright pleural line is visible between two ribs. A lines are seen as deeper replications of the pleural line.

Lung sliding

The pleural line should be closely examined for the phenomenon of lung sliding. The two pleural layers slide over one another with respiration, which is demonstrated by the pleural layers moving backwards and forwards. Little blebs (white or black) are seen within the pleural line or just below it that will move to and fro with the respiratory cycle (Video 1.10.1 Lung ultrasound, sonoanatomy, and standard views). The space below the pleura will shimmer. Lung sliding is easiest to appreciate with the depth reduced, the gain turned down, and the probe on the highest-frequency setting.

Lung sliding can also be demonstrated with M-mode. Placing M-mode through the pleural line will demonstrate the ‘seashore’ sign. The relatively immobile soft tissue above the pleura will yield clear horizontal lines (the sea), while the moving pleura will generate a chaotic ‘sandy’ appearance below the pleural line (Figure 14.4).


Figure 14.4 M-mode appearance of normal lung sliding: seashore sign.

Figure 14.4 M-mode appearance of normal lung sliding: seashore sign.

The presence of lung sliding immediately excludes a pneumothorax at that point in the chest with 100% specificity. The absence of sliding signifies that the pleura are either separated (pneumothorax, effusion), stuck together (infection, ARDS, pleurodesis), or that there is no respiration (one-lung intubation, pneumonectomy). In M-mode, absent sliding appears as a series of horizontal, straight lines above and below the pleural line that has been termed the barcode or stratosphere sign (Figure 14.5 and Video 1.10.2 Lung ultrasound, sonoanatomy, and standard views). Present, but diminished, sliding will be seen with low tidal volumes, hyperinflated lungs, and ARDS. The lungs expand more at their bases than their apices, and so sliding is easier to visualize at the lung bases.


Figure 14.5 M-mode appearance of no lung sliding: stratosphere sign.

Figure 14.5 M-mode appearance of no lung sliding: stratosphere sign.

A-lines

The acoustic impedances of soft tissue, bone, and air are very different. When US hits a boundary of differing acoustic impedances, it will be reflected back to the transducer. This explains why nothing can be visualized deep to the ribs and the pleura (assuming there is no fluid in or around the lung). The transducer itself will reflect some of the returning US waves, and these will bounce backwards and forwards between the probe and the pleura. This causes reverberation artefacts below the pleural line that appear as exact copies. Their spacing will be the same as between the probe and pleural line. These are termed A-lines and signify that there is air below the parietal pleura. Hence, they are present both in normal lungs and when there is a pneumothorax. Sometimes there are highly echogenic interfaces between soft tissue layers, which can cause artefactual lines to appear below the pleura. These can be differentiated from A-lines by their spacing.

B-lines

These artefacts are laser-like lines resembling comet tails, which extend from the pleural line to the depths of the image. They move backwards and forwards with lung sliding and obliterate A-lines when they pass across them (Figure 14.6). Their aetiology is debated, but it has been suggested that they result from the juxtaposition of thickened interlobular alveolar septa and air within the alveoli producing a reverberation artefact. When looked at carefully, B-lines are seen to be composed of closely repeating horizontal lines. The generation of B-lines requires that the parietal and visceral pleura layers are in opposition, and their presence therefore excludes a pneumothorax.

B-lines are present in any disease affecting the interstitium and are therefore seen in pulmonary oedema, ARDS, and lung fibrosis. Their location can be unilateral, bilateral, disseminated, localized, homogenous, or non-homogenous, depending on the pathology. The most common cause is pulmonary oedema. Increasing severity yields more B-lines, which become closer together. When severe, B-lines will coalesce, yielding a ‘white lung’ appearance below the pleural line that can be misinterpreted as B-lines being absent. Importantly, no A-lines will be seen and the pleural space will be much whiter than usual.

Up to two B-lines between ribs can be considered normal (particularly at the lung bases), while three or more, or B-lines close together in a transverse image, are considered pathological. It should be noted that whenever there is a juxtaposition of air and fluid within the lung, comet artefacts are generated and they are therefore seen bordering consolidation and atelectasis. Although they look similar, they are only termed B-lines if they originate from the pleural line.

Lung pulse

If the pleura are apposed to one another, then the cardiac pulsation will be transmitted through the lung to the parietal pleura. This can be seen with US as a small amount of lung sliding in time with the heartbeat. It can be visualized in 2D and M-mode. It can be appreciated as separate from the sliding of respiration by asking a subject to hold their breath while examining the pleura. The use of this sign is that in instances where there is no sliding due to lack of lung ventilation, it demonstrates that there is no pneumothorax, as by definition the pleural layers have to be touching. If unsure whether lung pulse is present on the 2D image, M-mode may be used which yields the lung pulse as ‘T lines’ (Figure 14.7). Lung pulse is seen over areas of atelectatic lung and over the non-ventilated lung following endobronchial intubation.


Figure 14.7 M-mode appearance of the lung pulse. T lines (highlighted with arrows) represent pleural movement from transmitted cardiac pulsation on the background of a motionless lung, which produces a stratosphere sign appearance in between.

Figure 14.7 M-mode appearance of the lung pulse. T lines (highlighted with arrows) represent pleural movement from transmitted cardiac pulsation on the background of a motionless lung, which produces a stratosphere sign appearance in between.

Postero-lateral

In the postero-lateral zones, the same signs should be looked for as with anterior scanning. The difference is that the diaphragm is now an important component of the exam. Locating it and differentiating between supra- and sub-diaphragmatic structures are vital. The depth will need to be great enough to extend to the mediastinum (around 15 cm is appropriate). The diaphragm is highly reflective to US waves and produces a bright, curved line that moves with respiration. The left of the image, as you look at it, will be the lung; the diaphragm should be right of centre, and the liver or spleen will be on the right (Figures 14.8 and 14.9). When scanning the left side, it is important to ensure that the spleen, rather than the stomach, is visualized in order to allow proper assessment of the diaphragm and any effusions. The spleen lies behind the stomach and will be revealed by either moving or angling the probe posteriorly. The depths of the image will be the mediastinum or spine, which will only be seen if an acoustic window from fluid is present.


Figure 14.8 Normal right posterior lateral scan. PL, pleural line; L, liver; D, diaphragm identified. Superior diaphragm not visible as obscured by normal aerated lung (veil sign).

Figure 14.8 Normal right posterior lateral scan. PL, pleural line; L, liver; D, diaphragm identified. Superior diaphragm not visible as obscured by normal aerated lung (veil sign).


Figure 14.9 Normal left posterior lateral scan. PL, pleural line; Sp, spleen; K, left kidney; D, part of diaphragm identified. Limited view of the diaphragm as the majority obscured by normal aerated lung.

Figure 14.9 Normal left posterior lateral scan. PL, pleural line; Sp, spleen; K, left kidney; D, part of diaphragm identified. Limited view of the diaphragm as the majority obscured by normal aerated lung.

With a normal air-filled lung, only the posterior portion of the diaphragm will be seen. Air in the costophrenic angle obliterates any structures below it. With respiration, the artefact of an air-filled lung will be seen coming across the screen like a curtain obliterating the view of the diaphragm and liver or spleen during inspiration. This has been termed the curtain or veil sign (Videos 1.10.2 Lung ultrasound, sonoanatomy, and standard views).

When there is a pleural effusion or a consolidated lung base, then the diaphragm is easy to visualize due to the US window provided (Figure 14.10). Diaphragmatic muscle thickness and excursion with respiration can be measured.


Figure 14.10 Effusion and collapsed lung with clear view of the diaphragm.

Figure 14.10 Effusion and collapsed lung with clear view of the diaphragm.

A collapsed or consolidated lung base has a very similar US appearance to the liver and spleen, and it is vital to differentiate the two by their relationship to the diaphragm. Similarly, it is easy to confuse ascites with a pleural effusion. Moving the probe around and looking from different angles is useful to clearly define what is what.

Pitfalls

The bright, motionless rib line may be mistaken for a lack of pleural sliding by having the probe in the transverse plane. Scanning in the longitudinal plane is recommended to produce the typical bats wing appearance that clearly differentiates the rib and pleural lines.

Use of too great a depth setting will make pleural sliding difficult to visualize.

Subcutaneous emphysema prevents transmission of US waves and visualization of the pleural line. It may be possible to massage the air out of the way using the probe in order to obtain images.

A full stomach may be misinterpreted as left basal consolidation or effusion. It is essential to identify the diaphragm to confirm that the lung is being imaged when scanning in the postero-lateral zones.

The spleen may be hypoechoic, which can be misinterpreted by novices as a pleural effusion. The position of the diaphragm must be identified to confirm an effusion.

Chapter 14

MCQs

Questions

1. When performing LUS:

  1. A The diaphragm can normally be located anteriorly at the seventh intercostal space

  2. B A phased array echo probe provides the best US images

  3. C The probe should be placed in a transverse orientation to eliminate rib shadows

  4. D Machine settings that minimize artefact generation yield clearer, and therefore more useful, images

  5. E When assessing anterior lung sliding, using a shallow depth, high frequency, and low gain will enhance the image

2. Regarding sonographic signs:

  1. A A-lines are reverberation artefacts from the soft tissue above the pleura

  2. B B-lines resemble comet tails, obliterate by A-lines, extend to the depths of the image, and move with lung sliding

  3. C The lung pulse is movement of the lung due to respiration

  4. D The whole diaphragm can only be clearly visualized if there is consolidation or an effusion present at the lung base when the probe is placed at the postero-lateral point

  5. E It is imperative to locate the diaphragm when performing basal scans, to avoid incorrect diagnoses

Answers

1. When performing LUS:

  1. A FALSE. It is located in the fifth intercostal space.

  2. B FALSE. Although this is designed to look between the ribs into the thorax, it does not produce as good images as curvilinear probes.

  3. C FALSE. Visualizing the ribs and pleura in the same image will avoid the mistake of thinking the rib line is the pleural line and falsely diagnosing a pneumothorax.

  4. D FALSE. Artefacts are important signs in LUS.

  5. E TRUE. High-frequency US provides the best resolution.

2. Regarding sonographic signs:

  1. A FALSE. Soft tissue interfaces can create artefacts similar to A-lines, but A-lines are artefacts from the pleural line.

  2. B TRUE. B-lines also become wider towards the bottom of the field.

  3. C FALSE. The lung pulse is caused by cardiac pulsation being transmitted through the lung.

  4. D TRUE. Air in the costophrenic angle will obscure the diaphragm.

  5. E TRUE. This will prevent other structures from being mistaken for consolidated lung, for example.