Chapter 2

Technique and protocols

Christian B. Laursen^1,2,3, Jesper R. Davidsen^1,2,4 and Fergus Gleeson⁵

¹Dept of Respiratory Medicine, Odense University Hospital, Odense, Denmark. ²Centre for Thoracic Oncology, Odense University Hospital, Odense, Denmark. ³Institute for Clinical Research, SDU, Odense, Denmark. ⁴South Danish Center for Interstitial Lung Diseases (SCILS), Odense University Hospital, Odense, Denmark. ⁵Dept of Radiology, Churchill Hospital, Oxford University Hospitals NHS Foundation Trust, Oxford, UK.

Correspondence: Christian B. Laursen, Dept of Respiratory Medicine, Odense University Hospital, Søndre Boulevard 29, 5000 Odense C, Denmark. E-mail: Christian.b.laursen@rsyd.dk

In order to ensure a high diagnostic accuracy of an US examination, knowledge of a number of aspects is needed: anatomy and physiology of the organ/structure assessed, US physics, functions of the US system and how to use them, scanning technique used, normal and abnormal findings, pitfalls and limitations of the examination, and integration of US findings into the clinical context. The use of a systematic approach is also essential, regardless of whether it is performed as a focused TUS (FTUS) or a diagnostic examination (TUS).

The primary aim of this chapter is to provide the US novice with a description of the basic concepts and approach needed to be able to perform a systematic FTUS examination, as well as some basic knowledge regarding a diagnostic examination (TUS). The reading of this chapter does not replace theoretical and practical training, or a competency assessment prior to performing unsupervised examinations in clinical practice.

The chapter will provide a brief overview of the basics of sonoanatomy of the thorax, a stepwise approach to the techniques and protocols for performing FTUS, and some of the basic differences and principles when performing TUS. Descriptions of more detailed sonoanatomy, pathology and specialised techniques are addressed in subsequent chapters in this Monograph describing specific conditions and examination techniques.

Basic sonoanatomy of the chest

Knowledge of the basic sonoanatomy of the thorax is an essential prerequisite in the performance of FTUS. The basic two-dimensional (2D)-mode findings are described in the following sections, with more detailed and specific sonoanatomy findings of TUS given in subsequent chapters in this Monograph. The following descriptions are typical findings when the transducer is placed in a longitudinal axis over an intercostal space (ICS) (figure 1).

Figure 1. A curved low-frequency transducer placed in zone L1. The orientation marker (arrow) is placed cranially to facilitate identification of the “bat sign” on the US screen.

Chest wall

Structures such as skin, s.c. tissue, muscles and connective tissue are visible just beneath the transducer. The connective tissue appears hyperechoic, and often more hyperechoic horizontal linear structures can be seen, representing connective tissue, such as muscle fascia. The two ribs aligning the ICS are visible as two hyperechoic lines with an underlying shadow (figure 2) [1, 2].

Figure 2. Normal sonoanatomy of the thorax. The muscles, fascia and s.c. tissue of the chest wall (CW) are placed just below the transducer. The superficial surface of the ribs (R) can be seen as hyperechoic horizontal lines with posterior shadowing (*). The visceral and parietal pleura can be seen as a hyperechoic horizontal line, the pleural line (PL), just below the ribs. The area below the pleural line (“L”) is due to artefact generation and does not represent the air-filled lung, which cannot be visualised using US.

Pleura

Placed just below and between the two ribs, a hyperechoic horizontal line is seen representing the visceral and parietal pleura. Most conventional clinical US machines are not able to differentiate the two pleural surfaces from each other. The combined surfaces are termed the pleural line (figure 2) [1, 3, 4].

The “bat sign”

Since several structures as well as the pleural line may appear as a horizontal hyperechoic (white) line on the US screen, it is essential to be able to identify which of these lines represents the pleura. LICHTENSTEIN et al. [4] described how to use the “bat sign” in order to facilitate correct identification of the pleural line. The term “bat sign” is used since the two ribs adjoining an ICS (hyperechoic surface with posterior shadowing) and the pleural line resemble a bat flying towards the US screen (figure 3). Thus, the following two steps should be performed prior to assessing the pleura line: 1) ensure that the scanning plane is vertical and the orientation marker is placed cranially (figure 1); and 2) use the “bat sign” sign to identify the pleural line (figure 3).

Figure 3. Identification of the “bat sign” is performed to identify the pleural line (PL) as being placed just below the two ribs (R). The ribs with posterior shadowing represent the bat’s wings and the pleural line represents the head of the bat.

Once this is done, the transducer can be rotated approximately 90° (depending on the angle of the ribs at the area being scanned) counterclockwise to avoid the ribs and visualise the entire pleural line in the ICS.

Assessment of the pleural line

The presence or absence of three findings is of importance when assessing the pleural line: 1) lung sliding, 2) the lung pulse and 3) B-line(s).

Lung sliding

Lung sliding is seen as a horizontal movement of the pleural line in synchrony with the respiratory cycle, indicating a sliding movement of the visceral pleura against the parietal pleura [3, 5]. Lung sliding is due to the up-and-down movement of the visceral pleura in synchrony with the piston-like respiratory movement of the diaphragm [6]. Several factors may affect the magnitude of lung sliding (e.g. lung zone scanned, patient tidal volume, underlying disease and intubation) [6–8]. When air separates the two pleural layers (e.g. pneumothorax (PTX)), the movement disappears and cannot be detected with US. In such cases, the pleural line represents only the parietal pleura, which is still visible but does not slide since it is fixed to the chest wall [5, 9]. Apart from PTX, other conditions may also cause absence of lung sliding (e.g. fibrotic involvement in interstitial lung diseases, prior pleural empyema or sequelae from a prior intrathoracic operation) [6–8, 10, 11].

Lung pulse

As well as lung sliding in synchrony with the respiratory cycle, the pleural line may also move in synchrony with the cardiac pulse. This movement, termed the “lung pulse”, is caused by the force of the cardiac pulsation being transmitted to the lung and hence to the visceral pleura [3, 7]. The lung pulse is not always present in healthy persons and is generally easiest to visualise in areas where the lung is in close contact with the pericardium and heart. Like lung sliding, the lung pulse indicates that the visceral and parietal pleural surfaces are juxtaposed at the location of the probe [7, 8, 12].

B-lines

A B-line has been defined as a hyperechoic, laser-like, vertical reverberation artefact originating from the pleural line. B-lines are continuous from the pleural line to the bottom edge of the screen and do not fade in intensity [3, 13, 14]. Other reverberation artefacts may also originate from the pleural line, but, in contrast to B-lines, they fade out relatively quickly and do not continue to the bottom edge of the screen (figure 4) [3, 4]. If lung sliding is present, B-lines move in synchrony with the sliding [15]. B-lines are discussed in more detail in another chapter in this Monograph [16].

Figure 4. A B-line can be seen to originate at the pleural line (PL) and extends vertically to the bottom of the screen without fading. Another vertical artefact can also be seen (“Z”). However, this quickly fades when extending vertically and does thus not fulfil the criteria for a B-line. Such artefacts have been termed Z-lines by LICHTENSTEIN et al. [7].

Lung

The normal air-filled lung cannot be visualised using conventional 2D-mode US. Due to the difference in acoustic impedance, the US signal is reflected at the surface of the lung (visceral pleura). The reflection is seen as a hyperechoic line with posterior shadowing. In persons with normal, aerated lung tissue, any structures that can be seen on the US screen below the pleural line are thus due to artefact generation [1, 3, 4].

Diaphragm and adjoining abdominal structures

Recognition and identification of the diaphragm and abdominal structures is an essential part of FTUS and TUS, as these structures serve as important anatomical landmarks when differentiating intrathoracic structures from abdominal structures.

Diaphragm

The superficial part of the diaphragm can be recognised as a hyperechoic “double line” located just below the ribs. The more hypoechoic area between the lines represents the muscle fibres of the diaphragm. As the diaphragm contracts, movement and thickening of the fibres can be observed [17]. The more central portion or tendinous component of the diaphragm can be visualised when using either the liver or the spleen as an acoustic window [18–21].

Liver and right kidney

In healthy persons, the liver can be visualised when scanning the lower part of the thorax on the right side. Excellent images of the liver can generally be obtained when scanning in the midaxillary line. The liver is seen as a large, hyperechoic solid structure. When breathing in and out, the motion of the diaphragm causes displacement of the liver. Just below the liver, the right kidney can be visualised [22, 23].

Spleen, left kidney and stomach

In healthy persons, the spleen can be visualised when scanning the lower part of the thoracic cavity on the left side. The spleen is seen as a hyperechoic solid structure. Just below the spleen, the left kidney can be visualised. It is difficult to identify the stomach when it is empty or air filled; however, it is easy to identify when filled with fluid as a hypoechoic collection of fluid. Often, small hyperechoic dots representing air bubbles are present within the fluid, and sometimes an air–fluid interface can also be seen [22, 23]. The stomach is typically seen in the lower part of the thoracic cavity on the left side.

Focused TUS

The purpose of FTUS is most often to diagnose or exclude acute, potentially life-threatening conditions [22]. In comparison, the purpose of TUS is to diagnose and exclude all conditions in the chest that potentially can be visualised using sonography. Therefore, TUS often includes assessment of all areas of the pleura and lungs, which can be visualised by transthoracic scanning. TUS is often time-consuming and requires good patient cooperation, which are not always compatible with an emergency setting. When performing FTUS, usually only a limited area of the surface of the pleura and lungs is evaluated, and it can therefore be performed quickly with minimal discomfort to the critically ill patient [24, 25]. The use of FTUS has been demonstrated to have a high diagnostic accuracy for many of the common conditions seen in a variety of emergency settings [3, 10, 24, 26–34]. When used as part of whole-body ultrasonography, FTUS has been shown to help identify patients with missed life-threatening conditions, and it significantly increased the proportion of correctly diagnosed and treated patients admitted to an emergency department with respiratory symptoms [35, 36]. FTUS, alongside other forms of focused US, should be used as an integrated part of the clinical assessment of these patients.

FTUS scanning protocols

Several different FTUS protocols and approaches have been described; however, no international consensus for the use of one specific protocol has been obtained [3]. Many protocols divide the thorax into a number of scanning points, areas or zones that are assessed using US, but the number of zones varies significantly for each protocol [4, 9, 12, 13, 31, 37–48]. The number of areas or zones scanned is of importance, since diagnostic criteria for some conditions (e.g. interstitial syndrome) are defined by the number of zones in which specific findings are represented. In this way, the number of areas or zones may potentially affect the diagnostic accuracy when compared with other protocols or with use in other settings [3, 24, 31, 49]. As well as the protocols describing FTUS, several other studies have described the use of FTUS principles as an integrated part of a whole-body ultrasonography approach in which several organs or structures are assessed in a given patient population or clinical setting [4, 34, 35, 50–54]. Many studies have used an eight-scanning-areas approach, as described by VOLPICELLI et al. [44], for assessing the anterior and lateral thoracic surfaces; similarly, the definition of interstitial syndrome is also based on these principles [3, 44]. However, this approach does not include assessment of the posterior thoracic surface, which is why posteriorly positioned pathology may be missed using this protocol [44]. LAURSEN et al. [35] modified the protocol to a 14-zone scanning protocol by adding assessment of the posterior surfaces using principles described previously by LICHTENSTEIN et al. [4], and this has subsequently been validated in prospective studies in a variety of settings [35, 36, 55, 56]. The use of this 14-zone FTUS approach alongside FoCUS and limited-compression ultrasonography of the deep veins has been validated for assessing patients with respiratory failure in an emergency department setting [35, 36]. Using the 14-zone approach, each hemithorax is divided into anterior, lateral and posterior surfaces, which can be further subdivided into smaller squares representing a scanning zone. Each of these scanning zones should be assessed using FTUS. Each of the scanning zones can be denoted from 1R to 7R on the right and from 1L to 7L on the left (figure 5) [35]. As described in subsequent sections, when assessing the thoracic lateral and posterior surfaces, it is of paramount importance to begin the assessment with identification of the upper abdominal structures and diaphragm. This is done in order to avoid mistaking abdominal structures as thoracic structures and vice versa (e.g. fluid in the stomach wrongly diagnosed as pleural effusion). To facilitate this approach, caudal zones (e.g. zones 3 and 5) have lower numbers than cranial zones (e.g. zones 4, 6 and 7) [35].

Figure 5. Focused TUS (FTUS) scanning zones using a 14-zone approach. The numbers of the zones denote the optimal scanning sequence. When scanning the lateral and posterior surfaces of the thorax, the examination should begin in the most caudal zones (e.g. zones 3 and 5) to ensure accurate identification of the border (diaphragm) between the chest and upper abdomen.

Predefined questions

FTUS should be performed in a focused manner in order to answer specific and clinically relevant yes/no questions [22]. The FTUS protocol aims to answer the following four predefined yes/no questions: 1) Is a PTX present? 2) Is a pleural effusion present? 3) Is interstitial syndrome present? 4) Is obvious pathology present?

Several studies have documented that FTUS has a high diagnostic accuracy for conditions included as part of the predefined questions [10, 28, 35, 36, 54, 57–61]. It is, however, of importance to underline that FTUS has not been validated for a number of other conditions (e.g. pulmonary embolism, malignancy, chest wall pathology), and it has not been determined whether there is a substantial difference in diagnostic accuracy when FTUS is compared directly with TUS. Any incidental FTUS findings should therefore generally be referred to TUS performed by a trained expert. The same principles apply when reporting FTUS findings; terms such as “normal examination” should generally be avoided, since they imply the performance of TUS. FTUS reporting should be limited to answering the predefined questions: “No signs of PTX, pleural effusion or interstitial syndrome.”

General principles and preparation prior to performing FTUS

Basic US principles have been addressed in the previous chapter in this Monograph [62]. A few key aspects regarding FTUS are mentioned in the following sections.

Choice of transducer

A curved low-frequency transducer is generally the preferred transducer for FTUS as it has an acceptable image quality for both superficial and deep structures and can be used to answer the focused questions. In some scenarios, the use of other transducers can be considered, such as a microconvex transducer, linear transducer or phased array transducer.

Microconvex transducer

A microconvex transducer is an acceptable all-around transducer for FTUS. Due to its small size, it is often easier to use when the patient can only be examined in the supine position (e.g. emergency setting, intensive care unit (ICU)) or when scanning children [3, 4].

Linear transducer

A linear transducer generates excellent images of superficial structures, such as ribs and the pleura, but deeper structures can be difficult to assess. It excels when the only clinically relevant question is to determine whether PTX is present, or as a supplement to a curved low-frequency transducer if doubt exists as to whether lung sliding is present or not.

Phased array transducer

Due to its limited visualisation of the pleural line, a phased array transducer is generally less suitable for FTUS than most other transducers but is an acceptable alternative if no other transducers are available [3].

Choice of pre-set, scanning mode and US machine software

If a dedicated FTUS pre-set is available, then this should be used. In the absence of such a pre-set, an abdominal pre-set can be used when using a curved transducer, or a musculoskeletal pre-set when using a linear transducer. The presence or absence of US artefacts plays a pivotal role when performing FTUS. Software (e.g. harmonics, cross-beam) minimising artefacts should generally be switched off when performing FTUS [3]. Normally, the only mode needed for FTUS is B-mode.

Patient positioning

Initially, the anterior and lateral surfaces of each hemithorax are scanned with the patient in the supine position. Subsequently, the patient is asked to sit up and the zones on the posterior surface are scanned. Some patients with acute respiratory failure cannot be placed in a supine position due to severe dyspnoea. Anterior, lateral and posterior surfaces are then scanned with the patient in the sitting position. The patient position will affect the position of free air (e.g. PTX) and free fluid (e.g. simple pleural effusion) in the pleural cavities, and patient positioning is therefore of importance when assessing the patient for these two conditions. The supine position is the recommended position for assessing the presence of PTX, since the air will tend to be placed anteriorly in the chest cavity, an area that is easily assessable using FTUS. If the patient is placed in the sitting position, a small PTX can be missed if the air is located solely at the apex of the chest cavity, an area that is more difficult to assess using US [3]. In comparison, patients should be placed in the sitting position when scanning to detect a pleural effusion, in which case the fluid will tend to be in the lower posterior zones. Some critically ill patients may, however, not be able to sit up to enable assessment of the posterior zones. In this case, the posterior surfaces can be scanned either directly or indirectly (see section on Supplementary scanning techniques).

FTUS examination technique

Once the machine has been prepared for the examination and the patient has been positioned in the most optimal position possible, the FTUS examination can be performed. The transducer is placed in the first of the scanning zones in the vertical plane above an ICS with the orientation marker facing cranially (figure 1). The operator then notes whether PTX, pleural effusion, multiple B-lines or other obvious pathology is present. Once this has been assessed, the transducer is moved to the next scanning zone, and this approach continues until all accessible zones have been scanned.

Specific transducer placement and findings for each scanning zone

Each zone scanned as part of the 14-zone FTUS protocol has some special characteristics and potential pitfalls. Some of these are described for each zone.

Zones R1 and L1

Transducer placement: the transducer should be placed in the second ICS a few centimetres lateral to the sternum. If the transducer is placed too far laterally, additional tissue (e.g. muscle, breast) will be present between the transducer and the pleura, possibly decreasing image quality.

Zone findings: these resemble the basic sonoanatomy pattern described previously (figure 2).

Zone R2

Transducer placement: when moving the transducer from zone R1 to R2, the transducer is moved to ICS4 a few centimetres lateral to the sternum. The anatomical landmark is the lower edge of the pectoralis muscle or breast tissue. In most persons, this point corresponds to the anterior phrenicocostal sinus, and both the pleural line and the superficial part of the diaphragm can be visualised. In some patients, pericardium or mediastinal tissue will be visible instead of the pleural line. Moving the transducer a few centimetres laterally will often allow visualisation of the pleural line.

Zone findings: zone R2 findings consist of the pleural line on the left of the screen and the diaphragm and liver on the right of the screen.

Zone L2

Transducer placement: when moving the transducer from zone L1 to L2, the transducer should gradually be moved caudally one ICS at a time until the pericardium and heart appear on the right side of the screen. This approach reduces the risk of misinterpreting the pericardium as the pleural line. In patients with hyperinflation of the lungs or emphysema, the lung may obscure the view of the pericardium and heart. Zone L2 findings then often resemble the findings in zone R2.

Zone findings: zone L2 findings consist of the pleural line on the left of the screen and the pericardium and heart on the right.

Zone R3

Transducer placement: assessment of zone R3 should always begin with identification of the liver, right kidney and diaphragm. Most often, this can be done by placing the transducer in the midaxillary line on the lower part of the chest. Once the liver has been identified, the pleural line can be identified either by asking the patient to take a deep breath or by gradually moving the transducer to an ICS cranially until the pleural line can be visualised.

Zone findings: zone R3 findings consist of the pleural line on the left of the screen and abdominal structures on the right of the screen (figure 6). When the patient takes a deep breath, the pleural line is seen moving in from the left with “removal” of the liver due to posterior shadowing and movement of the diaphragm. This finding is also termed the “curtain sign” since it resembles a curtain being drawn, obscuring the view to the abdominal structures [63–67].

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