Chapter 3
Chest wall and parietal pleura
Maged Hassan1,2,3 and Najib M. Rahman1,2,4
1Oxford Centre for Respiratory Medicine, Oxford University Hospitals NHS Foundation Trust, Oxford, UK. 2Oxford Respiratory Trials Unit, Nuffield Dept of Medicine, University of Oxford, Oxford, UK. 3Chest Diseases Dept, Faculty of Medicine, Alexandria University, Alexandria, Egypt. 4Oxford NIHR Biomedical Research Centre, Oxford, UK.
Correspondence: Maged Hassan, Oxford Centre for Respiratory Medicine, Churchill Hospital, Old Road, Oxford OX3 7LE, UK. E-mail: magedhmf@gmail.com
US examination of the chest wall and parietal pleura can reveal a number of pathologies and is useful in various clinical settings. Chest wall pathologies that are identifiable during US examination include haematomas, abscesses, rib abnormalities and lymph nodes. Additionally, useful information about pleural infection and neoplasia can be gathered using US. In addition to its diagnostic potential, US provides guidance for safe execution of percutaneous sampling and/or drainage of chest wall and pleural abnormalities.
Cite as: Hassan M, Rahman NM. Chest wall and parietal pleura. In: Laursen CB, Rahman NM, Volpicelli G, eds. Thoracic Ultrasound (ERS Monograph). Sheffield, European Respiratory Society, 2018; pp. 31–42 [https://doi.org/10.1183/2312508X.10006217].
Performing TUS for examination of the chest wall and pleura is indicated in a number of situations. In many instances, it is used to explore further and examine in more detail pleural or parietal abnormalities first seen on a chest CT scan. In other situations, TUS is used up front, particularly for evaluation of patients with localised thoracic lesions. It is the investigation of choice to first characterise palpable chest wall lesions that are picked up during clinical examination [1]. In patients with chest trauma, TUS can detect chest wall, pleural and lung injuries with high sensitivity [2]. TUS is especially valuable in delineating chest wall/pleural involvement by peripheral pulmonary masses, which has an important implication for staging. Finally, TUS is usually preferred to CT for guidance of invasive drainage and biopsy procedures due to its safety profile, live procedural ability and lower cost [3].
Technical considerations
The exact technique of chest sonographic examination, as well as the physics of TUS, is covered elsewhere in this Monograph [4, 5]. Generally speaking, examination of the chest wall and parietal pleura can be achieved with a combination of low-frequency probes (range 3.5–5 MHz), a convex array (curvilinear) or phased array (sector) probe, and higher-frequency linear array probes (range 5–7.5 MHz) [6–8].
Chest wall anatomy and pathology are ideally appreciated with a linear probe, given the superficial position of the chest wall [9]. Some sonographers find the phased array probe useful when examining the pleura and lung in patients with narrow intercostal spaces [6]. This probe is especially useful for moving structures, such as the heart. The convex probe, however, is generally the preferred probe for pleural and peripheral lung examinations, and is particularly superior to the phased array probe during guidance of invasive procedures [10, 11]. This is because the phased array probe has a narrow footprint, which misses large parts of the superficial layer of the chest wall and consequently cannot capture a portion of the invading needle.
Most of the TUS examination of the chest wall and pleura is conducted in the B-mode [7]. M-mode examination is not particularly useful except for exclusion of pneumothorax (PTX) following biopsy, given that it is a mode that captures movement. In addition to grey-scale imaging, colour Doppler is important for delineation of the vascular pattern of lesions, which is an essential component of characterisation and diagnosis. Doppler is used to identify the intercostal artery, which is necessary to minimise the risk of vascular injury during percutaneous invasive interventions [10, 12].
During TUS examination, the authors prefer to orient the scanning probe parallel to the ribs to obtain the maximum view of the pleura and to avoid the acoustic shadowing created by the ribs (figure 1b). The perpendicular probe orientation is useful mainly during examination of the acutely breathless patient in order to identify and quantify the number of intercostal comet tail artefacts, which is helpful in the diagnosis of pulmonary oedema [13]. As well as this indication, the parallel orientation should be followed during examination of the chest wall, pleura and lung to maximise the acoustic window.
Figure 1. Layers of the chest wall as seen using a high-frequency linear probe. a) Parallel orientation; b) perpendicular orientation. 1: skin; 2: s.c. fat; 3: muscles; 4: intermuscular septum; 5: outer rib cortex (with acoustic shadow seen deeper); 6: pleural band.
Chest wall
Anatomy
The chest wall is composed of four layers: the skin, s.c. fat, various layers of chest wall muscles, and ribs and intercostal muscle [14]. Very detailed anatomical images are made possible by high-frequency TUS (figure 1a). The skin appears as a thin isoechoic layer followed by s.c. fat, which is hypoechoic with interposing isoechoic fascial septa [12]. On TUS, muscles appear as generally hypoechoic with echogenic striae and dots depending on the orientation of the muscle fibres [12]. Different muscles are separated by dense fascial septa that appear hyperechoic. The outer cortex of ribs is hyperechoic, and since it causes reflection of most of the US waves, all deeper structures are obscured and only an anechoic shadow of the overlying rib (termed an acoustic shadow) is seen (figure 1b).
The intercostal vessels run between intercostal muscles. They run in the middle of the space in the most medial 5 cm of their course anteriorly and posteriorly. For the rest of the course, the neurovascular bundle is hidden under the lower border of the superior rib, which makes it elusive for TUS identification. Some subjects have an aberrant course, especially in the elderly, and excluding the presence of an unprotected artery in the pleural space is essential before interventional pleural procedures (figure 2).
Figure 2. Colour Doppler impulse indicating the position of the intercostal artery (arrow) just superficial to the pleura.
Pathology
The majority of chest wall lesions tend to be localised [12]. TUS has the potential to uncover lesions that are too subtle to be picked up on conventional CXRs. This makes TUS the investigation of choice in patients with localised pleuritic pain or a palpable lesion [15]. The range of chest wall lesions that can be identified by TUS is summarised in table 1 [1, 7, 10]. As mentioned previously, grey-scale images should ideally be complemented with colour Doppler scanning for identification of the vascular pattern of a given lesion [12].
Table 1. Pathological chest wall lesions identifiable by TUS
Soft tissue lesions |
Fluid collection (e.g. haematoma, abscess) |
Tumours |
Benign (e.g. lipoma, fibroma) |
Malignant (e.g. sarcoma, metastasis, invasion from lung/pleura) |
Lymph nodes (inflammatory, infective or malignant) |
Bony lesions |
Fractures |
Metastatic deposits |
Soft tissue lesions
Commonly identified lesions include lipomas, haematomas and lymph nodes. The combination of grey-scale findings and the colour Doppler pattern supplemented by clinical data can facilitate reaching a provisional diagnosis. A biopsy/aspiration is usually required for definitive diagnosis, and this is generally conducted with US guidance.
Fluid formations usually adapt to the shape of the surrounding more rigid structures [10]. Haematomas, commonly seen in trauma patients, can have various degrees of echogenicity depending on the erythrocyte content and can exhibit denser echoes due to organisation in older lesions [1]. Chest wall abscesses show heterogeneous echoes, which makes differentiation from organising haematomas challenging, particularly because of the tendency of undrained haematomas to be complicated by infection. The presence of a capsule surrounding the abscess facilitates differentiating the lesion from an organised haematoma [1].
Thoracic lymph nodes arise in the context of various inflammatory/infective conditions, lymphoma and lymph node metastases (table 1). Each of these three categories has distinctive sonographic features [7]. Characterisation depends on the grey-scale features, as well the vascular pattern seen by Doppler [16]. Reactive lymph nodes typically appear oval or triangular, with preserved architecture and an intact capsule [6, 7]. Their size rarely exceeds 20 mm and they have central regular vascularisation [1]. Lymph node metastases appear as round to oval structures with irregular borders and occasional infiltration of surrounding tissues [6, 17]. They show a “corkscrew” pattern of vascularisation [18]. In lymphoma, lymph nodes appear rounded or sharply bordered with irregular vascularisation but without infiltration of the surrounding tissues [1, 7]. It is important to note, however, that these findings are only suggestive of the aetiology and should be considered along with the clinical data. Histopathological confirmation is warranted in most cases, as US appearances can be nonspecific [19].
Malignant infiltration of the chest wall by distinct nodules or by extension from the lung and pleura is accurately delineated by TUS [20]. Malignant space-occupying lesions commonly appear hypoechoic with occasional hyperechoic parts (figure 3). Studying the vascular pattern of a chest wall swelling can suggest malignancy if an irregular pattern is identified by colour Doppler examination [1]. Colour Doppler is valuable during the planning of biopsies from a suspected swelling, which is an attractive target for invasive sampling due to its superficial position.
