Sonography of the pleura

Introduction

The CME review presented here explains the significance of pleural sonography and provides guidance in its application. Given the resolution of the devices used during the early days of sonography, in the 1980 s, the pleura could only be perceived as a white line. Due to the high impedance differences, the pleura can be particularly well delineated. The advent of high-resolution devices of more than 10 MHz, allowed even a normal pleura with a thickness of 0.2 mm to be assessed. This article presents the specifics of the examination technique with knowledge of pretest probability and describes the indications for pleural sonography. Pleural sonography is of great importance in emergency and intensive care medicine, as well as prehospital, outpatient and inpatient settings, the family practice and the specialty practice of the pulmonologist. Specific pleural sonography pediatric [1] [2] [3] as well as geriatric features are described. The ability to identify pneumothorax, even in difficult situations, and the evaluation of a pleural effusion are also presented. Because of today's high-resolution technology, both the pleura itself and small subpleural changes can be assessed and diagnosed. Direct and indirect sonographic signs and concomitants symptoms are described, and the specific clinical value of sonography presented. The significance and criteria of conventional brightness-encoded B-scan, color Doppler ultrasound (CDS) with or without spectral analysis of the Doppler signal (SDS), and contrast-enhanced ultrasound (CEUS) are outlined [4] [5] [6] [7] [8] [9] [10] [11]. Elastography [12] and ultrasound-guided interventions are also mentioned. One related other paper deals with the diseases of the lung parenchyma [13] and another paper deals with the diseases of the thoracic wall, diaphragm and mediastinum [14].

Note 1

A variety of sonographic techniques are available for diagnosis and intervention.

Pretest probability and questioning

Pretest probability for pleural and lung ultrasound (LUS) describes the probability of having a specific pleural or lung disease before performing an additional diagnostic procedure such as LUS. Whether LUS can correctly identify pleural or pulmonary disease in a patient depends on the probability of it being a pre-existing condition in the patient examined (a priori probability), as well as the intrinsic advantages and disadvantages of the test and therefore the pertinent questions to ask to allow an evidence-based approach. Sonography with knowledge of symptomatology and medical history is not without error, and certain findings may be misinterpreted as disease (false-positive result), or disease may not be correctly identified (false-negative result).

Note 2

Considering the pre-test probability and specific examination question is essential for the success of LUS.

Advantages and disadvantages of sonography

Advantages

Basically, LUS is the consistent imaging continuation of the percussion and auscultation findings of the chest in the sense of “ultrasound the stethoscope of the future” [15]. Important advantages include real-time examination, bedside implementation repeatable at any time, lack of radiation exposure, high spatial resolution in the near-field region, conclusions about perfusion using CDS, spectral analysis, and CEUS, and safe imaging guidance of interventions. Basically, sonography represents a “dialogic” examination method [16].

Note 3

In patients with dyspnea, lung and pleural ultrasonography is often sufficient for treatment decisions.

Note 4

In patients with oncologic issues, computed tomography is the method of choice for staging, and ultrasound is used as an adjunct to address specific questions.

Disadvantages

Relevant disadvantages are limited imaging of pulmonary pathologies due to total reflection at the lung surface resulting in artefacts, echoes triggered behind bony structures, lack of overview, lack of imaging of central hilar pathologies and limited ability to assess patients in the supine position. It is estimated that only about 70 percent of the pleura can be seen due to its bony surroundings.

Pleura

Examination technique

Some diagnostic questions can be answered with a focused examination guided by localized symptomatology, but the majority of indications for LUS require a systematic and fundamentally bilateral comparative examination approach. The specific protocols depend on both the specific question and the clinical patient situation. If an examination is only possible in the supine position (e. g., for patients on ventilators) and if pneumothorax or an interstitial syndrome is suspected, an 8-region protocol with bilateral examination ventral-cranial, ventral-caudal, lateral-cranial, and lateral-caudal is recommended. In contrast, for semi-quantification of the interstitial syndrome, examination of 28 intercostal spaces is preferred [17] [18] [19] [20]. In patients with suspected (COVID-19) pneumonia and pulmonary artery embolism, examination of the dorsal portions of the lungs is important, and consideration must be given to the small size of peripheral lung consolidations. Therefore, a protocol with 14 scan regions was proposed, including 3 dorsal-paravertebral regions on both sides in addition to the mentioned 8 ventral and lateral lung regions [21]. Further modifications have been described: A 4-region protocol (ventral-cranial + lateral-caudal on both sides), a 6-region protocol (3 scan zones each from ventral-cranial to median mid-chest to lateral-caudal on both sides), and a 12-region protocol with 3 cranial and 3 caudal scan zones each (parasternal, anterior axillary, and posterior axillary), which takes into account the fact that COVID-19 patients and those with ventilatory insufficiency can often only be examined in the supine position [8] [22]. This diversity of protocols makes it difficult to compare study results, findings, and scores. It is therefore recommended that the examination protocol used be indicated in the report and, if possible, that only one standard protocol be used at any given facility.

Another problem is that the expression of certain diagnostic artifacts of lung and pleural sonography, especially vertical artifacts, depends on the selected transducer (especially central frequency and bandwidth), instrument settings or presets, focus, time-gain compensation (TGC), and the use of artifact minimizing technologies (harmonic imaging, compounding). The available literature is limited and predominantly dates from the last 3 years [23] [24] [25] [26], which means that the International Consensus Guidelines published in 2012 were not yet able to provide any concrete recommendations [17]. International consensus recommendations from 2023 address the issue in detail and suggest the use of optimized LUS presets, but without detailing specifics [11]. It is known from experimental but not clinical studies that high mechanical index examinations can induce subpleural pulmonary capillary hemorrhage [27]. Whilst a clinically adverse effect remains to be proven and may be particularly apprehended in newborn examinations, capillary hemorrhaging involving the lung parenchyma itself could be misinterpreted as the starting point of B lines and thus yield false-positive results [11]. Therefore, the current consensus recommendations, following publications of the safety committees of national and international ultrasound professional societies, advise the use of a mechanical index of 0.4–1.0 (depending on age and thoracic wall thickness) and a limitation of the exposure time [11] [28].

Few conclusions worthy of generalization can be drawn from the published literature, which were first summarized in the proposals for standardization of LUS for the examination of COVID-19 patients [21] and have been summarized in [Table 1], with due consideration of current data.

Table 1 Proposals for the standardized imaging settings for LUS (after [11] [21] [23] [24] [25] [26].

The choice of convex or linear transducers depending on the patient's height; for detailed pleural imaging, high-frequency linear transducers are preferable. Single focus with the focus set on the pleural line. Turn off artifact suppression algorithms such as Tissue Harmonic Imaging, Frame Averaging (Persistence) and Compounding. Use the highest possible frame rate (i. e. no multi-focus, low persistence). Use a dynamic range of at least 60 dB. Avoid saturation phenomena (“white image”) by minimizing gain and MI. Limit the mechanical index (MI) to 0.4–0.7 (for neonates rather 0.4, for children and adults gradual reduction starting with 0.7 as long as image quality is maintained) and minimize exposure time to what is diagnostically required.

Normal sonography findings

The pleura consists of the pleura parietalis and the pleura visceralis. The normal pleura cannot be visualized by sonography. In the absence of pathology, neither pleural sheets can be visualized as a single mesothelial cell layer of only a few micrometers in diameter, even with high-frequency transducers. The pleural sheets can however indirectly be visualized as hyperechoic interface echoes. The reflex band of the aerated lung (lung interface line) can be seen as a fine line. Above this are intercostal muscles and fatty tissue, followed as a fine anechoic line by the pleural cavity and the hyperechoic visceral pleura, which cannot be delineated from the lung surface. Only in pathological processes (inflammatory and or neoplastic) does the then thickened pleura become directly visible as an anechoic structure that can be easily assessed, particularly with high-frequency linear transducers. In pathologically altered pleural sheets, there is usually increased fluid in the pleural cavity (often only locally), so that these can be easily visualized as a result of the higher impedance differences ([Fig. 1]). In the real-time B-scan, both pleural layers shift synchronously (“lung sliding”). This can be visualized in M-mode as well as in B-scan, better still with color Doppler or power Doppler.

Fig. 1 Normal pleural line with high-resolution linear probe. The normal pleura cannot be visualized by sonography. The reflex band of the aerated lung (lung interface line) can be seen as a fine line. Above this are intercostal muscles and fatty tissue, followed as a fine anechoic line by the pleural cavity and the hyperechoic visceral pleura, which cannot be delineated from the lung surface.

When assessing the pleura, it is recommended to start with the abdominal convex probe. In order to have the highest possible resolution of the pleura, the focus position should be shifted to the level of the pleural line. Although spatial resolution is limited with the convex probe, at least pleural sliding, regularity of pleural contour, any effusions, and acoustic phenomena emanating from the pleura and subpleural lung can be assessed ([Fig. 2]). Detailed assessment of the pleura should be performed with the high-resolution linear array transducer. Again, it is important to move the focus to the height of the pleural line to get the highest possible resolution. Regardless of the transducer type, it is important to differentiate the hyperechoic interfacial phenomena of rib cortices and pleura from each other in a section transverse to the rib process.

Fig. 2 Differentiation of the hyperechoic interfacial phenomena of rib cortices (*) and pleura (arrows) in a section transverse to the rib process. If the hyperechoic interfaces are connected with each other, a contour (yellow) is created, which is reminiscent of the silhouette of a flying bat (“bat sign”).

Indication(s), description of the typical situation

Described pathologies include tumors, thickening, interruption of the pleural line (fragmentation), and subpleural consolidations. Tumors are generally anechoic or hyperechoic, round or flat-lying processes, which are evaluated based on their margins and perfusion. Tumors can arise from both the parietal and visceral pleura. A pathologically altered pleura is notable due to the comet tail artifacts emanating from it ([Fig. 3]). Depending on the disease (common in viral infections), small (2–3 mm) subpleural consolidations may be apparent, also characterized by comet tail artifacts.

Fig. 3 Representation of different pleural artifacts. The left illustration A represents a normal Herring-Breuer inflation reflex with multiple A lines (arrows). The right illustration B shows a pathological irregular Herring-Breuer inflation reflex with vertical artifacts in terms of B lines. The underlying pathology of this localized interstitial syndrome needs to be established clinically. In this particular case, it involved a right-sided pleuropneumonia.

Pleurisy

The sonographic patterns of pleurisy are uncharacteristic and variable, and changes in the pleural line are often not distinguished from normal findings. The formation of a (minor) effusion is indicative ([Fig. 4]).

Fig. 4 Depicts different ultrasound findings in patients with a clinical diagnosis of pleurisy. In addition to pleural thickening, evidence of a small pleural effusion, lesions of varying sizes may be observed in the parietal pleura. The pathology underlying these sonographic findings needs be established clinically.

Pleural effusion

A pleural effusion is characterized by the presence of fluid between the parietal and visceral pleura. A small degree of effusion is physiological: 0.3 ml/kg [29]. A pleural effusion occurs when there is an imbalance between production and absorption. An effusion may be found free or encapsulated between the two pleural sheets. Free pleural effusion is found basally in the costophrenic angle. The effusion should be sought laterally dorsally in the supine patient and dorsally in the seated patient. Occasionally, a subcostal transhepatic scan is helpful when the patient is in a supine position ([Fig. 5]).

Fig. 5 Patient with right-sided chambered effusion, this can be visualized by intercostal (left) and subcostal transhepatic coupling (right).

Encapsulated pleural effusions are localized at the margins of various pleural pathologies ([Fig. 6]), but may also persist as residuals after interventional therapy for extensive pleural effusions. The encapsulated pleural effusion is assessed with the small part linear probe. A free pleural effusion needs to be located with the abdominal convex probe for a better overview. Particularly in overweight individuals with poor supine acoustic conditions, it is not always easy to determine whether the anechoic region above the diaphragm consists of fluid. The spine sign [30] [31] may help to confirm the diagnosis. The adjustment of the costophrenic angle must be done in such a way that the transverse processes of the spine are detected with their acoustic shadow. If these are only visible below the diaphragm and the lung and the lung extinguishes them during breathing like a curtain, the spine sign is negative; there is no pleural effusion. However, if the transverse processes are also visible above the diaphragm, there is pleural effusion ([Fig. 7]). If B lines resp. comet tail artifacts seen emanating from the pleura near the diaphragm, this represents lung tissue and a pleural effusion can be ruled out.

Fig. 6 Patients with localized effusion formation and different pathologies such as pleurisy (left), parainfectious pleural empyema (middle), and a pleural carcinomatosis (right) presenting with a pleural metastasis (m) (arrow).

Fig. 7 Showing the costophrenic angle to exclude and detect a small leaking pleural effusion; in the absence of effusion, the liver is reflected on the diaphragm (left). Detection of comet tail artifacts from the pleura adjacent to the diaphragm ensures that no pleural effusion is present (center). In the right illustration, the costophrenic angle is anechoic. The positive spine sign confirms that it is a pleural effusion. The transverse processes of the vertebral bodies are always seen in the abdominal region, but in the thoracic region they are only observed in the presence of pleural effusion or lobar pneumonia. An inflated lung leads to total reflection of the sound waves.

Volume determination

An accurate quantification of pleural effusions is often inadequate due to the complex spatial geometry and the often poorly defined position of the patient. Nevertheless, there are various formulas for estimating the volume [32] [33]. The simplest method is to multiply the lateral height extent of the effusion in centimeters measured from the costophrenic angle in the sitting position by a factor of 100 (volume in cubic centimeters) ([Fig. 8]). In another method [29], patients are examined lying down with the trunk slightly elevated by 15°. The probe is moved upward into the posterior axillary line. In cross-section to the body axis, the distance between pleura parietalis and pleura visceralis is measured at the base of the lung, at the end of expiration. The amount of pleural fluid is evaluated with the formula: Volume (ml) = 20 × distance between both pleural sheets (mm).

Fig. 8 Semiquantitative volume estimation in a patient with right-sided pleural effusion. A lateral effusion height of 7 cm is measured; multiplied by 10, this results in an effusion of approximately 700 ml.

However, determining the volume as accurately as possible is of little clinical relevance, although it is frequently performed. The indication for effusion puncture or drainage arises either for diagnostic reasons, due to a complicated effusion or empyema, or due to the patient's shortness of breath symptomatology. The severity of a patient's shortness of breath caused by a pleural effusion is not determined by the effusion volume alone, but is also determined by the patient's height, body mass index, and preexisting cardiopulmonary diseases, among other factors. In pre-existing heart failure with additional chronic obstructive pneumopathy, relief of as little as 300 mL may provide a clinical benefit to the patient. We therefore prefer a general classification such as angular, low basal, moderate, or marked pleural effusion and to establish the indication for puncture according to clinical criteria.

Type of effusion

Conclusions regarding the nature of the pleural effusion based on sonomorphology alone are limited. Fibrous changes as well as septations often occur in a chronic effusion; echogenic internal structures indicate hemorrhage or infection, but must be differentiated from artifacts. Echogenicity (anechoic, homogenous, or heterogenous echogenicity) can only give an indication of the cause of the effusion in conjunction with the clinical situation [34] [35]. While cardiogenic effusions or even a fresh hemothorax are often anechoic, echogenic effusions or those with internal echogenic reflexes may indicate an inflammatory or malignant etiology ([Fig. 9]), although many malignant effusions are also anechoic.

Fig. 9 Patients with anechoic (left), hyperechoic (middle) and polyseptated pleural effusion (right). Sonographic evaluation of the effusion in terms of echogenicity is not useful for the benign/malignant classification.

More important than echogenicity are: laterality, the absence of an encapsulation, local or diffuse pleural and pulmonary changes, and concomitant disease and age of the patient. Therefore, in addition to the effusion, the entire visible pleural cavity, the diaphragm, and the pleura itself should always be assessed ([Fig. 10], [11]).

Fig. 10 Patients with pleural effusion and sonographic imaging of lung pathologies: A confirmed lung metastases, B clinically confirmed pneumonia with air bronchogram and polyseptated effusion, C peripheral lung consolidations with pulmonary artery embolism confirmed on the CT scan, D pleural effusion with atelectasis and fluid bronchogram in confirmed central bronchial carcinoma (bottom right).

Fig. 11 Patients with malignant pleural effusions and pathologies involving the pleural region, the arrows indicate tumor structures: confirmed pleural metastasis from the right costodiaphragm angle of a bronchial carcinoma (top left), confirmed diaphragm metastasis imaged from a subcostal/transdiaphragm radiographic angle in a patient with a primary breast carcinoma (top right), visceral pleural metastasis in a bronchial carcinoma (bottom left), extensive pleural tumor structure in a mesothelioma (bottom right).

This allows the detection and also the ultrasound-guided biopsy of centrally located tumors, metastases on the diaphragm or pleural tumors, which elude ultrasound imaging when the lung is unfolded and filled with air. Unless there is a clinically clear reason for the pleural effusion (e. g., cardiogenic bilateral effusion in heart failure or unilateral effusion in chest trauma with rib fractures), the effusion must be punctured for diagnostic reasons and examined both biochemically and cytologically, and possibly also microbiologically [36].

Primarily, a transudate must be differentiated from an exudate in the case of non-traumatic effusion [37]. A usually bilaterally detectable transudate is due to volume overload as in heart failure and/or low oncotic pressure in albumin deficiency (e. g., liver cirrhosis, renal or intestinal loss). Exudates, on the other hand, are usually caused by pleural disease and thus are present only on the affected side. The differentiation can be made according to the Light criteria, published in 1972 [38] [39].

A threefold increase in the LDH concentration in pleural fluid compared to the norm occurs in the presence of an infection (incl. tuberculosis), rheumatological diseases, or neoplasia. The Light criteria are highly sensitive for the diagnosis of exudate, but misclassify up to 25 % of all transudates as exudates. Numerous additional details and alternatives have therefore been published. In particular, the determination of serum pleural gradient, cholesterol concentration and gradient, and tumor markers [40] [41] [42] [43] [44] must always be interpreted in the context of the patient's clinical picture. Effusion cytology has an overall sensitivity of only 50 %–80 % [45], but this increases with the volume of effusion sent for analysis and with repeat punctures.

Pneumothorax

Pneumothorax is air in the interspace between the parietal pleura and visceral pleura. Pneumothorax must be included in the differential diagnosis of any patient with dyspnea. Pneumothorax may occur spontaneously (for example, in patients with chronic obstructive pulmonary disease and emphysema, congenital connective tissue weakness, e. g., in Marfan syndrome, or cystic lung disease). Pneumothorax often occurs as part of trauma (stab injury causing an air bridge from the pleural cavity to the exterior or rib fracture causing an air bridge from the pleural cavity to the injured bronchi). Iatrogenic pneumothorax can occur both during external thoracentesis or by bronchial injury via bronchoscopy. Because of the elasticity of the lung, which collapses in the absence of negative pressure in the pleural cavity (estimated to be about 8 cm of water column), pneumothorax tends to increase with time. This is counteracted by natural pleural air reabsorption. The most sensitive method for detecting pneumothorax is computed tomography, which is considered the reference standard. In trauma patients, pneumothorax can be diagnosed by sonography with a sensitivity of 90 %, whereas chest radiography achieves a sensitivity of only 69 % [46]. However, only pleural sliding was assessed in this study and other sonographic criteria of pneumothorax ([Table 2]) which can be used to further increase diagnostic accuracy, were not included.

Table 2 Sonographic signs of pneumothorax.

Absence of lung sliding Detection of the lung point Missing comet tail artifacts and B lines Absence of lung pulse (M-mode or color Doppler)

In 2012, a diagnostic algorithm was proposed in a consensus conference [17]. In a slightly revised version, lung and pleural sliding are evaluated first ([Fig. 12], [13]). Direct signs (absence of lung sliding, evidence of lung point) have a higher value than indirect signs (absence of artifacts, absence of lung pulse (M-mode or color Doppler) ([Fig. 14]). The absence of lung sliding is typically found in the supine patient at the highest point approximately one hand width below the clavicle. However, this is only the case if there are no adhesions between the visceral pleura and the parietal pleura.

Fig. 12 Pneumothorax algorithm according to Volpicelli et al 2012, slightly modified by placing the lung pulse before the lung point. The lung pulse can almost always be detected in an inflated lung, but the lung point often cannot be found in a large pneumothorax.

Fig. 13 The left image shows the normal findings with normally aerated lungs (top B-mode, bottom M-mode). The M-mode image resembles a sandy beach (seashore sign). The right image shows the pleura with a transverse rib at the top. In color Doppler sonography, pleural sliding is apparent as a positive color sign (bottom right).

Fig. 14 Patient with pneumothorax and evidence of a lung point (arrows): Patient with right-sided mantle pneumothorax and evidence of a single lung point, the extent of the pneumothorax cannot be determined by sonography (left image). A small pneumothorax as evidenced by 2 lung points following the US guided puncture of a small pleural metastasis (M) (right image). The absence of a pleural effusion, increases the risk of a puncture-related pneumothorax.

Note 5

Direct signs of pneumothorax are superior to indirect signs.

On lung ultrasound, the skin, subcostal fatty tissue, musculature, and parietal pleura are visible as immobile, parallel lines (“waves”) in healthy individuals. The visceral pleura and lungs move so that they resemble a sandy beach with their grainy pattern (“seashore”) ([Fig. 15]). The presence of lung sliding or the seashore sign proves that there is no pneumothorax at this site. The skin layers, thoracic musculature, and parietal pleural line (“pleura parietalis”) are delineated as static (immobile) and parallel lines (“seashore”) as opposed to the visceral pleural line (pleura visceralis) and lung parenchyma (“waves”) which move with the respiratory cycle. The presence of comet tail artifacts and B lines, which originate from the visceral pleura, also excludes pneumothorax. If neither lung sliding nor B lines/comet tail artifacts are present, the lung pulse is sought. This shows the cardiac action transmitted, through the mediastinum and an inflated lung, all the way to the pleural line. A synchronous heartbeat pulsation is indicative of an inflated lung. It can be visualized in M-mode (regular warping of the horizontal lines to the pleural line) or by color or power Doppler ([Fig. 16], [17]).

Fig. 15 Patient with pneumothorax: In M-mode only horizontal lines are displayed (only “water without sand”) = stratosphere sign or barcode sign.

Fig. 16 The lung pulse is shown in M-mode. Recorded along with the ECG, there is a pleural line displacement at each QRS complex on the ECG. Evidence of a lung pulse means that the lung is inflated at that location.

Fig. 17 The lung pulse is shown in color. A color sign below the pleura results from both respiratory motion and lung pulse. Thus, a positive color sign means that the lungs are unfolded at this point.

A positive color sign in the color or power Doppler, caused either by the lung pulse or by pleural sliding, excludes pneumothorax.

On sonography, the size of the pneumothorax can only be estimated by means of the position of the lung point. The lung point is the site of transition of the pneumothorax into the lung which is normally adjacent to the thoracic wall. Accurate assessment of the extent of a pneumothorax is reserved for radiography or computed tomography. In extensive pneumothorax, the lung point is often not visible.

Solid pleural lesions and subpleural parenchymal changes (diffuse, circumscribed)

Pleural tumors usually occur in a localized fashion. They can be subdivided into primary pleural tumors or metastases [47]. The most common primary pleural tumor is a malignant pleural mesothelioma ([Fig. 18]). All other primary tumors such as solid fibrous pleural tumor ([Fig. 19]), sarcoma, and hemangioendothelioma are very rare. Malignant pleural mesothelioma is caused by asbestos exposure in 87 % of men and 65 % of women. The median survival after diagnosis is short [47]. Pleural mesothelioma may manifest in various forms, as plaques or planar structures and may involve the visceral or parietal pleura [48]. Pleural metastases occur in a large variety of tumors; frequently, the primary tumor is a lung or breast carcinoma ([Fig. 10], [11]).

Fig. 18 CT scan (Source: Prof. Dr. Andreas H. Mahnken, Direktor der Diagnostischen und Interventionellen Radiologie am Universitätsklinikum Marburg) and ultrasound of a patient with malignant pleural mesothelioma: A markedly thickened parietal pleura is seen in the apical region of the lung (upper images), and in the caudal diaphragm region the tumor (TU) breaches the diaphragm (lower images).

Fig. 19 Patient with a histologically confirmed solid fibrous pleural tumor: CT scan (Source: Prof. Dr. Andreas H. Mahnken, Direktor der Diagnostischen und Interventionellen Radiologie am Universitätsklinikum Marburg) and B-scan ultrasound.

Tumors may involve the visceral or the parietal pleura ([Fig. 20]). In pleural tumors, a biopsy is usually necessary so that histologic diagnosis and appropriate therapy can be initiated. Ultrasound-guided biopsy has a very high diagnostic accuracy and a low complication rate [49], as a puncture route can be chosen to avoid the interposition of ribs and intercostal vessels. It is best to biopsy a parietal pleural lesion at a site where pleural effusion is present to reduce the risk of pneumothorax ([Fig. 11], [14]).

Fig. 20 Top row: CT scan (Source: Prof. Dr. Andreas H. Mahnken, Direktor der Diagnostischen und Interventionellen Radiologie am Universitätsklinikum Marburg) and ultrasound, of a patient with a small visceral pleura/ lung metastasis (M), showing that the lesion slides with the respiratory cycle of the lung. US guided puncture (top right) resulted in the diagnosis of lung metastases from a primary malignant melanoma. The risk of pneumonia tends to be reduced in the case of lung metastases. Bottom row: Patient with squamous cell carcinoma and small parietal pleural metastasis, the lung slides over the pleural lesion (arrows) (lower left). US guided puncture (bottom center) resulted in the diagnosis of a lung metastasis. Post-intervention, a small pneumothorax was detected (see also Fig. 14) (bottom right).

Subpleural parenchymal changes may be diffuse or circumscribed. Diffuse changes include pulmonary diseases that lead to changes in the pleural reflex (pulmonary fibrosis with various causes such as sarcoidosis, amiodarone induced pulmonary fibrosis, systemic lupus erythematosus, and others) ([Fig. 21]). However, sarcoidosis and amiodarone induced pulmonary fibrosis are not primary pleural diseases, but diseases that extend from the lung to the pleura. Localized changes may include pleural scars after radiation therapy, inflammation, trauma, lung metastases, or tumors in contact with the visceral pleura ([Fig. 22]). Diffuse pleural changes also include less common causes such as IgG4 associated diseases that affect the pleura and correspondingly lead to pleural effusion. These may be suspected if lymphoplasmacytic infiltration is present in the pleural effusion or pleural biopsy [50] [51]. By sonography, pulmonary fibrosis presents with a picture of a severely altered pleura, which is thickened and fragmented, with partial subpleural small consolidations, and an interstitial syndrome with many comet tail artifacts.

Fig. 21 Patients with confirmed pulmonary fibrosis: in the upper row A, CT scan (Source: Prof. Dr. Andreas H. Mahnken, Direktor der Diagnostischen und Interventionellen Radiologie am Universitätsklinikum Marburg) shows dorsal fibrotic changes; on the right side, the corresponding ultrasound image shows multiple small nodular visceral lesions (arrows). In the lower row B, CT scan (provided by *BLINDED*, Marburg) depicts distinct changes, ultrasound shows nodular foci (arrows). Sonography is not suitable to show the extent of computer tomography fibrotic changes.

Fig. 22 Patients with confirmed pleural pathologies: change due to scarring (A), scleroderma (B); GvHD of the lung (C), tuberculosis (D), Mediterranean fever (E), neurofibroma (F); B-scan sonography of lesions is nonspecific with no characteristic features, a clinical classification is essential.

A differential diagnosis is not possible based on the sonographic appearance alone. However, sonography helps in certain diseases. Thus, pathologic lymph nodes may be found in sarcoidosis, which are readily accessible by biopsy, if superficial.

Fibrothorax

Fibrothorax is a scarring change of the pleura. Other terms include diffuse pleural thickening, pleural rind, and pleural fibrosis. Adhesion of both pleural layers can lead to respiratory distress due to the resulting restriction. The causes are diverse. More common, however, are benign changes such as a post pleurisy status (infection, tuberculous, drug-induced, uremic, rheumatism) or a post hemothorax status. If the parietal pleura is thickened and has focal calcifications, this indicates asbestos-induced fibrothorax [52] [53].

Important differential diagnoses are pleural calcifications after previous hemothorax or specific pleurisy. Biopsy is often necessary to force a differentiation of malignant from benign disease. However, histological confirmation of the diagnosis is often not very easy [54].

Interstitial syndrome – vertical reverberation artifacts

Based on clinical and experimental studies, vertical reverberation artifacts are heterogeneous artifacts with characteristics closely related to pleural and subpleural tissue composition. These artifacts may be short or long, bright, smooth, well or poorly defined, narrow or wide, and of varying shapes [55]. Isolated vertical artifacts are also observed in healthy, often older individuals (mostly in basal lung sections). A positive finding is considered to be present when an examined region displays three or more vertical reverberation artifacts between two ribs on a sagittal section (“interstitial syndrome”). The interstitial syndrome may be focal, unilateral, bilateral or ubiquitous. Ubiquitous means that in the supine patient at least 2 regions are positive for vertical repeating artifacts on both sides ventrally and laterally [17].

Adjustment

The vertical reverberation artifacts can in principle be visualized with all probe types, but in adults typically a convex probe with a frequency range between about 3 and 6 MHz should be used. Since the new ultrasound devices often have artifact suppression programmed into the presets, it is imperative that these settings be turned off, otherwise they also suppress reverberation artifacts. The vertical reverberation artifacts can be assigned to different diseases based on their presentation. They are divided into B lines and comet tail artifacts. To make this distinction, the pleura must be assessed with a high-frequency probe (10 MHz and above).

B-lines

B-lines are hyperechoic reverberation artifacts that originate from a smooth pleural line (assessed with a high-frequency probe) and extend to the lower edge of the image (more than 10 cm) without attenuation. They are always the same width, overshadow everything, and move with the lung sliding. If ubiquitous, this corresponds to pulmonary edema. They typically occur in cardiogenic pulmonary edema.

Comet tail artifacts

Comet tail artifacts are strongly echogenic reverberation artifacts that originate from an irregular, fragmented pleural line which often appears thickened (assessed with a high-frequency probe). They arise at a pathologically altered pleura, originate from subpleural consolidations or from the edge of lung consolidations. They vary in width, and end at different depths (less than 10 cm). If ubiquitous, they may be indicative of pulmonary fibrosis (e. g., sarcoidosis, pneumonitis, and others). Focal comet tail artifacts occur in pulmonary contusion, pleurisy, and other pleural diseases.

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