Congenital pulmonary airway malformation, previously known as congenital cystic adenomatoid malformation, accounts for 30–47 % of all fetal lung masses detected by US. CPAM is a hamartomatous malformation of the lung with abnormal branching of immature bronchioles that communicate with the tracheobronchial tree (Epelman et al. 2010; Correia-Pinto et al. 2010). Sonographic findings vary depending on the type of malformation (May et al. 1993). Stocker et al. initially classified CPAM into three histologic types (Stocker et al. 1977), which were later expanded into five types (Stocker 1994). The first three types are the forms most often recognized on US. Type 1, the most common, appears on US as single or multiple large cysts, often affecting the entire pulmonary lobe. Type 2 is seen as an echogenic mass with numerous small cysts. Type 3 malformations appear as homogenous echogenic masses without cysts.

Sonographic differentiation between CPAM and pulmonary sequestration may not always be possible. A systemic vessel arising from the aorta has also been described in patients with pulmonary airway malformation (Winters et al. 1997). Furthermore, hybrid lesions are often seen, consisting of CPAM associated with pulmonary sequestration in the same malformation; these represented around 50 % of cases in a pathologic series (Conran and Stocker 1999). Postnatal US study of CPAM is challenging. Since the abnormal lung communicates with the airways, the cysts, which contain fluid during fetal life, fill with air at birth, making their identification difficult on US. The air-filled cluster of cysts is seen as an area of increased echogenicity with a banded appearance (Fig. 5). This US finding, which is known as the “aurora sign,” has also been reported in several acquired interstitial lung diseases (Kohzaki et al. 2003). Therefore, CT and MRI are more suitable for postnatal study of the internal components of this malformation. Postnatal radiography, CT or MRI can be performed immediately if the patient is symptomatic or months later if asymptomatic to determine whether surgery is required.

Bronchopulmonary sequestration is a congenital malformation consisting of nonfunctioning lung tissue, which lacks a normal connection to the tracheobronchial tree and is usually supplied by a systemic artery that arises from the thoracic or abdominal aorta. It is the second most common cause of a fetal chest mass, and two different types are classically recognized: intralobar and extralobar. Intralobar sequestration has been found to occur in older infants and some authors have considered it an acquired lesion (Frazier et al. 1997). Extralobar sequestration has its own pleural covering, and venous drainage is into the systemic circulation (azygos-hemiazygos system, portal vein, or inferior vena cava). It has been considered a congenital condition. (May et al. 1993; Ko et al. 2000). Currently, this categorical classification is not universally accepted since mixed systemic and pulmonary venous drainage has been observed in several cases of extralobar sequestration. (Pumberger et al. 2003). Moreover, the intralobar type is also found in neonates and is now considered a congenital rather than an acquired lesion (Newman 2006).

On US study, sequestration is seen as a homogenous or heterogenous echogenic mass usually located in the lower pulmonary lobes, but sometimes occurring at or below the diaphragm. We recommend a subxiphoid approach to study the mass, search for an anomalous feeding vessel arising from the aorta, and investigate the systemic or pulmonary venous drainage (Fig. 6). The azygos may be enlarged in cases of systemic drainage to the azygos-hemiazygos system (Ko et al. 2000). Pulmonary venous drainage is better delineated by CT or MRI than by US (Fig. 7).

Solid pulmonary sequestrations, particularly those in infradiaphragmatic locations, should be differentiated from congenital neuroblastoma and adrenal hematomas (Manson and Daneman 2001; Langston et al. 2003; Vijayaraghavan et al. 2003). The presence of calcifications (common in neuroblastoma and exceedingly rare in neonatal sequestration) as well as the obstetric history (sequestration is detected in the second trimester and neuroblastoma in the third trimester) are the main distinguishing factors (Fig. 8).

Lobar overinflation is characterized by hyperinflation of one part of the lung (usually the apical and posterior segments of the left upper lobe) and is seen on US as an area of homogenous hyperechogenicity with normal pulmonary supply and an absence of cysts (Daltro et al. 2010). The increased echogenicity is believed to be secondary to fluid accumulation in the affected lung (Pariente et al. 2009). Differentiation between CLO and a microcystic form of CPAM or PS may be very difficult before birth. Diagnosis can be confirmed postnatally by chest X-ray and CT imaging (Seo et al. 2006).

Bronchogenic cysts are usually located in the subcarinal region, but they may also be found within the pulmonary parenchyma. Depending on their content, intrapulmonary cysts are seen as unilocular anechoic or weakly echogenic lesions. US is particularly helpful in cases where CT is inconclusive regarding the solid or cystic nature of the lesion (Fig. 9).

The pleura is a very superficial structure that is well visualized by US. It is composed of two membranes, the visceral and parietal pleurae, separated by a potential space seen as a thin hypoechoic band during respiration in real time. On intercostal longitudinal scans, the pleura is visualized as a highly echogenic linear structure that acquires a curving configuration in the transverse view (Fig. 23). The sonographist should be aware of certain features related to the pleura that are of great diagnostic value. In real-time sonography, the visceral pleura moves with the respiratory excursions, and recognition of this movement provides clues to the diagnosis of several conditions, such as pneumothorax and pleural infiltration by pulmonary or extrapleural tumors. The absence of visceral pleura movement is indicative of pneumothorax. Similarly, the absence of pulmonary tumor movement during respiration indicates that the pleura is infiltrated (Wernecke 2000).

The pleural surface is a strongly reflective structure, and when the US beam strikes it at certain angles, mirror artifacts (apparent duplication of adjacent structures) can occur. For example, intercostal muscles of the chest wall can reflect off the pleura and be projected over the lung, simulating pulmonary disease.

Sonography is considered highly sensitive for confirming suspected pleural effusion on AP chest plain films and is often used for this purpose (Eibenberger et al. 1991; Kocijancic et al. 2003). We believe sonography should replace the routine practice of lateral decubitus plain film confirmation in these patients.

On US imaging through an intercostal approach, pleural fluid is identified as a band-like collection separating the parietal and visceral pleura surfaces. When scanning through the abdomen, the collection is seen just above the diaphragm, blurring the costophrenic angle (Fig. 24). Several sonographic signs typical of pleural fluid help to distinguish pleural effusion from ascites. The three most important are the presence of septa within the collection that move with respiration, the “crus sign,” and the “bare area sign” (Seibert et al. 1998). The crus sign, which is pathognomonic of pleural collection, results from displacement of the diaphragmatic crus away from the spine due to interposition of fluid between this structure and the vertebral column (Fig. 25). The “bare area” refers to the posterior part of the right lobe of the liver, which is directly attached to the diaphragm without the covering layer of peritoneum. Peritoneal fluid cannot extend behind the right lobe at this point; thus, all fluid collections visualized behind the bare area are necessarily located in the pleural space.

One valuable advantage of sonographic study of pleural effusion is that it enables characterization of the nature of the fluid (Yang et al. 1992; Calder and Owens 2009). Simple effusions are anechoic, whereas complicated effusions present one or more of the following features: weakly echogenic debris with a swirling movement in real time (Fig. 26), mobile fibrin strands, septations (Fig. 27), and a honeycomb appearance (Fig. 28). Recognition of the septated nature of the pleural collection, information that influences patient management, is not usually provided by chest CT scans (Fig. 29) (Kurian et al. 2009).

The sonographic appearance of pleural effusion can be related to the classical division of pleural fluid into exudate or transudate, which depends on its protein content, pleural: serum LDH ratio, and other biochemical parameters. On sonography, both transudates and exudates can be anechoic, but collections presenting some of the complicated features mentioned above are always exudates.

Most exudative pleural effusions in pediatric patients are of infectious origin (Alkirinawi and Chernick 1996). These collections are known as parapneumonic effusion or empyema. The diagnosis of empyema is established when pleural fluid is grossly purulent, organisms are identified on gram stain or culture, pleural fluid white blood cell count is greater than 5 × 10⁹ cells/L, pH is below 7.0, or glucose level is less than 40 mg/dL.

There is still a great deal of controversy around the clinical management of parapneumonic effusion and empyema. (Feola et al. 2003; Wells and Havens 2003; Hogan and Coley 2008; Calder and Owens 2009). Two main treatment approaches are used: nonoperative, in which patients are treated with antibiotics alone or combined with chest drainage and fibrinolytic instillation, and operative, consisting of pleural debridement or decortication. The goal of both these treatment methods is to evacuate infected debris and re-expand the lung, and to reduce hospital stay and morbidity.

The presence of a honeycomb pattern on US is not a contraindication for pleural drainage and fibrinolytic therapy (Wells and Havens 2003). Surgery should be used only in patients who do not respond to this therapy.

In our experience, small exudates can rapidly (within 24 h) increase in volume and change their sonographic appearance, despite antibiotic therapy. Thus, close sonographic monitoring of children with empyema is recommended. In addition, US is a valuable tool for localizing loculations for thoracocentesis or thoracostomy tube placement (Coley 2002). The incidence of pneumothorax is considerably reduced when pleural taps are sonographically guided, particularly in the case of small or loculated collections (Raptopoulos et al. 1991; Shankar et al. 2000).

Opaque hemithorax on chest films is one of the main indications for chest US. In most cases, opaque hemithorax is caused by massive pleural effusion, but before pleural puncture is considered, US should be performed to confirm that the opacity is actually caused by fluid accumulation and not by a solid lesion or a mixture of both components (Fig. 30).

Primary tumors originating in the pleura, such as mesothelioma, are very rare in children, and neoplastic involvement of this structure is most often due to metastasis. Metastatic disease to the pleura often causes large pleural effusions that are probably due to impaired lymphatic drainage. This secondary pleural fluid, which can be profuse in some patients, may mask the tumor mass on chest X-rays, and in these cases, ultrasound is particularly helpful (Fig. 31). Metastatic involvement of the pleura can be caused by various intrathoracic or extrathoracic tumors, such as Wilms tumor, lymphoma, neuroblastoma, and rhabdomyosarcoma (Fig. 32).

Chest X-ray remains the method of choice for the diagnosis of pneumothorax, but several projections may be required to reveal its presence. In severely ill patients or trauma patients who cannot be easily moved, pneumothorax can be confirmed or ruled out by US without changing the patient’s position (Barillari and Kiuru 2010; Volpicelli 2011a).

Small pockets of air in the pleural space appear as bright, echogenic lines or points. In contrast to what is seen in the normal aerated lung, the image of air in the pleural space does not show comet tail artifacts (Fig. 33). A large pneumothorax can impede visualization of visceral pleura movement, an important indirect sign of air in the pleural space. The presence of air and fluid in the pleural space (hydropneumothorax) results in an air-fluid level on US that can produce a particular movement in real time known as the “curtain sign” (Ben-Ami et al. 1993; Lichtenstein et al. 2000).