(Radiographics. 2000;20:653-671.)
© RSNA, 2000
US in the Diagnosis of Pediatric Chest Diseases1
(CME Available in print version and on RSNA Link)
Ok Hwa Kim, MD,
Woo Sun Kim, MD,
Min Jung Kim, MD ,
Jin Young Jung, MD and
Jung Ho Suh, MD
1 From the Department of Radiology, Ajou University Hospital, 5 Wonchon-Dong, Paldal-Gu, Suwon 442-749, Kyonggi-Do, South Korea (O.H.K., M.J.K., J.Y.J., J.H.S.); and the Department of Radiology, Children's Hospital, Seoul National University, Seoul, South Korea (W.S.K.). Recipient of a Magna Cum Laude award for a scientific exhibit at the 1998 RSNA scientific assembly. Received March 11, 1999; revision requested May 10; final revision received January 7, 2000; accepted January 10. Address reprint requests to O.H.K. (e-mail: kimoh@madang.ajou.ac.kr).
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Abstract
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Most pediatric chest diseases are adequately evaluated with chest radiography. However, when chest radiography does not allow identification of the location and nature of an area of increased opacity, ultrasonography (US) can help establish the diagnosis. US may be helpful in evaluation of persistent or unusual areas of increased opacity in the peripheral lung, pleural abnormalities, and mediastinal widening; US is particularly useful in patients with complete opacification of a hemithorax at radiography. In cases of pulmonary parenchymal lesions, identification of air or fluid bronchograms at US and of pulmonary vessels at color flow imaging is useful for differentiating pulmonary consolidation or atelectasis from lung masses and pleural lesions. US allows characterization of pleural fluid collections as simple, complicated, or fibroadhesive, which is important information for planning thoracentesis or thoracotomy. Computed tomography and magnetic resonance imaging are superior to US in evaluation of the mediastinum, but US is a reasonable alternative in certain situations (eg, to avoid unnecessary investigation of a normal thymus simulating a mediastinal mass). In cases of chest wall lesions, US may enable localization of the site of origin to soft tissues or an extrapleural intrathoracic location. Osseous involvement, particularly rib involvement, is easily evaluated with US.
Index Terms: Children, respiratory system, 60.1298, 60.14 • Lung, abnormalities, 60.145, 60.146 • Lung, collapse, 60.74 • Lung, congenital malformation, 60.14, 60.145, 60.146 • Lung, consolidation, 60.21 • Pleura, fluid, 66.761 • Thorax, neoplasms, 47.30, 60.30 • Thymus, 675.134
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LEARNING OBJECTIVES FOR TEST 3
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After reading this article and taking the test, the reader will be able to:
• Discuss the characteristic US findings of pulmonary parenchymal diseases, including consolidation, congenital cystic lung diseases, and tumors.
• Summarize the characteristic US features of pleural effusion amenable to aspiration.
• Assess the role of US in the evaluation of mediastinal masses in children.
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Introduction
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Most pediatric chest diseases are adequately evaluated with chest radiography. However, interpretation of the location and nature of an area of increased opacity on chest radiographs is sometimes problematic, particularly in young infants with varied configurations of the normal thymus, and differentiation between pulmonary, pleural, and mediastinal lesions is not always easy. Under these circumstances, computed tomography (CT) and magnetic resonance (MR) imaging are frequently considered the best modalities for solving dilemmas encountered at chest radiography.
Ultrasonography (US) has been underused or often ignored as a diagnostic tool in the chest, especially in the lung, because air and the bony thorax were traditionally considered an obstacle to transmission of the ultrasound beam. However, since US has been gaining recognition as a highly useful tool in the evaluation of pleural lesions, its role in imaging of the lung and extracardiac mediastinum has expanded and its usefulness has been recognized (1–9). Technologic improvements in transducers as well as color flow imaging have made chest US even more useful by revealing the morphology of pulmonary, pleural, and mediastinal structures in more detail.
In this article, we describe the technique of chest US and review the characteristic US features of pulmonary parenchymal lesions, pleural diseases, mediastinal masses, and chest wall lesions. In addition, we illustrate the varied applications of US in the diagnosis of pediatric chest diseases.
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Technique of Chest US
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The optimal frequency of the transducer for chest US varies with the age of the patient, the location of the lesion, and the planned approach (7,10). Neonates and infants are best imaged with a high-resolution 5–10-MHz linear-array transducer; children and adolescents may require a 2–4- or 4–7-MHz sector or linear-array transducer. Transsternal, parasternal, and intercostal approaches are good for imaging of the lung, pleura, and anterior mediastinum (Fig 1). Sector transducers are used in subxiphoid and transdiaphragmatic approaches with the liver used as the acoustic window for evaluating juxtaphrenic paravertebral lesions. Suprasternal and supraclavicular approaches facilitate evaluation of the upper mediastinum and lung apexes.
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Figure 1. Transducer positions for imaging intrathoracic structures. A = supraclavicular, B = suprasternal, C = transsternal, D = parasternal, E = intercostal, F = subxiphoid, G = transdiaphragmatic.
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Every patient undergoes routine frontal or lateral chest radiography before US. US is performed in the supine, prone, or decubitus position, with the presumed location of the lesion based on the radiographic findings. Images are obtained in the transverse, longitudinal, and inclined transverse or inclined longitudinal planes to maximize demonstration of the lesion.
Color flow imaging may be helpful in characterizing the lesion by demonstrating the vascularity and flow pattern and in searching for anomalous vessels, such as occur in pulmonary sequestration.
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Pulmonary Parenchymal Lesions
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Consolidation and Atelectasis
When the lung is airless, as in consolidation or atelectasis, there is through transmission of the ultrasound beam, thus showing the lung with atypical internal architecture and echogenicity instead of the bright reflections of the aerated lung (11–14). The airless lung is similar in echogenicity and echotexture to the liver and spleen. Within the solid-appearing area of echogenicity, multiple bright dotlike and branching linear structures are found (Fig 2). These findings represent air in the bronchi and scattered residual air in alveoli within the consolidated or atelectatic lung. This appearance is termed a sonographic air bronchogram, which is analogous to the air bronchogram seen on standard chest radiographs.
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Figure 2. Sonographic air bronchogram in pneumonic consolidation. Intercostal oblique US scan of the left lower lobe shows echogenic lung containing bright dots and linear or branching structures (arrow), which represent air bronchograms. A small, hypoechoic pleural effusion (P) defines the boundary of the lung.
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In consolidation, the lung volume is increased by fluid or tissue, but the bronchi are spared and retain their normal branching pattern (15). In atelectasis, overall lung volume is decreased; as a result, supplying bronchi of the involved lung can be crowded together in very close apposition in one plane. In atelectasis, the US appearance of the air bronchogram is still present as long as bronchi are not obstructed. However, the scattered dotlike and branching pattern of the air bronchogram seen in consolidation may become crowded and parallel running (Fig 3). The latter finding may be seen in passive atelectasis of the peripheral lung due to a large pleural effusion, adhesive pleural thickening, or pneumothorax (13). In a practical sense, the distinction between consolidation and atelectasis is unimportant (11). The usefulness of US is in differentiation of pleural abnormalities from pulmonary parenchymal lesions; when both pulmonary and pleural lesions are present, distinction between these two lesion types is not always easy at chest radiography.
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Figure 3. Sonographic air bronchogram in passive atelectasis as a sequela of pneumonia and empyema. Transverse US scan of the left lower lobe shows conglomerated, parallel-running bright lines (black arrows) within echogenic lung, an appearance that represents an air bronchogram in atelectatic lung. A thick and echogenic rind of pleura (white arrows) denotes adhesive pleural thickening.
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Occasionally, when the bronchial tree is filled with fluid rather than air, as in mucoid impaction, US may demonstrate a branching pattern of anechoic or hypoechoic tubular structures within consolidated lung (Fig 4). Demonstration of fluid-filled bronchi, an appearance termed a sonographic fluid or mucus bronchogram, is a specific indicator of pulmonary parenchymal consolidation, equivalent to the air bronchogram (16).
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Figure 4. Sonographic fluid bronchogram in mucoid impaction in a 7-year-old boy. Chest radiography showed an oval area of increased opacity in the left lower lobe, which persisted for 1 month despite antibiotic therapy. The question was whether this area of increased opacity represented a true lung mass, parenchymal consolidation, or a posterior mediastinal mass. Transverse US scan of the left lower lobe obtained with the patient in the prone position shows an echogenic mass containing branching, hypoechoic, tubular structures (arrows), which represent bronchi filled with fluid or mucus; this appearance is reflective of parenchymal consolidation. Bronchoscopy showed tubular collections of impacted mucus in dilated left lower lobe bronchi.
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Sonographic air or fluid bronchograms may not be visible, particularly in the peripheral lung. In this situation, color flow US demonstrates the normally branching pattern of pulmonary vessels in consolidated lung (Fig 5). Identification of a normal pulmonary vessel is another indicator of pulmonary parenchymal consolidation (11,12). In a tumor mass, a normal pulmonary vessel may not be traced or can be distorted and displaced. However, anechoic pulmonary vessels within consolidated lung mimic fluid bronchograms at gray-scale US (11). Color flow imaging allows distinction of color-coded pulmonary vessels from anechoic or hypoechoic fluid-filled bronchi. In a practical sense, such distinction is not necessary because the existence of branching patterns of both fluid-filled bronchi and pulmonary vessels implies pulmonary parenchymal consolidation.
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Figure 5. Normal pulmonary vessel in consolidated lung. Transverse color US scan shows branching pulmonary vessels within an echogenic area of consolidation. Anechoic pleural fluid (P) with a color band (fluid color sign) represents an effusion with floating debris.
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In some patients, pneumonic consolidation may appear as a round area of increased opacity, thus mimicking a tumor (Fig 6). Differentiation between a tumor and a pseudomass is not always easy with chest radiography alone, and further evaluation with CT may be required. Firsthand application of US and identification of the characteristic US appearance of consolidation may eliminate the need for further imaging evaluation.
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Figure 6a. Round area of pneumonia in a 5-year-old boy. (a) Chest radiograph shows a masslike area of increased opacity in the right upper lobe. The lesion is attached to the chest wall. (b) Transverse US scan obtained with the patient in the prone position shows an echogenic mass containing a branching pattern of bright echoes; this is a typical sonographic air bronchogram. (c) Follow-up chest radiograph obtained 1 week later shows regression of the consolidation with pneumatocele formation.
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Figure 6b. Round area of pneumonia in a 5-year-old boy. (a) Chest radiograph shows a masslike area of increased opacity in the right upper lobe. The lesion is attached to the chest wall. (b) Transverse US scan obtained with the patient in the prone position shows an echogenic mass containing a branching pattern of bright echoes; this is a typical sonographic air bronchogram. (c) Follow-up chest radiograph obtained 1 week later shows regression of the consolidation with pneumatocele formation.
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Figure 6c. Round area of pneumonia in a 5-year-old boy. (a) Chest radiograph shows a masslike area of increased opacity in the right upper lobe. The lesion is attached to the chest wall. (b) Transverse US scan obtained with the patient in the prone position shows an echogenic mass containing a branching pattern of bright echoes; this is a typical sonographic air bronchogram. (c) Follow-up chest radiograph obtained 1 week later shows regression of the consolidation with pneumatocele formation.
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Congenital Cystic Lung Diseases
Pulmonary sequestration and cystic adenomatoid malformation are typical diseases included in the category of congenital cystic lung diseases (17,18). These two diseases have similar clinical and radiographic manifestations. Diagnostic pitfalls may be present in both diseases with a variety of radiographic findings; characteristic findings are persistent or chronic recurrent pneumonia, a multicystic pattern, or a homogeneous mass, especially at a lung base. The US findings are also similar: a complex echogenic mass with a cystic or homogeneously solid appearance (19–21). The cystic appearance is less common than the homogeneously solid appearance due to the superimposed echogenicity of small cysts in both diseases. When echogenic pulmonary parenchyma with or without cysts is discovered, particularly in the lower lobes, use of a subxiphoid approach to search for an anomalous vessel arising from the aorta may be helpful to differentiate pulmonary sequestration from cystic adenomatoid malformation (Figs 7, 8). Color flow imaging is useful to demonstrate anomalous vessels and trace them to their origin (10,22).
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Figure 7a. Intralobar pulmonary sequestration in a 2-year-old girl. (a) Chest radiograph shows an area of increased opacity in the left lower lobe with pleural effusion. US was performed to guide pleural tapping. (b) Longitudinal US scan of the left lung base shows echogenic lung containing multiple cysts. SP = spleen. (c) Transverse color US scan obtained with a subxiphoid approach shows an anomalous vessel (arrow) arising from the aorta (A) and supplying the mass (M).
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Figure 7b. Intralobar pulmonary sequestration in a 2-year-old girl. (a) Chest radiograph shows an area of increased opacity in the left lower lobe with pleural effusion. US was performed to guide pleural tapping. (b) Longitudinal US scan of the left lung base shows echogenic lung containing multiple cysts. SP = spleen. (c) Transverse color US scan obtained with a subxiphoid approach shows an anomalous vessel (arrow) arising from the aorta (A) and supplying the mass (M).
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Figure 7c. Intralobar pulmonary sequestration in a 2-year-old girl. (a) Chest radiograph shows an area of increased opacity in the left lower lobe with pleural effusion. US was performed to guide pleural tapping. (b) Longitudinal US scan of the left lung base shows echogenic lung containing multiple cysts. SP = spleen. (c) Transverse color US scan obtained with a subxiphoid approach shows an anomalous vessel (arrow) arising from the aorta (A) and supplying the mass (M).
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Figure 8a. Cystic adenomatoid malformation in a 13-year-old boy. (a) Chest radiograph shows patchy infiltration with small cystic areas in the left lower lobe. (b) Longitudinal US scan of the left lower lobe shows a honeycomb appearance with numerous small cysts within echogenic lung. This appearance is similar to that of pulmonary sequestration (see Fig 7b). (c) Cut section from the left lower lobe shows tiny cystic spaces containing fluid (arrows). The diagnosis of type II cystic adenomatoid malformation was established.
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Figure 8b. Cystic adenomatoid malformation in a 13-year-old boy. (a) Chest radiograph shows patchy infiltration with small cystic areas in the left lower lobe. (b) Longitudinal US scan of the left lower lobe shows a honeycomb appearance with numerous small cysts within echogenic lung. This appearance is similar to that of pulmonary sequestration (see Fig 7b). (c) Cut section from the left lower lobe shows tiny cystic spaces containing fluid (arrows). The diagnosis of type II cystic adenomatoid malformation was established.
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Figure 8c. Cystic adenomatoid malformation in a 13-year-old boy. (a) Chest radiograph shows patchy infiltration with small cystic areas in the left lower lobe. (b) Longitudinal US scan of the left lower lobe shows a honeycomb appearance with numerous small cysts within echogenic lung. This appearance is similar to that of pulmonary sequestration (see Fig 7b). (c) Cut section from the left lower lobe shows tiny cystic spaces containing fluid (arrows). The diagnosis of type II cystic adenomatoid malformation was established.
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Tumors
Most lung tumors that occur in childhood are metastatic. Primary malignant tumors of the lung, which include blastoma, mucoepidermoid carcinoma, bronchogenic carcinoma, hemangiopericytoma, and rhabdomyosarcoma, are extremely rare (23). The most common of these is pulmonary blastoma. Pulmonary blastomas are solitary masses located peripherally in the lung. As a result, they do not cause early symptoms and are frequently very large at presentation, occupying an entire lobe or hemithorax (24). US of the thorax is helpful in distinguishing a massive pleural effusion from a complex echogenic mass occupying an entire hemithorax (Fig 9).
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Figure 9a. Pulmonary blastoma in a 3-year-old girl. (a) Chest radiograph shows total opacification of the right hemithorax with contralateral shifting of the heart. The question was whether this appearance represented a massive effusion or a huge mass. (b) Transverse US scan shows a huge echogenic mass (arrows) with a central anechoic portion. An unexpected finding was the near absence of pleural effusion (P). (c) Contrast material-enhanced CT scan shows a huge mass with a necrotic portion replacing the whole right hemithorax.
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Figure 9b. Pulmonary blastoma in a 3-year-old girl. (a) Chest radiograph shows total opacification of the right hemithorax with contralateral shifting of the heart. The question was whether this appearance represented a massive effusion or a huge mass. (b) Transverse US scan shows a huge echogenic mass (arrows) with a central anechoic portion. An unexpected finding was the near absence of pleural effusion (P). (c) Contrast material-enhanced CT scan shows a huge mass with a necrotic portion replacing the whole right hemithorax.
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Figure 9c. Pulmonary blastoma in a 3-year-old girl. (a) Chest radiograph shows total opacification of the right hemithorax with contralateral shifting of the heart. The question was whether this appearance represented a massive effusion or a huge mass. (b) Transverse US scan shows a huge echogenic mass (arrows) with a central anechoic portion. An unexpected finding was the near absence of pleural effusion (P). (c) Contrast material-enhanced CT scan shows a huge mass with a necrotic portion replacing the whole right hemithorax.
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Pleural Diseases
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Pathologic processes that involve the pleura, either directly from isolated disease or indirectly from neighboring pulmonary lesions, manifest as pleural fluid collections, which are ideal for imaging with US because of their acoustic properties. The conventional chest radiographic findings of pleural fluid collections are variable and depend on the amount and age of the fluid collection (25). These findings range from complete opacification of the hemithorax to less striking but puzzling areas of increased opacity, especially in patients with loculated empyema or associated peripheral pulmonary lesions. If there is any doubt whether a pleural effusion exists, US allows easy distinction of pleural fluid from peripheral pulmonary infiltrates and also permits localization of pleural fluid for aspiration.
The different types of pleural effusion depend on the nature of the fluid collection: serous, purulent, hemorrhagic, or chylous (10,25). Serous fluid is usually a transudate, and purulent fluid is an exudate or empyema. At US, pleural fluid may be characterized as a simple effusion, a complicated effusion, or fibrothorax (pleural thickening or fibrosis) (26–29) (Table). A simple effusion appears as clear anechoic or cloudy hypoechoic fluid with or without swirling particles. A complicated effusion appears as septated or multiloculated, hypoechoic fluid partitioned by fibrin strands with no clear demarcation between the lung and pleural components. Fibrothorax appears as a thickened, echogenic rind of pleural plaque. The variable nature of fluid collections sometimes makes it difficult to determine whether thoracentesis is feasible on the basis of chest radiographs alone, even with use of the decubitus view. Characterization of pleural changes with US is very informative in guiding thoracentesis.
Useful features in distinguishing fluid amenable to aspiration from an organized or loculated effusion are changes in the shape of the fluid when the fluid shifts in response to a change in patient position or respiration, as well as demonstration of echogenic floating debris (28–30) (Fig 10). Both exudates and hemothorax may demonstrate echogenic debris. Color signal within a fluid collection in the pleural space, the so-called fluid color sign, is a useful sign for detection of an effusion with floating debris (30). The moving debris within the effusion may scatter the sound and produce color Doppler signals (Fig 5).
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Figure 10a. Simple pleural effusion with floating debris. (a) Chest radiograph shows an ill-defined area of increased opacity in the left lower lobe. (b) Longitudinal US scan of the left lower lobe shows a large amount of hypoechoic fluid containing swirling particles, an appearance indicative of a simple effusion amenable to aspiration. The subpulmonic location of the fluid is well visualized between the spleen (SP) and an echogenic area of lower lobe consolidation (L).
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Figure 10b. Simple pleural effusion with floating debris. (a) Chest radiograph shows an ill-defined area of increased opacity in the left lower lobe. (b) Longitudinal US scan of the left lower lobe shows a large amount of hypoechoic fluid containing swirling particles, an appearance indicative of a simple effusion amenable to aspiration. The subpulmonic location of the fluid is well visualized between the spleen (SP) and an echogenic area of lower lobe consolidation (L).
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As fluid collections organize, mobile, linear structures of fibrin bands or to-and-fro motion of septa tends to occur (3,26) (Fig 11). In some empyemas, the septa are so profuse that they produce a honeycomb appearance (Fig 12). These changes are predictive of significant difficulties with thoracentesis. The septated or multiloculated nature of pleural fluid may not be visible at CT. US performed with a high-resolution transducer is sensitive in demonstrating the internal derangement of fluid collections and may provide detailed information about the nature of pleural fluid. Fibrothorax appears as echogenic, solid-appearing pleural plaque with or without some loculation of fluid (26,28) (Fig 13). Demonstration of pleural thickening and fibrosis leads to deferral of thoracentesis and even thoracotomy (1). In these patients, surgical pleurectomy may be required.
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Figure 11. Complicated pleural effusion with fibrin bands. Transverse US scan shows anechoic fluid containing mobile echogenic bands. This type of fluid collection is amenable to thoracentesis.
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Figure 12a. Complicated pleural effusion with multiple loculi. (a) Intercostal oblique US scan shows thickening of the visceral and parietal pleura (arrows). The pleural space is filled with profusely septated fluid, which has a honeycomb appearance. This type of fluid collection is not really amenable to thoracentesis. (b) CT scan obtained on the same day shows a rind of hypoattenuating pleural effusion. The multiloculated nature of the effusion is not visualized.
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Figure 12b. Complicated pleural effusion with multiple loculi. (a) Intercostal oblique US scan shows thickening of the visceral and parietal pleura (arrows). The pleural space is filled with profusely septated fluid, which has a honeycomb appearance. This type of fluid collection is not really amenable to thoracentesis. (b) CT scan obtained on the same day shows a rind of hypoattenuating pleural effusion. The multiloculated nature of the effusion is not visualized.
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Figure 13a. Adhesive pleural thickening and fibrosis in a child with empyema. One week after thoracotomy, no more fluid was drained. (a) Chest radiograph shows a bandlike, pleural area of increased opacity along the left lateral chest wall. The question was whether this finding represented fluid that could be drained by repositioning the thoracotomy tube. (b) Transverse US scan shows thick, echogenic plaque filling the pleural space. In this situation, thoracotomy is no longer effective.
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Figure 13b. Adhesive pleural thickening and fibrosis in a child with empyema. One week after thoracotomy, no more fluid was drained. (a) Chest radiograph shows a bandlike, pleural area of increased opacity along the left lateral chest wall. The question was whether this finding represented fluid that could be drained by repositioning the thoracotomy tube. (b) Transverse US scan shows thick, echogenic plaque filling the pleural space. In this situation, thoracotomy is no longer effective.
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US can also demonstrate localized subpulmonic effusion in patients with an apparently elevated hemidiaphragm on plain chest radiographs (29). US often allows detection of associated lung or pleural masses hidden by pleural effusion. The patient with a completely opaque hemithorax is an ideal candidate for differentiation of massive pleural effusion from pleural or lung masses (Figs 9, 14).
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Figure 14a. Opaque hemithorax due to massive pleural effusion caused by pleural metastases in a 4-year-old boy. (a) Chest radiograph shows complete opacification of the right hemithorax. (b) Longitudinal US scan shows a massive pleural effusion containing echogenic masses (M). The patient had undergone left nephrectomy due to Wilms tumor, and the echogenic masses were metastases.
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Figure 14b. Opaque hemithorax due to massive pleural effusion caused by pleural metastases in a 4-year-old boy. (a) Chest radiograph shows complete opacification of the right hemithorax. (b) Longitudinal US scan shows a massive pleural effusion containing echogenic masses (M). The patient had undergone left nephrectomy due to Wilms tumor, and the echogenic masses were metastases.
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Mediastinal Masses
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Mediastinal US may have a screening role in the evaluation of mediastinal masses somewhere between the role of chest radiography and those of the more expensive imaging techniques (31–33). It is still uncommon to use US in the mediastinum. However, US may permit rapid clarification of radiographically equivocal findings. It may also partially replace CT and MR imaging in certain situations, for example, in young children with widening of the superior mediastinum to differentiate normal thymus from mediastinal masses and in critically ill patients in intensive care units (33).
In children less than 1 year old, the ossification centers of the sternum have not yet fused, and the mineral content of the bones and cartilage is lower than in older children (34). The thymus is also much larger relative to other structures. As a result, there are ample acoustic windows through the sternum, costal cartilages, and thymus, and US of the mediastinal structures is easily performed (7).
The Thymus: Normal and Abnormal
The thymus is located in the superior mediastinum under the sternum, anterior to the great vessels, and is easily identified in relation to the aorta and superior vena cava (8). In newborns and infants, the normal thymus is readily visible at US performed with suprasternal, transsternal, or parasternal approaches. The size, shape, and extent of the normal thymus in children are variable (35,36). The gland may span from the manubrium to the fourth costal cartilage. Occasionally, the normal thymus may bulge into the neck or extend down to the diaphragm. At US, the normal thymus has a homogeneous and finely granular echotexture with some echogenic strands (35,37,38). It is mildly hypoechoic relative to the liver, spleen, and thyroid gland. The normal thymus has a smooth, well-defined margin because each lobe is surrounded by a fibrous capsule. Color flow US shows that the normal thymus is hypovascular or nearly avascular (7).
The normal thymus sometimes has a confusing appearance on plain chest radiographs. Commonly encountered problems are a normal but prominent thymus mimicking a mediastinal mass (Fig 15) or upper lobe pneumonia or atelectasis. Under these circumstances, US allows easy identification of the normal thymus, thus enabling unnecessary further investigations to be avoided. A true apical lung lesion may be hidden behind the thymus and mimic an enlarged thymus (Fig 16). US allows differentiation of a concealed upper lobe lung lesion from the normal thymus because of their characteristic echogenicities.
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Figure 15a. Normal but prominent thymus simulating a mediastinal mass in a 5-month-old boy. (a) Chest radiograph shows a masslike area of increased opacity on the left side of the superior mediastinum. (b) Transverse US scan inclined to the left shows a normal left thymic lobe (T) with a round configuration. A = aorta.
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Figure 15b. Normal but prominent thymus simulating a mediastinal mass in a 5-month-old boy. (a) Chest radiograph shows a masslike area of increased opacity on the left side of the superior mediastinum. (b) Transverse US scan inclined to the left shows a normal left thymic lobe (T) with a round configuration. A = aorta.
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Figure 16a. Apical pulmonary consolidation mimicking a prominent thymus in a neonate with mild respiratory distress. (a) Chest radiograph shows a homogeneous area of increased opacity in the left upper lobe. The question was whether this area of increased opacity represented a normal but prominent thymus or a mediastinal mass or lung lesion. (b) Transverse US scan shows a triangular region of echogenic lung (L), which is sharply separated from a normal thymus (T). ST = sternum.
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Figure 16b. Apical pulmonary consolidation mimicking a prominent thymus in a neonate with mild respiratory distress. (a) Chest radiograph shows a homogeneous area of increased opacity in the left upper lobe. The question was whether this area of increased opacity represented a normal but prominent thymus or a mediastinal mass or lung lesion. (b) Transverse US scan shows a triangular region of echogenic lung (L), which is sharply separated from a normal thymus (T). ST = sternum.
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The thymus can protrude into the chest wall as a bulging mass owing to a congenital sternal defect (39) (Fig 17). Because the thymus is confined beneath the sternum, partial absence of the sternum, particularly a manubrial defect, may produce herniation of part of the thymus. US reveals absence of manubrial ossification. Virtual visualization of a herniated thymus during respiratory cycles is an advantage of US.
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Figure 17a. Herniation of the thymus into the anterior chest wall due to a manubrial defect. (a, b) Photographs show a neonate with an intermittently bulging mass (arrow in b) during crying. (c) Transverse US scan of a typical neonate shows the normal configuration of the ventral margin of the thymus (T), the manubrium (M), and assorted ribs (R). Manubrial ossification produces posterior acoustic shadowing through the midportion of the thymus. A = aorta. (d) Transverse US scan of the neonate with the bulging mass shows the thymus (T) herniating through a manubrial defect (arrows). The posterior acoustic shadowing produced by manubrial ossification is not seen. Assorted ribs (R) are widely separated. A = aorta.
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Figure 17b. Herniation of the thymus into the anterior chest wall due to a manubrial defect. (a, b) Photographs show a neonate with an intermittently bulging mass (arrow in b) during crying. (c) Transverse US scan of a typical neonate shows the normal configuration of the ventral margin of the thymus (T), the manubrium (M), and assorted ribs (R). Manubrial ossification produces posterior acoustic shadowing through the midportion of the thymus. A = aorta. (d) Transverse US scan of the neonate with the bulging mass shows the thymus (T) herniating through a manubrial defect (arrows). The posterior acoustic shadowing produced by manubrial ossification is not seen. Assorted ribs (R) are widely separated. A = aorta.
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Figure 17c. Herniation of the thymus into the anterior chest wall due to a manubrial defect. (a, b) Photographs show a neonate with an intermittently bulging mass (arrow in b) during crying. (c) Transverse US scan of a typical neonate shows the normal configuration of the ventral margin of the thymus (T), the manubrium (M), and assorted ribs (R). Manubrial ossification produces posterior acoustic shadowing through the midportion of the thymus. A = aorta. (d) Transverse US scan of the neonate with the bulging mass shows the thymus (T) herniating through a manubrial defect (arrows). The posterior acoustic shadowing produced by manubrial ossification is not seen. Assorted ribs (R) are widely separated. A = aorta.
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Figure 17d. Herniation of the thymus into the anterior chest wall due to a manubrial defect. (a, b) Photographs show a neonate with an intermittently bulging mass (arrow in b) during crying. (c) Transverse US scan of a typical neonate shows the normal configuration of the ventral margin of the thymus (T), the manubrium (M), and assorted ribs (R). Manubrial ossification produces posterior acoustic shadowing through the midportion of the thymus. A = aorta. (d) Transverse US scan of the neonate with the bulging mass shows the thymus (T) herniating through a manubrial defect (arrows). The posterior acoustic shadowing produced by manubrial ossification is not seen. Assorted ribs (R) are widely separated. A = aorta.
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Thymic enlargement occurs in both benign conditions and neoplastic infiltration; the latter cause is more common in childhood (40). Benign conditions include intrathymic hemorrhage, thymic cyst, hemangioma, and lymphangioma. Lymphangiomas are composed of dilated lymphatic sacs of variable size, which may appear unilocular or multilocular (Fig 18). With advances in high-resolution US, septated lesions are better evaluated with US than with CT or MR imaging. Lymphangiomas may undergo hemorrhage, in which case the lesion appears as a uniformly echogenic mass or multiple cysts containing echogenic debris.
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Figure 18a. Mediastinal lymphangioma involving the thymus in a 7-day-old neonate with mild respiratory distress. (a) Chest radiograph shows a full and enlarged mediastinal silhouette. (b) Transverse US scan shows multiple cysts replacing the whole thymus, which is a typical finding of cystic lymphangioma. The septa and the multicystic nature of the lesion were not demonstrated at subsequent MR imaging. ST = sternum.
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Figure 18b. Mediastinal lymphangioma involving the thymus in a 7-day-old neonate with mild respiratory distress. (a) Chest radiograph shows a full and enlarged mediastinal silhouette. (b) Transverse US scan shows multiple cysts replacing the whole thymus, which is a typical finding of cystic lymphangioma. The septa and the multicystic nature of the lesion were not demonstrated at subsequent MR imaging. ST = sternum.
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True thymic tumors such as thymoma and thymolipoma are very rare in childhood, but it is quite common to encounter thymic infiltration by leukemia (Fig 19) or lymphoma (40,41). At US, the abnormal thymus is characterized by an irregular or lobular margin, heterogeneous echogenicity, coarse echotexture, and calcifications. Langerhans cell histiocytosis is an infiltrative neoplasm that involves the thymus in young infants. A few reports have described Langerhans cell histiocytosis manifesting as an enlarged thymus containing punctate calcifications (42,43). In such cases, US may reveal an infiltrated thymus with calcifications (Fig 20).
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Figure 19. Enlarged thymus due to leukemic infiltration (acute lymphoblastic leukemia, T-cell type). Transverse US scan obtained with a suprasternal approach shows a lobulated thymic contour (arrows) with heterogeneous echotexture (see Fig 15b).
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Figure 20a. Langerhans cell histiocytosis of the thymus in a 1-year-old boy. (a) Chest radiograph shows widening of the anterosuperior mediastinum with a fuzzy margin. (b) Inclined transverse US scan obtained with a parasternal approach shows heterogeneous echotexture of the thymus (short arrows). The thymus contains discrete echogenic nodules (long arrows), which represent calcifications. A = aorta, S = superior vena cava. (c) Contrast-enhanced CT scan obtained at the level of the aortic arch shows an anterior mediastinal mass containing irregular hypoattenuating areas and punctate calcifications.
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Figure 20b. Langerhans cell histiocytosis of the thymus in a 1-year-old boy. (a) Chest radiograph shows widening of the anterosuperior mediastinum with a fuzzy margin. (b) Inclined transverse US scan obtained with a parasternal approach shows heterogeneous echotexture of the thymus (short arrows). The thymus contains discrete echogenic nodules (long arrows), which represent calcifications. A = aorta, S = superior vena cava. (c) Contrast-enhanced CT scan obtained at the level of the aortic arch shows an anterior mediastinal mass containing irregular hypoattenuating areas and punctate calcifications.
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Figure 20c. Langerhans cell histiocytosis of the thymus in a 1-year-old boy. (a) Chest radiograph shows widening of the anterosuperior mediastinum with a fuzzy margin. (b) Inclined transverse US scan obtained with a parasternal approach shows heterogeneous echotexture of the thymus (short arrows). The thymus contains discrete echogenic nodules (long arrows), which represent calcifications. A = aorta, S = superior vena cava. (c) Contrast-enhanced CT scan obtained at the level of the aortic arch shows an anterior mediastinal mass containing irregular hypoattenuating areas and punctate calcifications.
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Other Mediastinal Masses
In young infants, posterior mediastinal and neurogenic tumors make up the majority of mediastinal masses; in older children, the most common mediastinal mass is that due to lymphadenopathy resulting from leukemia or lymphoma (40). The latter tumor occurs in the middle mediastinum but frequently extends into the anterior mediastinum and involves the thymus. Anterior mediastinal tumors such as dermoid cyst, teratoma, thymoma, and tumors of thyroid origin are encountered less frequently than middle and posterior mediastinal tumors in childhood.
The right paratracheal zone is the most common site of adenopathy in tumoral or inflammatory conditions such as tuberculosis (Fig 21) and fungal infections. With massive adenopathy, nodal involvement become confluent and extends into the anterior mediastinum (44,45). At US, lymphadenopathy appears as discrete or conglomerated hypoechoic nodules. Posterior mediastinal lesions consist of various neurogenic tumors and neurenteric cysts (46). Juxtaphrenic paravertebral masses may be detected with a subxiphoid or transdiaphragmatic approach. In neurogenic tumors, US may demonstrate a lobulated or well-defined hypoechoic mass with granular or flecklike calcifications (Fig 22). Neurenteric cysts appear as well-defined, anechoic lesions with thin walls (Fig 23). When inflammation and hemorrhage occur, the cyst may contain echogenic debris due to proteinaceous fluid, mucus, or blood. Chest radiography allows localization of masses into the anterior, middle, or posterior mediastinum, whereas US allows characterization of masses as solid or fluid filled and detection of calcifications. Both modalities are thus valuable in arriving at the most likely diagnosis.
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Figure 21a. Mediastinal tuberculous lymphadenopathy in a 6-month-old boy with coughing and fever. (a) Chest radiograph shows an ill-defined area of increased opacity in the right upper lobe, which persisted despite antibiotic therapy. Obvious narrowing of the right intermediate bronchus may suggest mediastinal lymphadenopathy, but the question was whether the area of increased opacity represented pneumonic consolidation. (b) Transverse US scan obtained with a right parasternal approach shows multiple enlarged, hypoechoic lymph nodes (N) and a partly compressed brachiocephalic vein (BV). (c) Contrast-enhanced CT scan obtained at the level of the aortic arch (A) and brachiocephalic vein (B) shows markedly enlarged, conglomerated nodes with central low attenuation and peripheral rim enhancement, which reflect caseous necrosis. Note the compression of the brachiocephalic vein (arrow).
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Figure 21b. Mediastinal tuberculous lymphadenopathy in a 6-month-old boy with coughing and fever. (a) Chest radiograph shows an ill-defined area of increased opacity in the right upper lobe, which persisted despite antibiotic therapy. Obvious narrowing of the right intermediate bronchus may suggest mediastinal lymphadenopathy, but the question was whether the area of increased opacity represented pneumonic consolidation. (b) Transverse US scan obtained with a right parasternal approach shows multiple enlarged, hypoechoic lymph nodes (N) and a partly compressed brachiocephalic vein (BV). (c) Contrast-enhanced CT scan obtained at the level of the aortic arch (A) and brachiocephalic vein (B) shows markedly enlarged, conglomerated nodes with central low attenuation and peripheral rim enhancement, which reflect caseous necrosis. Note the compression of the brachiocephalic vein (arrow).
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Figure 21c. Mediastinal tuberculous lymphadenopathy in a 6-month-old boy with coughing and fever. (a) Chest radiograph shows an ill-defined area of increased opacity in the right upper lobe, which persisted despite antibiotic therapy. Obvious narrowing of the right intermediate bronchus may suggest mediastinal lymphadenopathy, but the question was whether the area of increased opacity represented pneumonic consolidation. (b) Transverse US scan obtained with a right parasternal approach shows multiple enlarged, hypoechoic lymph nodes (N) and a partly compressed brachiocephalic vein (BV). (c) Contrast-enhanced CT scan obtained at the level of the aortic arch (A) and brachiocephalic vein (B) shows markedly enlarged, conglomerated nodes with central low attenuation and peripheral rim enhancement, which reflect caseous necrosis. Note the compression of the brachiocephalic vein (arrow).
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Figure 22a. Incidentally discovered ganglioneuroblastoma in a 1-year-old boy. (a) Chest radiograph shows a juxtaphrenic paravertebral mass (arrow). (b) Transverse US scan obtained with a subxiphoid | |
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Abstract
LEARNING OBJECTIVES FOR TEST...
