(Radiographics. 2002;22:e1.)
© RSNA, 2002
Transthoracic US of the Chest: Clinical Uses and Applications1
Dow-Mu Koh, MRCP, FRCR,
Stephen Burke, MRCP, FRCR,
Neil Davies, FRCS, FRCR and
Simon P. G. Padley, MRCP, FRCR
1 From the Department of Radiology, Chelsea and Westminster Hospital, London SW10 9NH, England. Received June 4, 2001; revision requested August 1; revision received and accepted October 12. Address correspondence to D-M.K. (e-mail: dmkoh@globalnet.co.uk).
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Abstract
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Transthoracic ultrasound (US) of the chest is useful in the evaluation of a wide range of peripheral parenchymal, pleural, and chest wall diseases. Furthermore, it is increasingly used to guide interventional procedures of the chest and pleural space. The technique lends itself to bedside use in the intensive care unit, where suboptimal radiography may mask or mimic clinically significant abnormalities. The authors discuss the uses, techniques and applications of US of the chest. The sonographic appearances of pleural diseases (pleural effusion, pneumothorax, pleural mass, and mesothelioma), parenchymal diseases (pneumonia, neoplasms, heart failure, infarct, and rounded atelectasis), chest wall abnormalities (chest wall tumor and rib fracture), and diaphragmatic paralysis are discussed. The use of US in guiding biopsy, thoracocentesis, and other interventional procedures of the lung, pleural space, and mediastinum are also reviewed.
© RSNA, 2002
Index Terms: Index terms: Lung, diseases, 60.21 • Lung, US, 60.12983, 60.12984, 60.12985 • Lung neoplasms, 60.32, 60.33 • Pleura, diseases, 66.21 • Pleura, neoplasms, 66.30 • Pleura, US, 66.12983, 66.12984, 66.12985 • Pneumothorax, 60.73 • Thorax, diseases, 474.21 • Thorax, neoplasms, 474.30 • Ultrasound (US), guidance, 60.12985, 66.12985, 474.12985
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Introduction
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Examination of the chest is a rapidly developing application of ultrasound (US) and may be used to evaluate a wide range of peripheral parenchymal, pleural and chest wall diseases. The technique is particularly suited to bedside use in the intensive care unit, where suboptimal radiography may mask or mimic clinically significant abnormalities and where differentiation of pleural from parenchymal changes can be challenging (1). Furthermore, US is increasingly used to guide interventional procedures of the chest, such as biopsy and placement of intercostal chest drains. We present a review of the clinical uses and sonographic findings in a variety of pleural, parenchymal, chest wall, diaphragmatic, and mediastinal abnormalities.
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Technique and Instrumentation
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Transthoracic chest US can be performed with any modern US unit. A 2–5-MHz curvilinear probe allows visualization of the deeper structures, and the sector scan field allows a wider field of view through a small acoustic window. The chest wall, pleura, and lungs may be quickly surveyed with the curvilinear probe. Once an abnormality has been identified, a high-resolution 7.5–10-MHz linear probe can be used to provide detailed depiction of any chest wall, pleural, or peripheral lung abnormality. Both gray-scale and color Doppler imaging are useful for the assessment of pleural and parenchymal abnormalities.
Raising the arm above the patient's head increases the rib space distance and facilitates scanning with the patient in erect or recumbent positions. The posterior chest is best imaged with the patient sitting upright, while the anterior and lateral chest may be assessed in the lateral decubitus position (Fig 1).
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Figure 1a. Scanning positions for chest US. (a) The posterior chest is best scanned with the patient in the sitting positon. (b) The anterior and lateral chest can be examined with the patient in the lateral debcubitus position. (c) The mediastinum is evaluated via a suprasternal approach.
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Figure 1b. Scanning positions for chest US. (a) The posterior chest is best scanned with the patient in the sitting positon. (b) The anterior and lateral chest can be examined with the patient in the lateral debcubitus position. (c) The mediastinum is evaluated via a suprasternal approach.
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Figure 1c. Scanning positions for chest US. (a) The posterior chest is best scanned with the patient in the sitting positon. (b) The anterior and lateral chest can be examined with the patient in the lateral debcubitus position. (c) The mediastinum is evaluated via a suprasternal approach.
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Before performing the US examination, it is important to review the patient's chest radiograph to localize the area of interest. Maximum visualization of the lung and pleural space is achieved by scanning along the intercostal spaces. Scanning should be performed during quiet respiration, to allow for assessment of normal lung movement, and in suspended respiration, when a lesion can be examined in detail with gray-scale or color Doppler US. On gray-scale images, the echogenicity of a lesion can be compared with that of the liver and characterized as hypoechoic, isoechoic, or hyperechoic.
When color Doppler is used, the sensitivity of the Doppler should be set to low flow or the low-velocity scale (typically 0.25 m/sec). The wall filter is set to minimize rejection of small frequency shifts and to avoid interference from respiratory or cardiac movements (2). The color Doppler gain is increased until a uniform background colored "snowstorm" is obtained and then decreased until just a few random colored speckles remain.
When pulsed-wave Doppler is used to evaluate vascular flow within a lesion, it is important to maintain the Doppler angle at 60° or less. The pulsed-wave Doppler should be repeated at least twice to ensure reproducibility of the spectral waveform. The peak systolic velocity, end-diastolic velocity, resistive index, and pulsatility index are easily derived from the tracings.
Sonographic views of the upper anterior and middle mediastinum can be obtained via a suprasternal approach. The suprasternal approach allows adequate assessment of the upper mediastinum in 90%-95% of cases (3,4). This is performed with the patient in a supine position, with shoulders supported with a pillow and head extended backward. Views of the upper mediastinum should be obtained in the sagittal and axial planes. Color Doppler US is helpful in distinguishing the great vessels from any mediastinal mass (3). Visualization of the mediastinum via a parasternal or infrasternal approach is usually less reliable.
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Normal Anatomy
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The normal chest wall appears as a series of echogenic soft-tissue layers, representing the layers of muscles and the fascia planes. Below the soft tissue of the chest wall the ribs appear as curvilinear structures on transverse scans, associated with posterior acoustic shadowing. When the ribs are scanned along the long axis, the anterior cortex should appear as a continuous smooth echogenic line.
With a high-resolution linear probe, the visceral and parietal portions of the pleura can be seen as two echogenic lines deep to the ribs (Fig 2). The visceral pleura usually appears thicker than the parietal pleura (5). At real-time imaging, the visceral and parietal portions of the pleura are seen to slide over each other (5). With a 3.5-MHz curvilinear probe, differentiation between the visceral and parietal portions may not be possible, and they usually appear as echogenic bands measuring up to 2 mm thick. However, normal movement of the lung relative to the chest wall, which has been described as the "lung sliding" sign, should be apparent (Movie 1) (6).
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Figure 2a. (a) Normal US appearance of the chest. Below the relatively echogenic subcutaneous tissue, the intercostal muscles appear hypoechoic but contain mutiple echogenic fascia planes. The pleural interface appears as an echogenic line. The sharp change in the acoustic impedance at this interface results in reverberation artifacts (*), appearing as a series of horizontal lines parallel to the pleural interface. Vertical comet tail artifacts (+) can also be seen. (b) On the high-resolution scan, the visceral and parietal portions of the pleura can be resolved.
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Figure 2b. (a) Normal US appearance of the chest. Below the relatively echogenic subcutaneous tissue, the intercostal muscles appear hypoechoic but contain mutiple echogenic fascia planes. The pleural interface appears as an echogenic line. The sharp change in the acoustic impedance at this interface results in reverberation artifacts (*), appearing as a series of horizontal lines parallel to the pleural interface. Vertical comet tail artifacts (+) can also be seen. (b) On the high-resolution scan, the visceral and parietal portions of the pleura can be resolved.
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Beyond the pleura-lung interface, the lung is air-filled and does not allow further visualization of normal lung parenchyma. However, the large change in acoustic impedance at the pleura-lung interface results in horizontal artifacts that are seen as a series of echogenic parallel lines equidistant from one another below the pleural line (7). In addition, vertically oriented "comet-tail" artifacts can also be normally seen, originating at the pleura-lung interface. The visualization of these artifacts in a healthy adult is usually confined to the last intercostal space above the diaphragm. With a curvilinear probe, these artifacts are seen to fan out to the edge of the screen. The comet-tail artifact results from the fluid-rich subpleural interlobular septae, which are surrounded by air. Hence, these artifacts appear closely spaced, separated from each other by an average distance of 7 mm (7).
The aorta and superior vena cava can be recognized on the suprasternal view of the mediastinum. The diaphragm is best examined through the lower intercostal spaces and is seen as an echogenic line, 1 mm thick, above the liver and spleen. Normal downward movement of the diaphragm should be seen on inspiration.
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Diagnostic US of the Chest
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Diseases of the Pleural Space
Transthoracic US scanning is ideally suited for the evaluation of pleural diseases. US is useful for characterizing pleural effusions, for distinguishing between pleural effusions and pleural thickening, and in the evaluation of pleural masses
Pleural Effusions.
The classical appearance of a pleural effusion is an echo-free layer between the visceral and parietal portions of the pleura. The shape of the pleural effusion may vary with respiration and posture. In inflammatory effusions, lung sliding may be absent above the effusion as a result of lung adhesion between the visceral and parietal portions of the pleura (8).
The sonographic appearance of pleural effusion depends on the cause, nature, and chronicity of the collection (Fig 3). Four different appearances are recognized at US (9):
- Anechoic
- Complex but nonseptated
- Complex and septated
- Echogenic
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Figure 3a. (a) A large anechoic effusion with passive atelectasis of the underlying lung is seen. (b) In this complex and septated pleural effusion, the dependent part of the effusion shows increased echogenicity. Note the thick septations within the effusion (arrows). (c) In this patient with an empyema, note the diffusely echogenic pleural collection.
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Figure 3b. (a) A large anechoic effusion with passive atelectasis of the underlying lung is seen. (b) In this complex and septated pleural effusion, the dependent part of the effusion shows increased echogenicity. Note the thick septations within the effusion (arrows). (c) In this patient with an empyema, note the diffusely echogenic pleural collection.
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Figure 3c. (a) A large anechoic effusion with passive atelectasis of the underlying lung is seen. (b) In this complex and septated pleural effusion, the dependent part of the effusion shows increased echogenicity. Note the thick septations within the effusion (arrows). (c) In this patient with an empyema, note the diffusely echogenic pleural collection.
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Transudates are almost invariably anechoic. However, exudates may appear anechoic, complex, or echogenic. Effusions that are complex, septated, or echogenic are usually exudates. Mobile strands of echogenic tissue and septations are frequently observed in inflammatory effusions. Empyema can result in an echogenic collection that mimics a solid lesion. In comparison, malignant effusions are more frequently anechoic than echogenic (9). However, a firm diagnosis of malignant effusion can be made only if there is associated nodular pleural thickening (9).
It has been reported that thoracocentesis is usually successful in effusions that are anechoic, complex, or complex with movable septae (10). On the other hand, percutaneous aspiration or drainage of a complex effusion with fixed septae or an echogenic pleural collection is less likely to be successful (10). However, a recent study evaluating the use of computed tomography (CT) and US in the assessment of parapneumonic effusions and empyema found no correlation between the sonographic appearance of an effusion and its biochemical characteristics or clinical outcome, including the success of percutaneous chest drainage (11).
Small pleural effusions are readily detected and can be distinguished from pleural thickening. The "fluid color" sign is demonstrable on color Doppler scans in pleural effusions but is absent in pleural thickening (Movie 2) (12). The sign refers to the presence of color signal within the fluid collection that is believed to arise from transmitted respiratory and cardiac movements. The fluid color sign has a reported sensitivity of 89.2% and specificity of 100% in identifying small effusions (2).
The diagnosis of pleural effusion on a radiograph obtained with the patient in the supine position can be challenging, since it may be difficult to distinguish pleural from parenchymal opacities. Hence, thoracic US is particularly useful in the intensive care unit setting, where an undetected effusion may substantially impair cardiorespiratory function and may prompt sonographically guided thoracocentesis.
The volume of an effusion can be estimated with planimetric measurements of the square dimensions of an effusion in various longitudinal and transverse planes, or by measuring the subpulmonic effusion height (13,14). These have a reported correlation of 0.59 to 0.89 with the real effusion volume (13,14).
Yu et al compared US with CT in the assessment of 50 patients with unilateral opaque hemithorax (15). Forty-one of these patients had pleural effusions, with or without other abnormalities. US had a sensitivity of 95.1% for pleural lesions and 82.8% for parenchymal lesions (15). Not surprisingly, the depiction of mediastinal abnormality was poorer with US than with CT. However, in six patients, US showed pleural and parenchymal disease that was not identified at CT.
Pleural Thickening.
Pleural thickening appears as hypoechoic broadening of the pleura (Fig 4). Pleural thickening may occur in a variety of conditions. It is most frequently related to scarring, fibrosis, empyema, and pleuritis. Unlike pleural effusion, pleural thickening does not exhibit the fluid color sign (Movie 3).
At US examination, pleuritis is seen as an interruption of the pleural line and irregular hypoechoeic thickening of the visceral pleura (16). The chest radiograph is frequently unremarkable when disease is readily apparent at US. There may be an associated pleural effusion with or without increased vascularity at color Doppler US (16).
Previous asbestos exposure is a relatively common cause of pleural thickening and may be confirmed if calcified pleural plaques are evident. These plaques cause focal areas of dense reflectivity with dense posterior acoustic shadowing, often with evidence of adjacent noncalcified pleural thickening.
Pleural Masses.
Pleural masses may be benign or malignant. Benign pleural masses such as fibromas, lipomas, and neuromas are uncommon. These appear as well-defined rounded masses of variable echogenicity, depending on the fat content of the cells. A biopsy is usually required to reach a definitive diagnosis.
Malignant masses of the pleura include mesothelioma, lymphoma, and metastases. Mesothelioma is seen as irregular thickening of the pleura (Fig 5) that may appear nodular and is frequently associated with a large pleural effusion (5). However, CT remains the modality of choice in the preoperative staging of malignant mesothelioma (17).
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Figure 5a. (a) Malignant mesothelioma. CT scan shows lobulated pleural masses, with an area of chest wall invasion. (b) On the US scan, the pleural masses with chest wall infiltration (arrows) are clearly depicted.
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Figure 5b. (a) Malignant mesothelioma. CT scan shows lobulated pleural masses, with an area of chest wall invasion. (b) On the US scan, the pleural masses with chest wall infiltration (arrows) are clearly depicted.
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Subpleural lymphomatous deposits appear as wedge-shaped hypoechoeic infiltrates and may resemble pulmonary infarcts (5). The most common pleural metastases are from primary adenocarcinomas (Fig 6, Movie 4). Pleural effusions almost invariably accompany pleural metastases, but the volume of deposits on the pleural surface may be small and beyond the resolution of US. Pleural deposits more than 5 mm in diameter can be identified as oval echogenic nodules, frequently along the parietal or diaphragmatic pleura (5). Metastases may also appear as diffuse thickening of the parietal pleura and, to a lesser extent, the visceral pleura. Malignant pleural disease may invade the chest wall, with poor demarcation of the pleural mass and infiltration into the chest wall (5).
Color Doppler US of a malignant pleural mass (Movie 4) may reveal neovascularity with irregular tortuous vessels. At pulsed-wave US, the tumor vessels typically exhibit a low-resistance flow pattern, in keeping with neovascularization.
Pneumothorax.
Although a pneumothorax can usually be seen on a chest radiograph, a small pneumothorax may be overlooked on a radiograph of a supine patient obtained in the intensive care unit. Radiographs obtained in the intensive care unit are difficult to interpret because suboptimal technical factors, artifacts, and widespread lung changes can obscure or simulate pneumothorax.
The key sonographic signs used to diagnose pneumothorax include (Movie 5):
- Absent lung sliding
- Exaggerated horizontal artifacts
- Loss of comet-tail artifacts
- Broadening of the pleural line to a band
Bedside sonography is useful for excluding pneumothorax (6). Use of a combination of absent lung sliding and the loss of comet-tail artifact has a reported sensitivity of 100%, specificity of 96.5%, and negative predictive value of 100% (2). US was found to be more sensitive than radiography in the detection of pneumothorax, behind only percutaneous lung biopsy (18).
Although US is useful in the diagnosis of pneumothorax, the technique is unable to quantify the size of the pneumothorax. US may also be of limited use in patients with subcutaneous emphysema or pleural calcifications, because acoustic artifacts due to these conditions may limit visualization of the pleural interface (7).
Absent lung sliding should not be used as the sole criterion in the diagnosis of pneumothorax. Lung sliding may be absent in patients with previous pleurodesis, abestos-related diffuse pleural thickening, or adult respiratory distress syndrome in the absence of a pneumothorax (7).
Hydropneumothorax can also be identified with US (19). The "curtain sign" describes reverberation artifacts originating from the air within the pleura that obscures the underlying effusion during inspiration, allowing a confident diagnosis to be made.
Diseases of the Lung Parenchyma
In healthy individuals, visualization of the lung parenchyma is not possible because the large difference in acoustic impedance between the chest wall and the air within the lung results in near total reflection of the ultrasound waves. However, in parenchymal diseases that extend to the pleural surface, replacement of the air within the lung creates an acoustic window, allowing assessment of lung tissue.
Pneumonia and Lung Abscesses.
Lobar pneumonia, segmental pneumonia affecting the pleura, and pleurally based consolidation are detectable at US. In general, the size of the pneumonia appears smaller at US than on radiographs (20). This is because the periphery of the pneumonia is more air-filled, which results in more artifacts, thus limiting complete visualization of the extent of consolidation.
In the early phase of consolidation, the lung appears diffusely echogenic, resembling the sonographic texture of the liver (Fig 7). The shape of the pneumonia is rarely well defined, often showing irregular or serrated outlines. Branching echogenic structures are often (87% of patients) seen within the pneumonia and represent air bronchograms (21,22). Multiple lenticular echoes, representing air inlets and measuring a few millimeters in diameter and extending to the pleural surface, are also frequently observed. These lenticular echoes vary with respiration.
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Figure 7a. (a) US demonstrates an area of consolidation within the right lower lobe. The texture of the consolidated lung appears isoechoic to the liver. Multiple echogenic foci are seen within the consolidated lung and correspond to air-filled airways. (b) On the color Doppler scan, a pulmonary artery branch supplying the segment is clearly seen.
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Figure 7b. (a) US demonstrates an area of consolidation within the right lower lobe. The texture of the consolidated lung appears isoechoic to the liver. Multiple echogenic foci are seen within the consolidated lung and correspond to air-filled airways. (b) On the color Doppler scan, a pulmonary artery branch supplying the segment is clearly seen.
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Fluid bronchograms may also be observed in pneumonia (16%-92%) (5). These are identified as anechoic tubular structures, representing fluid-filled airways. The fluid bronchogram is seen in bronchial obstruction, which can result from either impacted secretions or a proximal tumor (22,23). Although the fluid bronchogram may be seen in isolated pneumonia, the presence of this sign in the appropriate clinical context should raise the suspicion of postobstructive pneumonitis. Indeed, US may be able to help in distinguishing the central obstructing tumor as a hypoechoic mass from the distal more echogenic consolidation (Fig 8) (23).
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Figure 8a. (a) CT scan shows a central perihilar mass (arrow) associated with collapse and consolidation of the left lower lobe. The margins of the mass, however, are not clearly demarcated. (b) The area of obstructive pneumonia also contains numerous tubular structures (arrows) of low attenuation, in keeping with fluid-filled bronchi. (c) US clearly demonstrates fluid-filled bronchi (long arrows) as anechoic branching structures. The central tumor (short arrows) appears as a well-circumscribed mass, slightly hypoechoic compared with the adjacent consolidated lung.
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Figure 8b. (a) CT scan shows a central perihilar mass (arrow) associated with collapse and consolidation of the left lower lobe. The margins of the mass, however, are not clearly demarcated. (b) The area of obstructive pneumonia also contains numerous tubular structures (arrows) of low attenuation, in keeping with fluid-filled bronchi. (c) US clearly demonstrates fluid-filled bronchi (long arrows) as anechoic branching structures. The central tumor (short arrows) appears as a well-circumscribed mass, slightly hypoechoic compared with the adjacent consolidated lung.
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Figure 8c. (a) CT scan shows a central perihilar mass (arrow) associated with collapse and consolidation of the left lower lobe. The margins of the mass, however, are not clearly demarcated. (b) The area of obstructive pneumonia also contains numerous tubular structures (arrows) of low attenuation, in keeping with fluid-filled bronchi. (c) US clearly demonstrates fluid-filled bronchi (long arrows) as anechoic branching structures. The central tumor (short arrows) appears as a well-circumscribed mass, slightly hypoechoic compared with the adjacent consolidated lung.
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Color Doppler US demonstrates normal or increased flow in the normal vessels of the consolidated lung, and may be helpful in distinguishing simple pneumonia from postobstructive pneumonia. The pulsatility index of blood flow in the consolidation is higher in cases of obstructive pneumonia (24).
As the disease progresses, the echogenicity of the pneumonia increases and becomes more heterogeneous. With successful treatment, reestablished ventilation within the consolidation gives rise to more air-inlet artifacts, and the area of pneumonia diminishes in size.
Although pneumonia is the most common cause of lung consolidation, its appearance is nonspecific. Infarction, hemorrhage, vasculitis, lymphoma, and brochoalveolar carcinoma can result in consolidation that appears similar to that of pneumonia at US. When the diagnosis is uncertain, US may be used to guide lung biopsy (25,26). This is especially useful in immunocompromised patients in whom pulmonary consolidation may pose a diagnostic problem.
Pneumonia resulting from pyogenic organisms can undergo necrosis leading to lung abscess formation. A lung abscess can be identified at US as a hypoechoic lesion with a well-defined or irregular wall (5,25). The center of the abscess is usually anechoic but may contain internal echoes and septations.
Neoplasms.
Primary Lung Neoplasms. Peripheral lung tumor appears as a homogeneous, well-defined mass that is usually hypoechoic but may be slightly echogenic. There is usually posterior acoustic enhancement (Fig 9, Movie 6) (25). Consolidation and fluid bronchograms may been seen adjacent to the mass.
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Figure 9a. (a) Chest radiograph obtained in a 40-year-old woman presenting with cough and weight loss. A mass associated with pleural effusion is seen at the periphery of the right upper zone. (b) CT scan shows the mass in the posterior segment of the upper lobe. US (Movie 6) reveals the mass as a hypoechoic, rounded lesion with posterior acoustic enhancement. Successful US-guided biopsy of the mass was performed.
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Figure 9b. (a) Chest radiograph obtained in a 40-year-old woman presenting with cough and weight loss. A mass associated with pleural effusion is seen at the periphery of the right upper zone. (b) CT scan shows the mass in the posterior segment of the upper lobe. US (Movie 6) reveals the mass as a hypoechoic, rounded lesion with posterior acoustic enhancement. Successful US-guided biopsy of the mass was performed.
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US is useful for assessing invasion of the chest wall by tumor (Fig 5) (25,27). High-resolution linear US probes are best suited for this purpose. US (sensitivity, 76.9%; specificity, 68.8%) is more sensitive than CT (sensitivity, 69.2%; specificity, 72.4%) in the evaluation of chest wall invasion (27). Extension of the tumor beyond the parietal pleura into the chest wall can be confidently determined if the mass is seen to breach the pleura, with loss of movement of the mass with respiration.
Color Doppler US has been shown to be useful in distinguishing malignant from benign pulmonary masses (28,–30). Color Doppler signal may be obtained in peripheral malignant masses in a substantial proportion (64%) of cases (28). Malignant masses are associated with neovascularity, which demonstrates low-impedance flow (28). A constant flow pattern has a high correlation with malignancy, whereas a pulsatile or triphasic flow pattern may be seen in both benign and malignant neoplasms (29). It has been shown that malignant tumors have a lower pulsatility index, resistive index, and peak systolic velocity but a higher end diastolic velocity compared with benign tumors. A resistive index of 0.52 ± 0.13 (sensitivity, 100%; specificity, 95%) and pulsatility index of 1.43 ± 0.13 (sensitivity, 97%; specificity, 95%) is reportedly useful in differentiating malignant from benign masses (28).
US is a valuable tool in the assessment of Pancoast or superior sulcus tumors (31). Visualization of the extent of the tumor can be limited at CT because of the orientation of the scan plane. US is able to depict the tumor mass, help assess any associated pleural or chest wall extension, and guide percutaneous biopsy (Fig 10).
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Figure 10a. Middle-aged man with Pancoast tumor. (a) Radiograph shows tumor within the left lung apex. (b) The mass is clearly visible on the US scan, appearing as a hypoechoeic mass that contains foci of strong reflectivity and acoustic shadowing and corresponding air-filled bronchioles. The mass can be easily biopsied under US guidance.
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Figure 10b. Middle-aged man with Pancoast tumor. (a) Radiograph shows tumor within the left lung apex. (b) The mass is clearly visible on the US scan, appearing as a hypoechoeic mass that contains foci of strong reflectivity and acoustic shadowing and corresponding air-filled bronchioles. The mass can be easily biopsied under US guidance.
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Understandably, mediastinal nodal disease resulting from pulmonary neoplasm often escapes detection at US, even though sonography of the superior and anterior mediastinum may demonstrate enlarged nodes (32). In patients presenting with signs and symptoms of superior vena cava obstruction, color Doppler US may also show collateral veins and tumor vascularity, associated with a mediastinal soft-tissue mass.
Metastasis. Peripheral pulmonary metastasis may be detected at sonography (33), apppearing as multiple subpleural echogenic nodules measuring about 1–2 cm in diameter. Color Doppler imaging demonstrates the high vascularity of these lesions and their low-resistance flow pattern. After successful chemotherapy, residual nodules showed diminished vascularity at color Doppler imaging (33).
Pulmonary Embolism.
An area of pulmonary infarction may be recognized at US as a peripheral wedge-shaped hypoechoic region (34–36). Early infarcts are less well defined, becoming more demarcated with time. A central hyperechoic structure, corresponding to a bronchiole, may be visualized. In addition, a congested vessel leading into the infarct may be seen. As in sonography of pneumonia, the area of pulmonary infarct demonstrable at US is usually smaller than that seen at angiography or scintigraphy.
Although the technique has been advocated with enthusiasm by some (34–36), with a reported sensitivity of 77%–98% and specificity of 66%–83%, the experience has not been universal. With the increasing use of pulmonary CT angiography, sonography is unlikely to change current clinical practice in the initial diagnosis of pulmonary embolism.
Heart Failure.
The presence of bilateral, widespread comet-tail artifacts has been reported as a useful sign for distinguishing patients with heart failure (Movie 7) from those with chronic obstructive airway disease in the intensive care unit (37). Comet-tail artifacts were reportedly absent in 92% of patients with chronic obstructive airway disease (37).
Rounded Atelectasis.
The sonographic appearance of rounded atelectasis has been described (38). It appears as a pleurally based mass, associated with thickening of the adjacent pleural and extrapleural fat. An echogenic line, representing the scarred, invaginated pleura, can be seen extending from the pleura into the mass in 86% of cases (Fig 11) (38).
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Figure 11a. (a) CT scan shows an area of rounded atelectasis within the left lung base. (b) US demonstrates this abnormality as a hypoechoic mass with acoustic enhancement. Note the characteristic invagination of the pleura, appearing as an echogenic line contiguous with the area of pleural thickening.
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Figure 11b. (a) CT scan shows an area of rounded atelectasis within the left lung base. (b) US demonstrates this abnormality as a hypoechoic mass with acoustic enhancement. Note the characteristic invagination of the pleura, appearing as an echogenic line contiguous with the area of pleural thickening.
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Arteriovenous Malformation.
Pulmonary arteriovenous malformations (PAVMs) are frequently located at the periphery of the lung and may therefore be detected on US scans (39). Wang et al (39) described the sonographic appearance of PAVM in a series of six patients. PAVMs appeared as well-defined hypoechoic lesions associated with posterior acoustic enhancement. On Doppler images, these lesions showed high vascularity, with low-impedance flow characteristics. US revealed decreased or absent flow in these lesions after effective arterial embolization.
Diseases of the Chest Wall
Soft-tissue, nodal, and rib abnormalities can be diagnosed with US.
Soft-Tissue Disease.
US is sensitive for the detection of soft-tissue masses arising within the chest wall. Most of these lesions are benign, such as lipomas (Fig 12), sebaceous cysts, hematomas, and abscesses.
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Figure 12a. Lipoma of the chest wall. (a) T1-weighted coronal magnetic resonance (MR) image shows a high-signal-intensity subcutaneous left chest wall mass. (b) Low signal intensity is seen on the short-inversion-time inversion-recovery coronal (fat-suppressed) image. This MR appearance is characteristic of a lipoma. (c) At US, the lesion shows mixed echogenicity. Note the underlying ribs (arrows) appearing as curvilinear echogenic structures with strong posterior acoustic shadowing.
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Figure 12b. Lipoma of the chest wall. (a) T1-weighted coronal magnetic resonance (MR) image shows a high-signal-intensity subcutaneous left chest wall mass. (b) Low signal intensity is seen on the short-inversion-time inversion-recovery coronal (fat-suppressed) image. This MR appearance is characteristic of a lipoma. (c) At US, the lesion shows mixed echogenicity. Note the underlying ribs (arrows) appearing as curvilinear echogenic structures with strong posterior acoustic shadowing.
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Figure 12c. Lipoma of the chest wall. (a) T1-weighted coronal magnetic resonance (MR) image shows a high-signal-intensity subcutaneous left chest wall mass. (b) Low signal intensity is seen on the short-inversion-time inversion-recovery coronal (fat-suppressed) image. This MR appearance is characteristic of a lipoma. (c) At US, the lesion shows mixed echogenicity. Note the underlying ribs (arrows) appearing as curvilinear echogenic structures with strong posterior acoustic shadowing.
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Unfortunately, sonography of chest wall masses is frequently nonspecific, showing a mass of variable echogenicity (Fig 13).
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Figure 13a. (a) T1-weighted and (b) T2-weighted axial MR images images through the upper chest show an intramuscular mass of intermediate T1 signal intensity and high T2 intensity. Small areas of low intensity are just visible within the mass on the T2-weighted image. This appearance would be in keeping with a hemangioma, which was proved at surgery. (c) The US findings for this mass are nonspecific, showing heterogeneous echoes with a more hypoechoic center. No increased vascularity was noted at color Doppler imaging.
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Figure 13b. (a) T1-weighted and (b) T2-weighted axial MR images images through the upper chest show an intramuscular mass of intermediate T1 signal intensity and high T2 intensity. Small areas of low intensity are just visible within the mass on the T2-weighted image. This appearance would be in keeping with a hemangioma, which was proved at surgery. (c) The US findings for this mass are nonspecific, showing heterogeneous echoes with a more hypoechoic center. No increased vascularity was noted at color Doppler imaging.
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Figure 13c. (a) T1-weighted and (b) T2-weighted axial MR images images through the upper chest show an intramuscular mass of intermediate T1 signal intensity and high T2 intensity. Small areas of low intensity are just visible within the mass on the T2-weighted image. This appearance would be in keeping with a hemangioma, which was proved at surgery. (c) The US findings for this mass are nonspecific, showing heterogeneous echoes with a more hypoechoic center. No increased vascularity was noted at color Doppler imaging.
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Lymph Nodes.
Lymph nodes, particularly within the axilla and supraclavicular fossa, are easily examined with US. Sonography can be helpful in distinguishing reactive (inflammatory) lymph nodes from those infiltrated by a malignant process.
Reactive lymph nodes are oval or triangular in shape, demonstrating an echogenic fatty hilum that may become even more prominent with inflammation. Malignant lymph nodes usually appear plump, rounded, hypoechoic, with loss of the fatty hilum (Fig 14) (40). Irregularity in the borders of these lymph nodes suggests extracapsular spread (5). At color Doppler US, increased vascularity may be demonstrable within these infiltrated lymph nodes (Fig 14). Enlarged nodes in lymphoma also appear rounded and hypoechoic but are usually well defined.
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Figure 14a. (a) Hyperplastic lymph node shows preservation of the normal central echogenicity due to fat. (b) On the power Doppler image, no appreciable vascularity is detected within the lymph node. (c) In another patient with metastatic breast carcinoma, abnormal enlarged lymph nodes are detected within the axilla, appearing rounded and diffusely hypoechoic. There is loss of the normal echogenic center. (d) In the same patient, abnormal vascularity is detected within these lymph nodes on the power Doppler image.
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Figure 14b. (a) Hyperplastic lymph node shows preservation of the normal central echogenicity due to fat. (b) On the power Doppler image, no appreciable vascularity is detected within the lymph node. (c) In another patient with metastatic breast carcinoma, abnormal enlarged lymph nodes are detected within the axilla, appearing rounded and diffusely hypoechoic. There is loss of the normal echogenic center. (d) In the same patient, abnormal vascularity is detected within these lymph nodes on the power Doppler image.
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Figure 14c. (a) Hyperplastic lymph node shows preservation of the normal central echogenicity due to fat. (b) On the power Doppler image, no appreciable vascularity is detected within the lymph node. (c) In another patient with metastatic breast carcinoma, abnormal enlarged lymph nodes are detected within the axilla, appearing rounded and diffusely hypoechoic. There is loss of the normal echogenic center. (d) In the same patient, abnormal vascularity is detected within these lymph nodes on the power Doppler image.
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Figure 14d. (a) Hyperplastic lymph node shows preservation of the normal central echogenicity due to fat. (b) On the power Doppler image, no appreciable vascularity is detected within the lymph node. (c) In another patient with metastatic breast carcinoma, abnormal enlarged lymph nodes are detected within the axilla, appearing rounded and diffusely hypoechoic. There is loss of the normal echogenic center. (d) In the same patient, abnormal vascularity is detected within these lymph nodes on the power Doppler image.
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Rib Abnormalities.
After chest trauma, US may be used in the diagnosis of rib fracture. Sonography is best performed along the line of the rib and over the site of maximum tenderness.
US is more sensitive than radiography in the detection of rib fracture (41,42). Fracture appears as a gap, step, or displacement of the cortex of the rib (Fig 15). The fracture may be associated with a localized hematoma, effusion, or soft-tissue swelling. Subtle crack fractures may exhibit a small reverberation artifact known as the "light-house phenomenon" or "chimney phenomenon" (5). During the acute healing phase, increased echogenicity is seen filling in the space of the rib fracture, representing callus formation. With time, calcification of the callus may cast a small acoustic shadow. When union and remodeling are completed, a slight contour abnormality of the cortex may be all that is discernible.
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Figure 15. When a normal rib is scanned along its long axis, the anterior cortex appears as a smooth, continuous echogenic line. In this example of an acute rib fracture, a visible gap with loss of continuity of the anterior cortex of the rib is seen. A small, hypoechoic hematoma bridges the gap.
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Bony metastases to the ribs can sometimes be visualized at US. Infiltration of the bone appears as a hypoechoic mass, replacing the normal echogenicity of the rib. There is disruption of the echogenic cortical line, which may be associated with abnormal acoustic transmission (5).
Diaphragmatic Abnormalities
There is wide variability in the normal movement of the diaphragm during respiration. There is normally asymmetry in the movement of the two leaves of the diaphragm.
Diaphragmatic Paralysis.
Diaphragmatic paralysis may be identified as paradoxical movement of the diaphragm with respiration (43). A paralyzed diaphragm may appear atrophic, with less contraction and shortening on inspiration than occurs in the normal diaphragm (Movie 8).
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US-guided Thoracic Intervention
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US is increasingly used to guide interventional procedures of the chest. Percutaneous pleural and lung biopsy can be performed under US guidance, with either the freehand technique or a needle guide. The majority of biopsies at our institution are performed with the freehand technique. Effusions and other pleural collections may also be safely drained or aspirated under US guidance. A range of self-retaining pigtail catheters (8–14-F) is suitable for this purpose.
Interventional Procedures of the Pleural Space
Thoracocentesis and Catheter Drainage.
US can be used to safely guide thoracocentesis or catheter drainage of effusions due to a wide range of causes, especially in the intensive care unit (44).
A recent review found small-bore catheters (8–14-F) to be an effective treatment for parapneumonic effusion, empyema, malignant effusion, and pneumothorax (45). The smaller tubes are also much better tolerated than are the wide-bore intercostal chest drains that have been traditionally used by clinicians for the initial treatment of parapneumonic effusions. Image-guided placement of small-bore catheters has been successful in the treatment of empyema (46). The deployment of small-bore catheters in the thorax is also associated with a lower complication rate compared with that of the wide-bore chest drains (45).
Access to the deepest part of the pleural collection can be made with an 18- or 16-gauge needle under direct US visualization. This allows direct thoracocentesis of the effusion or insertion of a 0.035-inch guide wire, which is used to guide serial incremental dilatation of a tract and deployment of the drainage catheter.
Parapneumonic effusions are effectively treated in this way, with the drainage catheter left in situ for 5–10 days on average. The reported success of radiologically guided drainage procedures ranges from 72% to 88% (47). Chemical pleurodesis can be considered prior to removal of a catheter from a malignant effusion.
Pleural Biopsy.
US can be used to guide biopsy of the pleura, either with a standard Abrams needle or an automated cutting needle device (18- or 20-gauge). US-guided pleural biopsy with an automated cutting needle is an effective method of obtaining material for histopathology. The diagnostic yield of US-guided biopsy of a solid pleural lesion is about 80% (Movie 9).
A study comparing the use of US-guided Tru-cut needle biopsy and unaided Abrams needle biopsy showed a higher sensitivity (70%–86%) and specificity (100%) with the US-guided Tru-cut biopsy in the diagnosis of pleural malignancy and tuberculosis (48). US-guided core biopsy also has a sensitivity of 77%, specificity of 80%, and positive predictive value of 100% in the diagnosis of malignant mesothelioma (49).
Chest Wall Interventional Procedures
Chest Wall Biopsy.
Chest wall biopsy may be performed to evaluate an indeterminate soft-tissue mass or, in the context of a lung malignancy, to detect the presence of chest wall invasion by tumor. US-guided cutting needle biopsy of the chest wall has also been proposed for the preoperative assessment of chest wall invasion by lung tumor (sensitivity, 61.5%; specificity, 100%; accuracy, 82.8%) (27).
Pulmonary Interventional Procedures
Lung Cancer.
Peripheral lung tumors that are in contact with or near the pleural surface can be safely biopsied under US guidance. The technique may sometimes be limited by an adequate acoustic window. Real-time visualization of the mass at US allows accurate needle placement. Masses as small as 1 cm in diameter have been successfully sampled. The shortened procedure time is especially helpful in less cooperative patients (50).
US has also been used to guide biopsy of Pancoast tumors arising from the lung apices, with a reported diagnostic yield of 83% (5). Use of color Doppler to evaluate the area prior to biopsy helps to avoid accidental injury to the subclavian vessels.
Biopsy may be performed by using either a needle guide with the US probe or by means of freehand needle insertion (Movie 10). The passage of the tip of the needle can be seen in real time, confirming sampling of the lesion. US has a reported sensitivity of 97% and accuracy of 98% for targeted diagnosis of peripheral lung cancer (51). In tumors exhibiting central necrosis, US is particularly helpful in directing biopsy to the solid viable portions of the tumor, with improved sensitivity (52). Biopsy with a cutting needle is preferred to fine-needle aspiration because of its higher diagnostic yield, and because it allows identification of histologic subtypes, it has a higher specificity for benign lesions.
Overall, US-guided percutaneous lung biopsy is extremely safe, with an overall complication rate of about 1%–2% (5,25). Pneumothorax and hemoptysis are the most commonly encountered complications. However, in most cases, no treatment is needed. US, in common with other imaging modalities, may show the development of a pneumothorax at the time of biopsy.
Pneumonia and Lung Abscess.
Yang et al found US-guided biopsy of pulmonary consolidation helpful in determining its cause, and reported a diagnostic yield of as high as 93% (25). Color Doppler allows the path of biopsy to be selected, avoiding major vessels within the lung. US-targeted biopsy of consolidation is especially useful in the immunocompromised population, in whom the cause of consolidation is wide and varied, and in whom presentation of disease may be cryptic.
In patients with a discrete lung abscess, obtaining microbiologic confirmation may be difficult. US-guided fine-needle aspiration of the abscess cavity allows specimens for microbiologic testing to be obtained, with a reported success rate of 94% (53). Drainage of lung abscesses is rarely warranted and is best achieved under CT guidance.
Mediastinal Intervention
Biopsy of Mediastinal Masses.
US can be used to guide biopsy of mediastinal masses and lymph nodes located in the anterior and upper mediastinum (Fig 16, Movie 11). These can be visualized at US via a suprasternal or parasternal approach. Biopsy of these masses has been achieved under US guidance. The reported diagnostic yield of core biopsies ranges from 84% to 100% (54). Color Doppler imaging helps in the identification and avoidance of the major vessels within the mediastinum.
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Figure 16a. (a) Chest radiograph demonstrates widening of the mediastinum. (b) CT shows a mass within the anterior mediastinum. US was used to guide biopsy of this lesion. (c) Histopathologic section revealed the presence of small round and polygonal cells within the sample, characteristic of a seminoma. Special markers confirmed the mass to be of thymic origin.
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Figure 16b. (a) Chest radiograph demonstrates widening of the mediastinum. (b) CT shows a mass within the anterior mediastinum. US was used to guide biopsy of this lesion. (c) Histopathologic section revealed the presence of small round and polygonal cells within the sample, characteristic of a seminoma. Special markers confirmed the mass to be of thymic origin.
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Figure 16c. (a) Chest radiograph demonstrates widening of the mediastinum. (b) CT shows a mass within the anterior mediastinum. US was used to guide biopsy of this lesion. (c) Histopathologic section revealed the presence of small round and polygonal cells within the sample, characteristic of a seminoma. Special markers confirmed the mass to be of thymic origin.
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Acknowledgments
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The authors thank Dr Nick Green, Dr Colum Prendergast (Lister Hospital, Stevenage, England) and Matt Treherne (Lister Hospital, Stevenage, England) for their help and kind assistance.
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References
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Yu CJ, Yang PC, Chang DB, Luh KT. Diagnostic and therapeutic use of chest sonography: value in critically ill patients. Am J Roentgenol 1992; 159:695-701.
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Abstract
Introduction
