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| CECIL |
| TEXT BOOK of MEDICINE |
Section IX Respiratory Diseases
| 84 IMAGING IN PULMONARY DISEASE Paul Stark • |
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IMAGING OF THE LUNGS, MEDIASTINUM, AND CHEST WALL
Epidemiology
Worldwide, chest radiography is the most commonly performed imaging procedure; more than 75 million chest radiographs are performed every year in the United States alone. Chest radiographs provide useful information about the patient's anatomy and disease at a minimal monetary cost and with radiation exposure that most experts agree is negligible. Although many novel imaging techniques are available, the plain chest radiograph remains invaluable in the initial assessment of disorders of the lung, pleura, mediastinum, and chest wall.
Imaging Techniques
Chest radiographs, although classically obtained with cassettes and x-ray film, are now commonly acquired by digital imaging with electronic display at workstations and distribution of data through networks. Regardless of the image processing approach used, the standard chest radiograph is performed at 2 m from the x-ray tube focal spot to the image detector, in frontal and lateral projections. If possible, the films should be obtained with the patient inhaling to total lung capacity. These images provide views of the lungs, mediastinum, and chest wall simultaneously.
Portable Radiography
Although bedside or portable radiography accounts for a large number of chest radiographs, the images obtained are generally of lower technical quality, cost more, and are more difficult to interpret. Lung volumes are low, thereby leading to crowding of vascular structures, and the low kilovoltage technique required for the portable equipment yields films with overexposed lungs and an underpenetrated mediastinum. The anteroposterior projection and the slightly lordotic angulation of the x-ray beam combine to distort the basilar lung structures and magnify the cardiac silhouette. Recumbent studies also make recognition of pleural effusions or pneumothoraces more difficult.
Computed Tomography
Computed tomography (CT) has multiple advantages over conventional radiography. It displays cross-sectional anatomy free of superimposition, with a 10-fold higher contrast resolution. Multislice CT scanners acquire a continuous, volumetric, isotropic data set with possibilities for high-quality two-dimensional or three-dimensional reformatting (volume rendering) in any plane. High-resolution CT of the lung parenchyma is an important application; narrow collimation of the beam combined with an edge-enhancing high spatial frequency algorithm results in exquisite detail of normal and abnormal lungs, and correlation with pathologic anatomy is high.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) depends on the magnetic properties of hydrogen atoms. Magnetic coils and radio frequency coils lead to induction, excitation, and eventual readout of magnetized protons. The molecular environment of hydrogen atoms will affect the rate at which they release energy; this energy yields a spatial distribution of signals that is converted into an image by computer algorithms, similar to CT. Because of its soft tissue specificity, MRI has applications in the assessment of chest wall invasion, mediastinal infiltration, and diaphragmatic involvement by lung cancer or malignant mesothelioma.
Positron Emission Tomography
Fluorodeoxyglucose positron emission tomography (FDG-PET) uses labeled fluorodeoxyglucose to image the glycolytic pathway of tumor cells or other metabolically active tissues with affinity for glucose. This technique has proved helpful in studying intrathoracic tumors and has facilitated the work-up of solitary pulmonary nodules.
Ultrasonography
Outside the heart, ultrasonography plays only a limited role in thoracic imaging. Its primary use is to localize pleural effusions and to guide their drainage.
Evaluation of Chest Images
Images of the chest are best evaluated by examining regions of the lung for specific findings and relating these findings to known diagnostic groups. A number of critical radiographic features should be considered, with an appreciation for the known causes of these changes.
Diffuse Lung Disease
Diffuse lung disease is an overall term for a number of related abnormal parenchymal radiographic patterns. Although radiologists have attempted to separate alveolar from interstitial lung disease radiographically, this distinction is no longer recommended because the correlation between the radiographic localization to a compartment and the actual histopathologic findings is relatively poor. For example, nodular patterns can be produced by either interstitial or alveolar disease. Conversely, so-called alveolar disease processes can induce an interstitial reaction. Ground-glass opacities can be induced by either alveolar or interstitial disease. Air bronchograms, the presumed paradigm of air space disease, can be identified in a small percentage of patients with predominantly interstitial lung disease, such as sarcoidosis, pulmonary lymphoma, and pulmonary calcinosis.
Because of such limitations, a graphically descriptive approach that combines analysis of predominant opacities, assessment of lung expansion, and distribution and profusion of disease yields a differential diagnosis. The term infiltrate should be avoided; instead, pulmonary opacities are classified as large (i.e., exceeding 1 cm in largest dimension) or small (i.e., less than 1 cm in diameter).
Large Opacities
Large opacities (Table 84-1) are characterized according to their distribution. Diffuse homogeneous opacities are typical for diffuse alveolar damage (Fig. 84-1A), increased permeability (noncardiogenic) pulmonary edema, diffuse viral pneumonia, or Pneumocystis jiroveci pneumonia. Multifocal patchy opacities (see Fig. 84-1B) are found in multifocal bronchopneumonia, recurrent aspiration, or vasculitis. Lobar opacities without atelectasis are typically seen in lobar pneumonia. Lobar opacities with atelectasis often result from obstruction of a lobar bronchus by foreign bodies, tumors, or mucus plugs. Perihilar opacities are seen in hydrostatic pulmonary edema due to left-sided heart failure (Fig. 84-2), renal failure, volume overload, or pulmonary hemorrhage.
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| FIGURE 84-1 A, Patient with diffuse alveolar damage. Chest radiograph shows diffuse homogeneous opacification of both lungs with clearly visible air bronchograms. B, Patient with acute varicella pneumonia. Chest radiograph demonstrates multiple “acinar” nodules with tendency for confluence, yielding multifocal patchy parenchymal opacification. |
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| FIGURE 84-2 Patient with hydrostatic pulmonary edema due to left-sided heart failure. Chest frontal radiograph demonstrates classic “batwing” distribution of pulmonary edema. |
Small Opacities
In contrast to the large pulmonary opacities, a number of radiographic patterns characterize small pulmonary opacities in diffuse lung disease. It is helpful to differentiate small nodular, linear, reticular, or combined patterns (Table 84-2). Micronodular opacities, which include nodules 1 mm and smaller in diameter, can result from talc granulomatosis in intravenous drug abusers (Chapter 32), alveolar microlithiasis, rare cases of silicosis, talcosis, coal workers' pneumoconiosis (Chapter 93), and beryllium-induced lung diseases (Chapter 93) as well as from occasional cases of sarcoidosis (Chapter 95) or hemosiderosis. The nodular pattern includes nodules up to 1 cm in diameter. Frequent causes include infections or inflammatory granulomas such as miliary tuberculosis (Chapter 345), sarcoidosis (Chapter 95), fungal diseases, extrinsic allergic alveolitis, and Langerhans cell histiocytosis (Chapter 92).
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Linear Patterns
Linear patterns, also called Kerley's lines, are mostly a reflection of thickened interlobular septa. Kerley's A lines, which radiate 2 cm to 4 cm from the hilum toward the pulmonary periphery and particularly toward the upper lobes (Fig. 84-3), reflect thickening of the axial interstitial compartment and can be a feature of left ventricular failure or allergic reactions. Kerley's B lines, which reflect thickening of the subpleural interstitial compartment, typically are about 1 cm in length and 1 mm in thickness and usually found in the periphery of the lower lobes, abutting the pleura. The B lines are characteristic of subacute and chronic left ventricular failure (Chapter 57), mitral valve disease (Chapter 75), lymphangitic carcinomatosis, viral pneumonia, and pulmonary fibrosis (Chapter 92). Kerley's C lines, which are rarely diagnosed by radiologists, result from thickening of the lung parenchymal interstitium and form a reticular pattern on chest radiographs.
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| FIGURE 84-3 Patient with known transfusion reaction. Chest radiograph displays ground-glass opacification of both lungs and bilateral Kerley's A lines, presenting as long linear structures extending from the hilar regions into the pulmonary periphery. |
Reticular Patterns
Reticular patterns are small polygonal, irregular, or curvilinear opacities on chest radiographs (Fig. 84-4). The differential diagnosis varies according to the timeline of the pathologic change. Acute onset of a reticular pattern can occur in interstitial edema (e.g., due to left-sided heart failure), atypical pneumonitides (e.g., viral or mycoplasmal pneumonia), early exudative changes in a connective tissue disorder (e.g., systemic lupus erythematosus; Chapter 287), and acute allergic reactions (e.g., transfusion reactions [Chapter 183] or reactions to Hymenoptera stings). The common chronic processes resulting in a reticular pattern are idiopathic interstitial pneumonias (Chapter 92), connective tissue diseases (particularly scleroderma and rheumatoid lung), asbestosis (Chapter 93), radiation pneumonitis, end-stage hypersensitivity pneumonitis, drug reactions, lymphangitic spread of cancer, end-stage granulomatous infection, lymphoma in its bronchovascular form, Kaposi's sarcoma in its bronchovascular manifestation, and sarcoidosis.
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| FIGURE 84-4 Diffuse reticular lung disease. Chest radiograph in a 94-year-old patient with diffuse reticular opacities due to idiopathic pulmonary fibrosis with honeycombing and traction bronchiectases. The lung volumes are typically reduced by a decreased pulmonary compliance. |
Honeycombing
Honeycombing, which is an indication of end-stage interstitial lung disease (Chapter 92), reflects a restructuring of pulmonary anatomy accompanied by bronchiolectasis. Honeycombs form a multilayer of small subpleural spaces between 3 and 10 mm in diameter. They can be distinguished from paraseptal emphysema by their thicker wall and multiple layers.
Alveolar Pattern
An alveolar (Chapter 91) or air space pattern is characterized by acinar nodules, 0.6 to 1 cm in diameter. These nodules encompass an acinus, in the strict anatomic sense, and surrounding peribronchiolar lung tissue. Other patterns include ground-glass opacities (a reflection of incomplete alveolar filling), coalescent large opacities, consolidation involving whole lobes or segments, opacification in a bronchocentric distribution, air bronchograms, and air alveolograms. These radiographic features are helpful in placing a disease into a particular radiologic category, but the radiographic pattern called alveolar does not simply correspond to exclusive histologic alveolar filling because the interstitial compartment is involved as well in most cases. A more accurate description is parenchymal rather than alveolar opacification or consolidation.
Bronchial Patterns
Bronchial patterns, as best depicted by diffuse bronchiectasis (Chapter 90), are seen on conventional radiographs as linear, tubular, or cystic lucencies and opacities that follow the expected path of bronchi, so-called tramlines because they resemble tram tracks. Mucoid impaction, as seen in patients with asthma, allergic bronchopulmonary aspergillosis, or plastic bronchitis, leads to opacities described as toothpaste, cluster of grapes, or finger-in-glove. The “dirty lung” pattern seen in smokers with chronic bronchitis (Chapter 88) results from bronchial wall thickening, peribronchial fibrosis, respiratory bronchiolitis, and pulmonary arterial hypertension.
Vascular Patterns
Arterial patterns reflect changes in pulmonary perfusion. The term caudalization reflects the normal blood flow distribution pattern in an upright person in which the basilar pulmonary vessels are two to three times wider than the upper lobe vasculature. Cephalization, in which the ratios of diameters of vessels are reversed, is frequently seen in recumbent persons, in whom it may be considered normal; however, when it is present in individuals imaged in the upright position, it indicates left ventricular failure, mitral valve disease, or basilar emphysema (Fig. 84-5). Equalization, or balanced flow with well-demonstrated vessels to upper and lower lung zones, is found in hyperkinetic circulation due to anemia, obesity, pregnancy, Graves' disease, or left-to-right shunts. Equalization or balanced flow with oligemia can be seen in hypovolemia, diffuse emphysema, or right-to-left shunts. Centralization reflects dilation of central pulmonary vessels, with accompanying normal or diminished peripheral circulation. Typically, it is seen in pulmonary arterial hypertension (Fig. 84-6). Lateralization of flow, favoring one lung over the other, also called asymmetrical perfusion, is visible with unilateral emphysema, unilateral bronchiolitis obliterans (Swyer-James-McLeod syndrome), or unilateral obstruction of the pulmonary artery. Locally enlarged vessels occur in patchy emphysema, multiple pulmonary emboli, arteriovenous malformations, and nonuniform bronchiolitis obliterans. This pattern produces a mosaic perfusion on high-resolution CT scanning.
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| FIGURE 84-5 Patient with left ventricular failure. Chest frontal radiograph shows cephalization of pulmonary blood flow. |
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| FIGURE 84-6 Patient with primary pulmonary arterial hypertension. Chest frontal radiograph shows centralization of flow with pulmonary artery aneurysms and peripheral pulmonary oligemia. |
Lung Volume
Conventional radiographs and CT scans are taken during a breath hold at full inspiration. Lung volume is inferred from the relative size of the low-attenuation gas-containing regions compared with what one would expect for an equivalently sized chest wall (Table 84-3). Lung volumes larger than expected are commonly found in patients with diffuse emphysema (Fig. 84-7) (Chapter 88), chronic asthma (Chapter 87), or diffuse bronchiolitis and in highly trained athletes. With a few rare exceptions, chronic diffuse infiltrative lung diseases (Chapter 92) lead to loss of volume.
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| FIGURE 84-7 Patient with severe emphysema. Chest radiograph shows hyperexpansion of both lungs with bullous changes at the right lung base and leftward mediastinal shift. |
Anatomic Distribution
The anatomic distribution of disease can significantly facilitate the approach to diagnosis (Table 84-4 and Fig. 84-8). Upper zone lung disease predominates in tuberculosis, fungal disease, sarcoidosis, pneumoconiosis (except asbestosis), Langerhans cell histiocytosis, ankylosing spondylitis, cystic fibrosis, cystic P. jiroveci pneumonia, radiation pneumonitis, and end-stage hypersensitivity pneumonitis. Basilar lung disease is preferentially found in bronchiectases, aspiration, desquamative interstitial pneumonia, nonspecific interstitial pneumonitis, usual interstitial pneumonitis, drug reactions, asbestosis, scleroderma, and rheumatoid arthritis. However, any diffuse lung process will eventually progress to involve both lungs irrespective of zonal boundaries.
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| FIGURE 84-8 A, Basilar pulmonary disease. Chest radiograph in a 48-year-old patient with known scleroderma. Bibasilar fine reticular opacities and parenchymal bands are visible in both lower lobes. B, Apical lung disease. Chest radiograph in a 42-year-old patient with ankylosing spondylitis. Severe architectural distortion with cicatrizing atelectasis of both upper lobes, retraction of both pulmonary arteries cephalad, and bilateral bulla formation containing fungus balls are evident. |
Lymph Nodes
Enlarged lymph nodes that are visible on chest CT scans and, when larger, on chest radiographs can provide diagnostic information (Table 84-5). The following entities can be associated with diffuse lung disease and concurrent enlarged lymph nodes: sarcoidosis (Chapter 95); lymphoma; fungal disease; tuberculosis (Chapter 345); pneumoconioses (Chapter 93), particularly silicosis and beryllium-associated lung disease; lung cancer; and metastatic malignant disease other than lung cancer.
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Pulmonary Nodules
Solitary pulmonary nodules are covered in Chapter 201. The majority of patients with multiple pulmonary nodules have metastatic disease from primary cancers either within or outside the lung (Fig. 84-9). These lesions have a predilection for subpleural lung regions, including the interlobar fissures. In patients with human immunodeficiency virus infection, Kaposi's sarcoma and lymphoma can induce the formation of such nodules. Infectious processes that present with multiple nodules include multiple abscesses from recurrent aspiration (Chapter 94) or septic emboli (Chapter 76); tuberculous and nontuberculous mycobacterial granulomas (Chapters 345 and 346); fungal processes, including histoplasmosis (Chapter 353), coccidioidomycosis (Chapter 354), and cryptococcosis (Chapter 357); and infection with flukes, such as Paragonimus westermani (Chapter 377). Noninfectious inflammatory conditions that can present with multiple pulmonary nodules include Wegener's granulomatosis (Chapter 291), rheumatoid nodules (Chapter 285), sarcoidosis (Chapter 95), and amyloidosis (Chapter 296).
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| FIGURE 84-9 Multifocal pulmonary opacities. Chest radiograph in a 70-year-old patient with known carcinoma of the thyroid gland widening the superior mediastinum and displacing the cervical trachea to the right. Bilateral large and small pulmonary nodules and masses due to metastatic tumor are present. |
Pleural Disease
Pleural diseases are covered in Chapter 100. Abnormalities of the pleural space easily can be displayed by conventional radiographic methods supplemented by CT scanning. The volume of pleural effusions can be reliably estimated on standard posteroanterior films: 75 mL obscures the posterior costophrenic sulcus, 150 mL obscures the lateral costophrenic sulcus, 200 mL produces a rind of 1 cm in thickness on decubitus films, 500 mL obscures the diaphragm and is also visible on supine films, and 1000 mL reaches the level of the fourth anterior rib on upright chest radiographs. An effusion of 200 mL or more can be sampled by thoracentesis. The smallest amount visible on decubitus films is 10 mL. With care, as little as 175 mL of effusion can be detected on supine films. Free layering effusions produce a veil of opacity or filter effect superimposed on the aerated lung; pulmonary vessels are clearly visible through the added opacity generated by the effusion, and air bronchograms are absent.
Subpulmonic Effusions
Subpulmonic effusions elevate the lung base, mimicking an elevated hemidiaphragm. The highest curvature point of the pseudodiaphragm is shifted laterally. Large effusions can lead to diaphragmatic inversion. Separation of the lung base from the gas-containing stomach is indicative of a subpulmonic effusion, particularly when the stomach gas bubble is displaced inferomedially. Loculated pleural effusions suggest the presence of pleural adhesions. Such encapsulated collections have obtuse angles of interface with the chest wall and have a sharply defined border with the adjacent lung.
Pleural Plaques
Pleural plaques result from parietal pleural accumulation of hyalinized collagen fibers (Fig. 84-10); their presence suggests asbestos exposure (Chapter 93). Plaques preferentially involve the parietal pleura adjacent to ribs six through nine and the diaphragm. They are less pronounced in the intercostal spaces and spare the costophrenic sulci as well as the apices. Calcifications are visible on chest radiographs in 20% and on CT scans in 50% of individuals with plaques. Imaged in profile, pleural plaques produce focal areas of apparent pleural thickening. Over the diaphragm, they appear as curvilinear calcifications or scalloping. Pleural plaques viewed en face can simulate lung disease. Their appearance has been likened to holly leaves, sunburst patterns, “geographic” patterns, or stippled or irregular structures. Rare visceral pleural plaques that occur in interlobar fissures can mimic pulmonary nodules.
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| FIGURE 84-10 Patient with known prior occupational asbestos exposure. Chest radiograph shows extensive bilateral calcified plaques seen en face, in profile, and along the diaphragmatic contour. |
Diffuse Pleural Thickening
Diffuse pleural thickening is a response observed after exposure to any of a number of stimuli including infection, inflammation, trauma, tumor, thromboembolism, radiation, and asbestos. Severe involvement results in formation of a generalized pleural peel with smooth margins and usually less than 2 cm in thickness. Radiologically diffuse pleural thickening is characterized by a smooth, noninterrupted pleural opacity involving at least a quarter of the chest wall circumference, obliterating the costophrenic sulci and encompassing also the apices. The CT criteria for diffuse pleural thickening include a thickness of at least 3 mm.
Malignant Disease
Malignant tumors of the pleura are more common than benign ones, and metastatic disease is more frequent than a primary pleural mesothelioma. Primary tumors originate from pleural membranes. Pleural invasion by lung cancer, subpleural plaques in lymphoma, hematogenous dissemination to the pleura, and direct pleura seeding are other mechanisms of pleural involvement by tumor. Benign pleural tumors include lipomas, fibrous tumors, and neurogenic tumors. Lipomas are most common; their diagnosis is facilitated by CT scanning. Fibrous tumors of the pleura originate from pluripotent mesenchymal cells found in the visceral pleura or, less commonly, in the parietal pleura. They can induce paraneoplastic syndromes such as hypertrophic osteoarthropathy (Chapter 189) or hypoglycemia and only rarely invade or metastasize. In nearly half of these patients, the tumor can be on a pedicle and be mobile as a patient changes position.
Pneumothorax
Pneumothorax means gas in the pleural space (Chapter 100). The most important radiologic feature of a pneumothorax is a visceral pleural line or edge that is convex or straight toward the chest wall and produces a lucent separation of the visceral and parietal pleura (Fig. 84-11). In a majority of cases, no pulmonary vascular structures are visible beyond the visceral pleura. On upright chest radiographs, gas is primarily found in the apicolateral pleural space. Expiratory chest radiographs are not necessary for the detection of small pneumothoraces because all pneumothoraces are visible on inspiratory films. On supine chest radiographs, pleural gas accumulates in a subpulmonic location; it outlines the costophrenic sulcus, forming the deep sulcus sign. A tension pneumothorax leads to marked shift of the mediastinum to the contralateral side and to flattening or inversion of the ipsilateral hemidiaphragm.
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| FIGURE 84-11 Patient with spontaneous tension hydropneumothorax. Chest radiograph shows complete atelectasis of the left lung with a large pneumothorax and a left basilar gas-liquid level. The patient had primary tuberculosis. |
Imaging of the Mediastinum
The mediastinum encompasses midline thoracic structures that are delineated by mediastinal pleura, the diaphragm, the sternum, the spine, and the thoracic inlet. The mediastinum is commonly divided into an anterior compartment, a visceral middle compartment, and a paraspinal, posterior mediastinal compartment (Table 84-6). Each compartment contains specific pathologic entities.
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Imaging Techniques
On well-penetrated chest radiographs, the anterior junction line, the posterior-superior junction line, the azygoesophageal stripe, the pleuroesophageal stripe, the paratracheal stripe, and the para-aortic and the paraspinal stripes or lines should be assessed (Fig. 84-12). Mediastinal masses need to be detected and localized first. Their obtuse angles of interface with the mediastinal pleura as well as extension into both hemithoraces indicate the mediastinal origin of such lesions.
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| FIGURE 84-12 Chest radiograph with superimposed mediastinal stripes. Yellow: right paratracheal stripe. Light blue: right and left paraspinal stripes. Red: azygoesophageal stripe. Brown: pleuroesophageal stripe. Purple: anterior junction line complex. Pink: left subclavian artery border. Light green: posterior-superior junction line. Dark green: para-aortic line. |
CT facilitates localization of a mass to a specific mediastinal compartment. Once it is known whether the mass is predominantly fat, cystic, soft tissue, or calcified, the differential diagnosis can be limited. MRI of the mediastinum has a role in diagnosis of vertebral disease or neurogenic tumors with extension into the spinal canal. It is as good as CT in diagnosis of aortic aneurysms and dissections (Chapter 78).
Mediastinal Compartments
The anterior mediastinum is actually a potential space that may contain the fatty replaced thymus and small normal lymph nodes. Space-occupying lesions in this compartment typically include thymomas, lymphomas, teratomas and other germ cell tumors, substernal thyroid goiters, lipomas, and other connective tissue tumors as well as hemangiomas or lymphangiomas (Fig. 84-13A).
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| FIGURE 84-13 A, Patient with anterior mediastinal teratoma. Chest radiograph shows a mediastinal contour abnormality due to projection of the mass into the right hemithorax. Note the obtuse angle of interface formed by the pleura covering the mass with the mediastinum. B, Patient with Castleman's giant lymph node hyperplasia. Chest frontal radiograph shows large subcarinal middle mediastinal mass that projects lateral to the right atrium. C, Patient with paraspinal ganglioneuroma. Chest radiograph shows right lower paraspinal contour abnormality widening the right paraspinal region and encompassing the height of three thoracic vertebrae. |
The middle mediastinum is subdivided into the subcarinal space, paratracheal region, retrotracheal region, aortic-pulmonic window region, and retrocardiac space. Characteristic lesions are enlarged lymph nodes and bronchopulmonary foregut malformations (see Fig. 84-13B).
In the retrotracheal region, aberrant right subclavian arteries, posterior descending goiters, esophageal tumors, diverticula, or thoracic duct cysts can be found. In the aortic-pulmonic window, ductus diverticula, bronchopulmonary foregut malformations, or aortic or pulmonic artery aneurysms can form compartment-specific space-occupying lesions.
The paraspinal region is considered radiologically to belong to the posterior mediastinum. Important masses in that space include neurogenic tumors that originate from the sympathetic chain or from segmental nerve roots (see Fig. 84-13C). Extramedullary hematopoiesis in patients with severe anemia can result in paravertebral masses formed by hypertrophied bone marrow that extrudes from ribs or vertebral bodies. Enlarged lymph nodes due to lymphoma or metastatic disease are occasionally seen in a paraspinal location. Vertebral disease, including bacterial or tuberculous spondylitis, tumors, and post-traumatic hematomas, can widen the paraspinal region and produce contour abnormalities.
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