13 Radiographic Signs and Differential Diagnosis

1. The Opaque Hemithorax (Table 13.1)

Diagnostic Approach

Complete opacification of a hemithorax frequently indicates the presence of significant disease. The differential diagnosis includes:

• Inflammatory or neoplastic unilateral pulmonary infiltration
  
• Unilateral pulmonary atelectasis or agenesis
  
• Pneumonectomy
  
• Pleural disease and most commonly the presence of a large pleural effusion

The chest radiograph shows the opaque hemithorax, any associated mediastinal displacement, and occasionally may suggest the correct diagnosis. Computed tomography (CT) ± bronchoscopy are standard 2nd line investigations for further assessment.

Comparison of the volume of the opaque hemithorax to that of the unaffected side is important when considering the differential diagnosis (Fig. 13.1):

• An increase in the volume of the hemithorax is recognized by contralateral mediastinal displacement, widening of the intercostal spaces, and depression of the hemidiaphragm. A large opaque hemithorax may indicate either a massive pleural effusion or a large intrathoracic mass. While a pleural effusion may lead to complete opacification of the hemithorax in the supine position, some aerated lung is usually visible at the apices on upright views.
  
• Ultrasound will show the presence of an effusion and allow image-guided diagnostic aspiration. Computed tomography, however, is the imaging modality of choice for assessment of underlying pulmonary and mediastinal abnormality.
  
• A decrease in the volume of the hemithorax is manifest by ipsilateral mediastinal displacement, narrowing of the intercostal spaces, and elevation of the hemidiaphragm. A small opaque hemithorax may indicate a restrictive pleural process, pulmonary atelectasis, pulmonary hypoplasia, or past pneumonectomy. A fibrothorax may show a rind of calcification at its pulmonary interface. Complete lung atelectasis is most commonly secondary to central bronchial carcinoma. Pulmonary hypoplasia is associated with marked ipsilateral mediastinal displacement (see Chapter 2, p. 57).
  
• An opaque hemithorax of normal volume may indicate extensive unilateral pulmonary consolidation or it may be seen when a pleural effusion is associated with underlying pulmonary atelectasis.

Table 13.1 The opaque hemithorax

Pleural effusion Pleural thickening and fibrothorax Pleural mesothelioma and metastatic pleural carcinomatosis Pneumonia Atelectasis Tuberculosis Pulmonary aplasia, agenesis, and pneumonectomy Diaphragmatic hernia Thoracic scoliosis and chest wall deformity

Fig. 13.1 The opaque hemithorax.

Complete pulmonary atelectasis results from occlusion of a main bronchus by carcinoma or less commonly by an aspirated foreign body, mucus plug, stricture, or bronchial tear. The volume of the hemithorax is reduced and there is ipsilateral mediastinal displacement (Fig. 13.2). A concomitant pleural effusion, however, may restore thoracic symmetry (Fig. 13.3).

Pneumonia

It is rare for pneumonic infiltration to involve the entire lung. A well-penetrated chest radiograph may show small residual areas of normal lung and air-filled bronchi traversing the consolidation (air bronchogram). The diagnosis is supported by clinical findings of cough, sputum production, pyrexia, and leukocytosis, and by identifying the causative organism in sputum samples or from blood cultures.

Fig. 13.3a–c Opaque right hemithorax. Chest radiograph (a) shows opaque right hemithorax. CT images (b and c) show the large effusion with extensive abnormal soft tissue infiltrating the mediastinum and right hilum and with right lung atelectasis. Diagnosis was bronchial carcinoma and left midzone pulmonary metastasis is noted.

The involved hemithorax is small and there is ipsilateral mediastinal displacement (see Chapter 2, p. 58) (Fig. 13. 4).

Thoracic Scoliosis and Chest Wall Deformity

Extreme scoliosis with posterior unilateral rib prominence and thoracic deformity can occasionally mimic an opaque hemithorax, particularly on underexposed views. This appearance may be accentuated by the secondary ventilatory restriction and pulmonary underexpansion seen in some of these cases.

Fig. 13.4 Left pneumonectomy: Chest radiograph shows opaque left hemithorax with ipsilateral mediastinal displacement.

2. Lobar and Segmental Opacification

Diagnosis becomes more difficult and merits further evaluation when radiographic abnormality persists. Bronchial carcinoma is a frequent cause of persistent atelectasis and/or consolidation, particularly in older individuals from “at risk” categories (see Chapter 6, p. 157). Bronchial obstruction by mucus plugs, foreign bodies, and benign tumors are less common causes of persistent lobar/segmental opacification. Pulmonary infarction may also lead to segmental opacification which is slow to resolve (see Chapter 7, p. 189).

Differential Diagnosis (Table 13.2)

Pseudosegmental Opacities

Loculated pleural effusions, interlobar effusions, chest wall tumors, and large pulmonary angiomata and tumors occasionally mimic segmental opacities in their shape and location but they never have an air bronchogram or alveologram.

Lobar and Segmental Consolidation

An inflammatory or neoplastic process may spread within the alveolar spaces of a segment until it reaches the segmental or lobar boundary. Aerated lung is replaced by fluid and cellular infiltrate. Residually aerated alveoli may appear as small “foamy” lucencies (positive air alveologram) and residual air within the bronchi forms a pattern of branching radiolucent channels (positive air bronchogram) within the consolidation. Usually the anatomic shape of the segments remains unchanged although a slight volume increase can occur in entities such as Klebsiella pneumonia producing a convex border with the adjacent normal lung.

Table 13.2 Lobar and segmental opacification

(Figs. 13.5–13.10)

Classic lobar pneumonia caused by pneumococci or Klebsiella pneumoniae has become a rarity. Because bacterial pneumonia spreads from alveolus to alveolus through the pores of Kohn, lobar and segmental boundaries create a reasonably effective barrier. Because of this, the opacity exhibits at least one sharp border, even if the consolidation does not involve the entire segment (peripheral consolidation).

Tuberculous Pneumonia

Tuberculous pneumonia has a predilection for the upper lobes and the apical segments of the lower lobes. Often there are accompanying features of past tuberculosis such as apical pleural thickening and fibrocirrhotic changes in the apical parenchyma. Any upper lobar segmental opacity persisting for weeks should raise suspicion of tuberculous pneumonia, particularly in at-risk groups of individuals.

Bronchioloalveolar Carcinoma

Some bronchial carcinomas, especially those of bronchioloalveolar type, spread within the alveoli of a lung lobe or segment and may be indistinguishable from pneumonic consolidation.

Fig. 13.5 CT coronal reformat shows partial right lower lobe consolidation with loss of definition of the right hemidiaphragm.

Fig. 13.6 Right upper lobe consolidation with a minor degree of associated atelectasis leading to slight superior displacement of the right minor fissure.

Fig. 13.7a, b Pneumonic consolidation involving the anterobasal left lower lobe.

Pulmonary Infarction

Pulmonary infarcts appear as subtle segmental opacities which occur predominantly in the subpleural lung of the lower lobes. Left ventricular failure usually is a prerequisite for the development of infarction following pulmonary embolism.

Lobar and Segmental Atelectasis

• Absorption atelectasis (obstructive atelectasis) in which there is initial occlusion of the bronchial lumen followed by absorption of air in the distal lung: Airway obstruction may be caused by tumor, mucus plugs, foreign bodies, inflammatory bronchial narrowing or occlusion, or extrinsic compression (e.g., by lymph node enlargement in “right middle lobe” syndrome). In most cases there is incomplete collapse of the involved segment or lobe because of collateral air drift through the pores of Kohn.
  
• Relaxation atelectasis (compression atelectasis): In this case lung expansion is prevented by a pneumothorax or large pleural effusion with resulting pulmonary atelectasis. This form of atelectasis may persist for a short time following aspiration of the effusion or pneumothorax.

Fig. 13.8a, b Right middle lobe pneumonia. The right heart border is not visualized.

Fig. 13.9 Pneumonic consolidation involving the anterior segment of the right upper lobe lies adjacent to the minor fissure.

Volume loss in a lobe tends to have a specific radiographic appearance (Fig. 13.12):

• Right upper lobe atelectasis is manifest as a wedge/ triangular-shaped opacity lying adjacent to the superior mediastinum (Fig. 13.14).
  
• Right middle lobe atelectasis is associated with obliteration/“loss” of the right cardiac border.
  
• Right and left lower lobe atelectasis lead to obliteration/“loss” of the respective hemidiaphragm (Fig. 13.15a, b) and in left lower lobe atelectasis, a wedge-shaped opacity may be visible in the left paravertebral/retrocardiac region.
  
• Left upper lobe atelectasis leads to “hazy” opacification of the left hemithorax (Figs. 13.16a, b, see also Fig. 6.24a–c).
  
• Lingular segment atelectasis leads to obliteration/ loss of the left cardiac border.

Atypical appearances may be due to collateral aeration of isolated boundary zones or may occur in cases where fibrous adhesions prevent uniform volume loss.

Fig. 13.11 Evolution of round atelectasis (from Hanke and Kretschmar 1983).

Complete atelectasis appears radiographically as a homogeneous shadow but initially some portions of the segment may still be aerated. The following radiographic features characterize atelectasis:

• The shape and location of the opacification (see Fig. 1.33).
  
• Concavity of the segmental margin. This is clearly appreciated only at sites where the segment borders on the lobar boundary or interlobar fissure.
  
• Hyperexpansion of adjacent lung characterized by decreased vascular markings and local hypertransradiancy.
  
• Displacement of interlobar fissures and vascular shadows.
  
• Ipsilateral shift of the mediastinum, elevation of the hemidiaphragm, narrowing of the intercostal spaces, and transmediastinal herniation of the opposite lung. These changes are seen with lobar or complete pulmonary atelectasis.

Fig. 13.12 Lobar atelectasis.

Fig. 13.13a, b Right upper lobe consolidation and atelectasis due to endobronchial tumor. There is superior displacement of the right minor fissure and the major fissure is displaced anteriorly.

Fig. 13.14 Right upper lobe atelectasis due to an endobronchial mucus plug.

Fig. 13.15a, b Right lower lobe atelectasis. There is opacification of the right cardiophrenic angle with “loss” of the right hemidiaphragm.

Fig. 13.16a, b Left upper lobe atelectasis secondary to central bronchial carcinoma.

Breast carcinoma and other tumors occasionally metastasize to the wall of a lobar or segmental bronchus (see Fig. 6.34 a, b).

Benign Endobronchial Tumors

Bronchial carcinoid is the most common benign tumor to display endobronchial growth and lead to distal atelectasis. A small number of cases will develop bronchoceles distal to the obstructing tumor and branching opacities may then be the predominant radiographic finding. Other endobronchial tumors including chondroma, which occasionally shows flocculent calcification, hamartomas, and amyloid tumors are comparatively rare.

Endobronchial Mucus Plugging

This is characteristically seen in severe asthma and in patients with superimposed Allergic Bronchopulmonary Aspergillosis (ABPA).

Bronchial Rupture and Hematoma

Severe thoracic injuries may rupture a main or lobar bronchus with submucosal hematoma and distal atelectasis.

Inflammatory Bronchial Strictures and Extrinsic Compression

Inflammatory bronchial strictures may be seen in entities such as tuberculous infection and Wegener’s granulomatosis.

The right middle lobe bronchus is particularly susceptible to compression by adjacent tuberculous lymph nodes resulting in distal atelectasis.

3. Opacification which does not Conform to Anatomic Boundaries

Inhomogeneous and Regionally Confluent Air Space Opacification

Differential Diagnosis (Table 13.3)

Acute Bronchopneumonia

This disease is characterized by patchy air space shadowing characteristically involving the mid-to-lower lung zones. Acute bronchopneumonia is usually due to bacterial infection (Fig. 13.17).

Table 13.3 Pulmonary opacification not conforming to lobar and segmental boundaries

Aspiration Pneumonia

Fungal pneumonias and particularly semi-invasive aspergillosis (Fig. 13.18) in older individuals with a mild degree of immunocompromise due to comorbidities may manifest as patchy air space consolidation.

Pulmonary Edema

Early-stage alveolar edema may have an inhomogeneous patchy distribution in the mid-to-lower lung zones. There are characteristically features of associated interstitial edema with peribronchial cuffing and interlobular septal thickening.

Pulmonary Contusion

Patchy, confluent opacities representing sites of intra-alveolar edema and hemorrhage are seen in the traumatized lung. Pulmonary contusions typically resolve in 3 to 6 days except when infection supervenes.

Pulmonary Hemorrhage

Diffuse pulmonary hemorrhage is associated with underlying disease entities including Antiglomerular Basement Membrane Disease (AGBMD) and Wegener’s Granulomatosis. Initial multifocal opacification in a peribronchovascular distribution may progress to areas of confluent consolidation.

Churg-Strauss Allergic Granulomatosis

Chest radiographs may show transient, recurrent pneumonic infiltrates in addition to pericardial and pleural effusion in patients with this systemic necrotizing vasculitis.

Pulmonary Neoplasia

Both bronchioloalveolar carcinoma and pulmonary lymphoma may be manifest radiographically as patchy air space consolidation in a peribronchial distribution.

Radiation Pneumonitis and Fibrosis

Air space consolidation characteristically conforms to the radiation portal in radiotherapy-induced pneumonitis. Consolidation over time may organize leaving some degree of interstitial fibrosis (Fig. 13.19).

Fig. 13.17a, b Multifocal airspace consolidation not conforming to segmental or lobar boundaries.

Fig. 13.18 Aspergilloma: Mycetoma with air crescent is seen within right upper lobe cavity.

Sarcoidosis (Fig. 13.20)

See Chapter 3, p. 91.

Adult Respiratory Distress Syndrome

See Chapter 8, p. 208.

Bilateral Symmetric Hazy Opacification

• Bilateral posterior pleural effusions in a supine patient (Fig. 13.21 a, b).
  
• Hyaline membrane disease—Acute respiratory distress syndrome in the newborn (Fig. 13.22).
  
• Adult respiratory distress syndrome—Diffuse alveolar damage (Fig. 13.23).
  
• Pulmonary infection, edema, hemorrhage (Fig. 13.24).
  
• Alveolar proteinosis (Fig. 13.25).

Fig. 13.20a, b Sarcoidosis. The chest radiograph (a) shows bilateral hilar and mediastinal lymph node enlargement with areas of pulmonary consolidation. CT shows bilateral hilar and mediastinal lymph node enlargement with areas of consolidation consistent with the “alveolar” variant of sarcoidosis.

Fig. 13.21a, b Pleural effusions in supine patient: Hazy opacification of both hemithoraces on AP supine projection (a) is due to bilateral pleural effusions and underlying pulmonary atelectasis (b).

Fig. 13.22 Hyaline membrane disease with bilateral “ground-glass opacification” in a preterm infant. Air bronchograms are visible.

Fig. 13.23 Adult respiratory distress syndrome in acute pancreatitis. Initial chest radiograph shows diffuse, bilateral pulmonary opacification.

Fig. 13.24 Noncardiogenic pulmonary edema in a patient with renal impairment.

Fig. 13.25 Alveolar proteinosis. Chest radiograph shows “hazy” opacification of both lungs with associated interstitial thickening in patient with alveolar proteinosis.

4. Opacification involving the Upper Zone and/or Apicomediastinal Angle

Differential Diagnosis (Fig. 13.26, Table 13.4)

Vascular and Mediastinal Lesions

Vascular Ectasia

Ectasia of superior mediastinal vessels leads to mediastinal widening with a curved, sharply defined lateral border. Dilatation and elongation are particularly common in elderly patients and may involve the left brachiocephalic vein, subclavian vein, superior vena cava, and innominate artery. Computed tomography will confirm the vascular ectasia when there is diagnostic uncertainty.

Fig. 13.26 Upper zone opacification.

Table 13.4 Upper zone and apicomediastinal angle opacification

Aneurysmal dilatation of the aortic arch may produce a large mediastinal density, generally with a convex lateral border on the posteroanterior (PA) chest radiograph (see also Chapter 11, p.283). Aneurysmal dilatation of a subclavian/anomalous right subclavian artery may lead to unilateral superior mediastinal widening.

Retrosternal Goiter and other Mediastinal Soft Tissue Lesions

There is widening of the superior mediastinum and the trachea may be narrowed or displaced. Thyroid nodules may show flocculent calcification. See also Chapter 11, p. 285.

Pulmonary and Pleural

Upper Lobe Pneumonia

Pneumonia involving the apical segment of the right upper lobe or the apicoposterior segment of the left upper lobe appears as a triangular paramediastinal opacity. Consolidation in the anterior segment of the right upper lobe abuts the minor fissure.

Upper Lobe Tuberculosis (Fig. 13.27)

Tuberculous infection should be excluded in all cases of persisting upper lobe consolidation (see also Chapter 3, p. 72).

Fig. 13.27 Right upper lobe tuberculosis.

Fig. 13.28 Radiation pneumonitis posttreatment of left hilar/ upper lobe bronchial carcinoma.

Fig. 13.29 Apical pleural thickening.

Bronchial Carcinoma

Bronchial carcinomas may occur in the superior sulcus of the lung and infiltrate locally (Pancoast tumor). The chest radiograph may demonstrate destruction of adjacent ribs. More commonly, bronchial carcinoma arising from and occluding the upper lobe bronchus leads to distal upper lobe consolidation and atelectasis (Fig. 13. 28).

Silicosis

Silicotic nodules sometimes may be concentrated in the upper lobes. Progressive massive fibrosis (PMF) complicating silicosis has a marked predilection for the upper lobes.

Pulmonary Langerhans Cell Histiocytosis

Early-stage nodular PLCH characteristically involves the upper-to-mid zones with relative sparing of the lung bases (see also Chapter 6, p. 154).

Mucoid Impaction (Bronchocele Formation)

Congenital bronchial atresia occurs most commonly in the apicoposterior segmental bronchus of the left upper lobe. The airway distal to the atresia is dilated and filled with mucus and appears as an elongated, partially branched opacity in the left upper lobe (bronchocele). Allergic bronchopulmonary aspergillosis also leads to mucus plugging of bronchi and characteristically colonizes dilated upper lobe bronchi.

Fig. 13.30 Pleural line associated with an azygos lobe.

Apical Pleural Thickening

An apical pleural peel may extend to involve the mediastinal pleura and reach a thickness of several millimeters (Fig. 13.29).

Azygos Lobe and Fissure

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