Atelectasis is defined as incomplete air filling and underexpansion of the lung. It may involve the entire lung or appear localized to a lobe, segment, or subsegment. The collapsed lung may displace the diaphragm, fissures, hila, mediastinum, or other anatomic borders. Regions of increased radiopacity and approximation of vascular markings, ribs, or other structures are apparent occasionally. Atelectasis is always secondary to underlying pathology; therefore its presence should prompt a vigorous search for a cause. Although several mechanisms of collapse have been identified, obstruction of the airway is the most common.

DISEASE	COMMENTS
Adhesive (adhesion of the alveolar interior walls)
Hyaline membrane disease Postoperative	Decreased surfactant production in the neonate.
	Tissue adhesions related to cardiac or thoracic surgery.
Cicatrizing (scarring, fibrosus, and contraction of lung interstitium)
Connective tissue disease	Widely disseminated radiopacities resulting from tissue fibrosus related to rheumatoid arthritis, scleroderma, idiopathic pulmonary fibrosis, and so on.
Infections(FIG. 27-1) [p. 1147]	Scattered linear and circular radiopacities in the upper lung fields correlating to a history of granulomatous infections (e.g., tuberculosis, histoplasmosis, coccidioidomycosis); lesions have a tendency to form irregular thick-walled cavities.
Radiation	Nonanatomic regions of involvement that correlate to history and site of past therapeutic radiation exposure.
Compressive (intrapulmonary lesions compressing normal lung tissue)
Bullous emphysema [p. 1150]	Large, thin-walled air sacs associated with emphysema and typically found in the upper lung; these sacs may compress adjacent normal lung tissue.
Pulmonary mass	Space-occupying mass lesion within the lung (e.g., neoplasms, sarcoidosis).
Obstructive (intrinsic or extrinsic airway obstruction)
Broncholithiasis	Occurs occasionally when a radiodense, calcified lymph node from prior granulomatous infection erodes through the bronchial wall and obstructs the lumen of the airway.
Foreign bodies	Seen in infants, children (e.g., marble, peanut), and adults (e.g., dentures, tooth fragments after trauma, nails or screws held in mouth, meat) after aspiration of a foreign body; usually a characteristic history exists; occasionally patients exhibit an asymptomatic period of hours to days after incident.
Mucus plugs	Most often associated with asthma, chronic bronchitis, surgery, and neurologic suppression of cough reflex.
Tumors(FIG. 27-2) [p. 1187]	Bronchogenic carcinoma is a common cause of obstruction in patients over the age of 50 years; these patients may exhibit concurrent enlargement of the involved hilum or mediastinum. Bronchial carcinoid tumors are more common in younger patients.
Passive (extrapulmonary lesions compressing normal lung tissue)
Body wall lesions	Expansile rib lesions, pleural-based disease, and intercostal soft-tissue lesions.
Pleural space filling	Space-occupying lesions in the pleural space (e.g., edema, chyle, hemorrhage, air).
Subsegmental, platelike, discoid (linear radiopacity in periphery of lung)
(FIG. 27-3)
Hospitalization	Resulting from hypoventilation because of painful breathing, anesthesia, pleural effusion, pneumonia, and so on.
Pulmonary embolism [p. 1165]	Commonly involves collapse as a nonspecific finding of acute pulmonary embolism, often associated with pleural effusion; when the collapse occurs in the late manifestations of the disease, it represents pulmonary scar formations.

FIG. 27-1 Right lower lobe collapse caused by destruction and fibrosis from past infection. Air in dilated bronchi (black arrows) is an important clue to fibrosis and resulting bronchiectasis as the cause of the collapsed lobe. White arrow points to displaced major fissure.

FIG. 27-2 Right lower lobe collapse resulting from bronchial carcinoma. Although the obstructing bronchial lesion is not seen, metastatic bone disease is present in the right eighth rib, and there are multiple old fractures of the right ribs. A, Posteroanterior radiograph. B, Lateral radiograph. C, Computed tomography scan in a different patient with right lower lobe collapse.

FIG. 27-3 Discoid atelectasis showing a typical bandlike shadow.

Chest wall and pleural-based lesions often are marked by a characteristic radiographic appearance of a radiopaque convexity extending inward, toward the lung, with sloping superior and inferior tails. The outer border of the lesion is incompletely defined, confirming an extrapulmonary location. A variety of etiologies are responsible, including infections, primary neoplasms, metastasis, and trauma.

DISEASE	COMMENTS
Abscess [p. 1376]	Most commonly results from Staphylococcus infection, tuberculosis, and actinomycosis; associated characteristics include rib involvement, pulmonary infiltrate, pleural effusion, and a painful, red, subcutaneous mass.
Hematoma	Suggested by the presence of pleural effusion, history of trauma, evidence of trauma (e.g., rib fracture), or hemoptysis; retrosternal hematomas occasionally are seen in the lateral projection on those patients experiencing automobile accidents, in which the steering wheel inflicts blunt trauma to the chest.
Pleural fluid [p. 1162]	Free or loculated transudate, exudate, blood, chyle, etc., presenting as a well-defined density in the pleural space.
Rib fracture	Secondary to blunt or penetrating trauma to normal ribs or less forceful trauma to ribs affected by pathology (e.g., tumor or infection). Fracture occurs as a thin, vertical radiolucent line with offset of rib cortices, often best seen on an oblique projection; acute fracture warrants a careful search for associated hemothorax or pneumothorax.
Rib tumors (FIG. 27-4)	Most common benign tumors: osteochondromas, followed by enchondromas and osteoblastomas; metastatic bone disease, chondrosarcoma, and multiple myeloma produce multiple lytic regions; nonneoplastic lesions such as fibrous dysplasia commonly involve the ribs.
Skin lesions	Soft-tissue densities (e.g., nipples, moles, neurofibromas) typically appear with incomplete borders because of region of contact with the skin. Examination of the skin surface or repeat radiographs using a radiopaque marker confirms questionable lesions.
Soft-tissue benign tumor	Variety of tissue types, appearing as a smooth protruding mass from body wall into lung field; lipomas are most common and may present as intrathoracic or extrathoracic lesions.
Soft-tissue malignant tumor	Often visible, painful mass with associated bone destruction; the most common malignant soft-tissue neoplasms of the chest wall in adults are fibrosarcoma and liposarcoma; metastasis, mesotheliomas, and bronchogenic (Pancoast) tumors may involve the pleura; a past history of radiation therapy is a risk factor.

FIG. 27-4 Multiple myeloma of the rib appearing as an extrapleural lesion (arrowheads) and pathologic rib fracture. A, Peripheral pulmonary radiopacity extending inward from a broad-based chest wall origin (extrapleural sign). B, The appearance is better demarcated on computed tomography scans (arrowhead). C, Involvement of the skull also is noted in this patient (arrowheads).

The right hemidiaphragm typically is one intercostal space higher than the left. With proper inspiratory effort, the right hemidiaphram should be below the level of the posterior portion of the tenth rib or the anterior portion of the seventh rib. The position of the left hemidiaphram is more variable than that of the right because of the subjacent gastric air bubble. Unilateral or bilateral changes of the diaphragm’s position may suggest underlying pathology of the thorax or abdomen.

Diffuse alveolar disease comprises bilateral, widely disseminated opacifications that may appear very dense (homogenous consolidation) and correlate to complete filling of the alveolar space. Alternatively, a patchy and fluffy appearance (heterogeneous consolidation) denotes incomplete filling of the alveolar space.

The opacifications of lung vary in size and exhibit a tendency toward coalescence. Within the regions of opacification, characteristic tubular radiolucent shadows are seen occasionally, representing air-filled bronchi transversing water-filled parenchyma (air-bronchogram sign).

The diseases are divided into acute and chronic forms based on history and serial radiography. Although an exact timeline is difficult to develop, patterns that remain largely unchanged for several weeks or months are chronic.

DISEASE	COMMENTS
Acute diffuse alveolar disease
Adult respiratory distress syndrome [p. 1213]	Increased capillary permeability resulting from a wide variety of systemic and pulmonary insults (e.g., drug abuse, fractures, smoke inhalation, shock), causing excessive fluid accumulation in the alveolar spaces.
Near-drowning(FIG. 27-10)	May be prevented by laryngeal spasm; otherwise fluid may enter the lung from drowning or submersion events.
Pneumonia [p. 1169]	Related to aggressive infections of the lung; patients typically are extremely ill, exhibiting fever, productive cough, difficulty breathing, and malaise; although many organisms may be associated with this presentation, gram-negative infections are notorious for producing this appearance.
Pulmonary edema(FIG. 27-11) [p. 1164]	Accumulates within the interstitium during the early stages; the resulting linear densities are termed
	Kerley’s lines and are subdivided by position within the lung; lymphatic drainage is overwhelmed as fluid accumulates, and edema spills over to the alveolar lumen, relating an alveolar pattern; the appearance may be “cloudlike,” diffusely involving the lung fields bilaterally; as pulmonary edema progresses, the alveolar pattern is seen centrally with interstitial extensions to the periphery.
Pulmonary hemorrhage	Associated with anticoagulant therapy, pulmonary contusion, or less commonly Goodpasture’s syndrome; hemoptysis and patient history are indicative.
Chronic diffuse alveolar disease
Alveolar proteinosis(FIG. 27-12)	Bilateral confluent radiopacities, less commonly diffuse nodular pattern; appearance may be transient, and there are surprisingly few associated clinical symptoms apart from dyspnea.
Radiation pneumonitis	Diffuse pulmonary damage with accompanying history of therapeutic radiation.

FIG. 27-10 Bilateral, symmetric, air-space radiodense shadows in a near-drowning victim.

FIG. 27-11 Cardiogenic alveolar edema. A, Butterfly pattern. B, Bibasilar edema in a different patient showing septal lines and a left pleural effusion. Note also the bronchial wall thickening and thickening of the minor fissure (arrow).

FIG. 27-12 “Bat’s wing” shadowing in alveolar proteinosis.

Solitary or multiple localized alveolar disease is marked by extensive filling of the alveolar spaces with space-occupying lesions (e.g., blood, edema, pus, protein, cells), relating a complete (homogenous) or partial (heterogeneous) consolidation or opacification of a segment, lobe, or entire lung.

This pattern differs from the previous pattern of diffuse alveolar pattern because the involved pulmonary region is localized, unilateral, and peripherally well defined. However, similar to the previous pattern of lung disease, it may be separated into acute and chronic presentation based on the patient’s history and available serial radiographs. Although an exact timeline is difficult to develop, patterns that remain largely unchanged for several weeks or months are chronic.

DISEASE	COMMENTS
Acute localized alveolar disease
Bronchioloalveolar (alveolar cell) carcinoma(FIG. 27-13) [p. 1187]	May appear as well-defined consolidation similar to pneumonia; however, it does not respond to appropriate management of pneumonia.
Pulmonary contusion	Appearance corresponding to alveolar collections of edema and atelectasis of the lung in response to bronchial obstruction; the obstruction may develop slowly (e.g., bronchogenic carcinoma, bronchial carcinoid tumors) or rapidly (e.g., foreign body, mucus plug).
Obstructive pneumonitis	Consolidation of lung parenchyma (lung, lobe, or segment) resulting from filling of the normally airfilled alveolar sacs with exudate and inflammatory cells of similar radiopacity to water. Causative agents include bacteria, fungi, and viruses; clinical features of malaise, fever, and purulent expectorant typically accompany the radiographic findings; the consolidated area becomes patchy and fades to normal after successful treatment.
Pneumonia(FIG. 27-14) [p. 1169]	Transient densely radiopaque or incompletely consolidated patchy regions that extend from the body wall and represent blood and edema in the alveolar sacs; history of blunt trauma to the chest and presence of rib fractures are strongly indicative.
Chronic localized alveolar disease
Atelectasis [p. 1143]	Incomplete inflation of a lung (or segment) possibly appearing as a region of increased radiopacity resulting from increased lung density; concurrent finding of incomplete inflation (hilar, diaphragm, or mediastinum displacement) and clinical absence of fever and purulent cough distinguish atelectasis from pneumonia.
Lymphoma [p. 1203]	Appearance of radiopacities that represent pulmonary infiltrate of neoplasm or superimposed infection secondary to immunosuppression after treatment; often lymphoma presents with mediastinal widening; pulmonary involvement occurs as a result of direct lymphatic extension from the mediastinum.
Pulmonary infarct(FIG. 27-15) [p. 1181]	Radiopaque parenchymal density (classically pleural-based triangular appearance with apex toward hilum) located most commonly in the peripheral lower lung field. Over time the lesions tend to resolve from the periphery, inward to the center, preserving the radiopacity’s triangular configuration (“melting” sign).
Radiation pneumonitis	Patchy, irregular areas of incomplete consolidation resulting from tissue “weeping” and edema produced from tissue irradiation; a history of radiation is suggestive; the location corresponds to the radiation port.
Sarcoidosis [p. 1218]	Patchy, irregular, radiopaque areas representing noncaseating, granulomatous deposits in the interstitium; it appears as an alveolar pattern; parenchymal involvement only is an uncommon presentation of the disease; more commonly, findings include concurrent enlarged hilar lymph nodes or adenopathy that precedes the pulmonary disease; sarcoidosis is more common in blacks and women.
Tuberculosis [p. 779]	Transmitted by repeated contacts with infected individuals through inhalation; tuberculosis is not easily contracted in immunocompetent individuals; consequently, the immunocompromised and those of low socioeconomic scale (e.g., acquired immunodeficiency syndrome [AIDS] patients, alcoholics, elderly people, homeless persons) are vulnerable. The radiographic features of the primary infection include consolidation with pleural effusion; any lung segment may be involved and the findings typically resolve without complication; reactivation infections typically involve the upper lobes, often with scarring and a tendency toward cavitation.

FIG. 27-13 Localized pattern of bronchioloalveolar carcinoma appearing as air-space consolidation in the right lower lung.

FIG. 27-14 A, Posteroanterior chest radiograph demonstrating partial consolidation of the lateral portion of the right middle lobe. B, The partially consolidated segment is noted as a radiodense zone anterior to the oblique fissure (arrowheads) on the overexposed lateral projection.