Radiography of Interstitial Lung Disease

1

What radiographic features distinguish interstitial diseases from airspace diseases?

Two primary characteristics radiographically distinguish interstitial diseases from airspace diseases. First, interstitial diseases displace little of the air within the lung, whereas airspace diseases displace large amounts of air. Interstitial diseases change the overall opacity of the lung very little, whereas airspace diseases in most cases dramatically increase the opacity (whiteness) of the lung on chest radiography. Second, interstitial diseases appear as increases in small nodules (generally <5 mm in diameter) or thin lines (<5 mm in width) (or both) within the lung, whereas airspace diseases appear as indistinctly marginated patches of opacity. See Figure 19-1 for the normal appearance of the lungs on frontal chest radiography.

2

What factors influence the likelihood of one interstitial disease over another interstitial disease?

Three primary factors affect the likelihood of a given interstitial disease:

• 1.

  The relative incidence of a given disease in the general population. Typical presentations of common diseases are most likely, followed by atypical presentations of common diseases, typical presentations of uncommon diseases, and atypical presentations of uncommon diseases.

• 2.

  The clinical history of the patient.

• 3.

  The radiographic pattern of the interstitial disease.

3

What is the most common interstitial abnormality identified on chest radiography?

Interstitial pulmonary edema, usually caused by congestive heart failure, is the most common interstitial abnormality encountered in daily practice. A diagnosis of interstitial pulmonary edema should be considered in all cases of interstitial abnormality detected on a chest radiograph. In many cases, it might be advisable to diurese the patient and repeat the chest radiograph as the first diagnostic test.

4

Name the most common interstitial abnormalities other than interstitial pulmonary edema.

Idiopathic pulmonary fibrosis and sarcoidosis are the most common chronic interstitial disorders in the United States and should be among the first diagnoses considered when encountering a chest radiograph with an interstitial abnormality.

5

What radiographic characteristics help determine the diagnosis of interstitial disorders?

Interstitial abnormalities may be roughly subdivided into abnormalities that produce small round opacities ( nodular interstitial diseases ) and abnormalities that produce small networks of holes ( reticular interstitial diseases ). In this chapter, we describe one nodular pattern of interstitial disease and the three following reticular patterns of interstitial disease: the peripheral reticular pattern, the linear pattern, and the cystic pattern.

6

What is the appearance of a nodular interstitial pattern on chest radiography?

A normal chest radiograph typically shows many small nodular opacities that represent normal blood vessels end on. In most cases, these small nodules can be recognized as blood vessels because they overlap with a small line of similar diameter, which represents an adjacent branch of the pulmonary vascular tree. The nodular pattern of interstitial lung disease appears as increased numbers of small nodular opacities (<10 mm in diameter) that are randomly distributed throughout the lung parenchyma. These small nodules do not overlap with the normal vascular lines of the lung ( Figures 19-2 and 19-3 ).

7

What disorders cause nodular interstitial diseases?

Four broad groups of disorders cause nodular interstitial diseases:

• Granulomatous lung diseases

• Nodular pneumoconioses

• Small metastases

• Smoking-related lung diseases

8

Discuss granulomatous lung diseases that cause nodular interstitial disease.

Sarcoidosis is the most common granulomatous interstitial lung disease to cause a micronodular pattern. This idiopathic disorder typically manifests in middle-aged individuals, especially African Americans. Miliary infections also cause a small nodular pattern and are typified by miliary tuberculosis but also include miliary spread of histoplasmosis, cryptococcosis, coccidioidomycosis, and blastomycosis. These infections typically affect immunocompromised individuals, such as patients with human immunodeficiency virus (HIV) infection, patients who have undergone organ transplantation, or patients with a history of long-term corticosteroid use. Extrinsic allergic alveolitis or hypersensitivity pneumonitis not only causes a granulomatous interstitial fibrosis, but may also produce an interstitial nodular pattern of lung disease.

9

Discuss pneumoconioses and tumors that lead to nodular interstitial lung disease.

Pneumoconioses that may produce a micronodular interstitial pattern are silicosis, coal workers' pneumoconiosis, talcosis, and berylliosis. Pneumoconioses are diffuse interstitial lung diseases caused by inorganic dusts, most often related to occupational exposures. Mining, sandblasting, gravestone engraving, and pottery are some occupations in which workers may be exposed to silica dust with resultant silicosis; as identified in the name, coal workers' pneumoconiosis is seen in coal miners. Berylliosis is an uncommon chronic pneumoconiosis that may be encountered in individuals who mine beryllium, who manufacture beryllium ceramics, or who previously manufactured beryllium lighting (these types of lights are no longer manufactured because of the high risk of acute and chronic berylliosis). Talcosis may occur as a result of the mining of talc or excessive inhalation of talcum powder and in intravenous drug abusers. Thyroid carcinoma is the prototypic tumor that produces thousands of tiny micronodular metastases and may appear as a nodular interstitial lung disease. Breast cancer may also produce this pattern of metastasis; other primary tumors rarely produce a micronodular pattern of lung metastasis.