The pulmonologist and radiologist commonly recognize a variety of abnormal patterns of diffuse parenchymal lung disease on the conventional chest radiograph. Identification of these patterns, along with recognition of other associated findings, can be extremely useful in guiding the development of a differential diagnosis . The diagnostic approach to diffuse lung disease based on interpretation of the conventional chest radiograph will be discussed here, using examples to illustrate many of the radiographic features.
CRITIQUE OF PATTERN USE
The traditional approach to radiographic assessment of diffuse lung disease first involves determining whether the pulmonary parenchymal process is located within the interstitium or the alveolar spaces. However, although radiographic criteria for both types of processes have been established over the years, the correlation is relatively poor between the accuracy of the radiologic localization (to either the airspaces or the interstitium) and the actual pathologic findings. Specific issues include the following:
- Nodular patterns can be produced by either interstitial or alveolar disease.
- Interstitial pneumonias usually also involve the alveolar compartment.
- So-called alveolar disease regularly involves the interstitium as well. The paradigm of pure alveolar disease is pulmonary alveolar proteinosis, yet high resolution computed tomographic (HRCT) scanning has shown that, even in this entity, the interlobular and intralobular septa are thickened, forming the “crazy paving” pattern on thin-section CT.
- Air bronchograms, the ultimate radiographic sign of air space disease, can be detected in a small percentage of cases of histologically pure or predominant interstitial lung disease, such as sarcoidosis, pulmonary lymphoma, and pulmonary calcinosis.
- Ground glass opacities are formed by either alveolar or interstitial disease.
Because of these limitations, a more descriptive approach has been promulgated by radiologists. This approach takes into account an analysis of predominant opacities, an analysis of lung volumes, distribution of disease, and the presence of associated findings [2-4] (table 1).
An attempt to avoid the usage of histologic terminology was made by the International Labor Office (ILO) in graphically describing the radiographic findings of pneumoconioses. The ILO classification system uses standard chest radiographs, graphic rather than anatomic descriptors, and a semiquantitative scheme for profusion of opacities (based on a 12-point scale) . This scheme has an acceptable interobserver variability.