Radiology 2002; 225:639-653.
Hansell DM.
Diseases that primarily affect the small vessels of the lung are difficult to diagnose. Many conditions are characterized by involvement of small pulmonary vessels, and pathologically they can be conveniently divided into occluding and inflammatory types. The former, typified by chronic pulmonary thromboembolism and primary pulmonary hypertension, are relatively cryptic in terms of imaging. In contrast, inflammatory vasculitides, which often cause pulmonary hemorrhage and infarction, result in florid but nonspecific radiographic abnormalities. The spectrum of thin-section computed tomographic abnormalities encountered in the inflammatory vasculitides is wide: For example, in Wegener granulomatosis the pattern ranges from cavitating nodules to lobar consolidation to ground-glass opacity. This review highlights some of the less obvious imaging manifestations of occlusive and inflammatory diseases of the small pulmonary vessels.
The imaging, and understanding, of conditions that affect the small vessels of the lung is a challenge. Many of the difficulties encountered in considering diseases of the small vessels are mirrored in small-airways diseases: Problems include the definition of anatomic criteria for what constitutes a "small vessel," the utility of clinical versus pathologic classification, the nonspecific and often late clinical manifestation of such diseases, the continuum between small- and large-vessel involvement, and, finally, the lack of specific radiographic findings in most of these diseases.
Although some diseases affecting the small vessels elude a precise clinical or histologic diagnosis, for reasons explored later, an increasing array of thin-section computed tomographic (CT) features can be used to corroborate small-vessel disease as the dominant pathologic process. While acknowledging the less-than-definitive diagnostic role of CT in diseases of the small vessels of the lung, this review will include discussion of those conditions in which an understanding of the pathology can be usefully brought to bear on the interpretation of thin-section CT findings in this difficult group of disorders.