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Background
Centrilobular nodules are frequently encountered on computed tomography (CT) of the chest. They can be seen in a variety of settings including as an incidental finding in an asymptomatic patient or a critically ill patients in the hospital. They are the result of disease processes involving the small airways or due to vascular causes. Unlike hematogeneous or perilymphatic nodules, centrilobular nodules typically spare the pleural surfaces and fissures. Their distribution can be focal, multifocal/patchy, or diffuse, and they can also be further characterized by their density such as solid or ground glass. Certain appearances on CT, such as tree-in-bud, clustered small nodules, or branching densities, jacks (those used in the childhood game), V- or Y-shaped densities, typically represent mucus-impacted small airways due to small airway infection, which is, by far, the most common etiology. Alternatively, diffuse centrilobular ground glass nodules are generally seen in the setting of hypersensitivity pneumonitis or smoking-related interstitial lung disease. However, it is important to be aware of other less frequently encountered entities that can also manifest as centrilobular nodules.

Educational Goals / Teaching Points
This exhibit will outline the anatomic distribution of these nodules within the secondary pulmonary lobule, provide a systematic approach to arriving at a differential diagnosis for centrilobular nodules at imaging, and will review both common and uncommon entities that may result in this finding.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
This exhibit will review the structure of the secondary pulmonary lobule as well as an anatomic approach to multinodular disease on high-resolution chest CT and how to decide if one is viewing a case of centrilobular nodules. Common and uncommon causes of centrilobular nodules will be discussed. The disease processes will be illustrated with both CT images as well as pathology photomicrographs.

Conclusion
It is important for the radiologist to be familiar with the pattern of centrilobular nodules when interpreting chest CTs and be aware of ancillary clues that may point to a specific diagnosis particularly when coupled with the clinical history. In cases where there may be an overlap of imaging findings, the radiologist should also know when to suggest an appropriate differential diagnosis, helping guide further workup in these patients.