It should also be noted that, in individual cases, HRCT scan findings may be sufficiently characteristic, especially when coupled with close clinical correlation, to obviate the need to perform a biopsy. Patients with classic HRCT scan findings of sarcoidosis, as well as those with subacute HP for which biopsy confirmation may not be required, should be included in this category. In patients with a known history of smoking, the finding of scattered, tiny, ill-defined centrilobular upper lobe nodules is sufficiently characteristic to warrant a clinical diagnosis of RB, obviating the need for more invasive diagnostic procedures, while the finding of scattered centrilobular opacities associated with bizarrely shaped cysts predominantly involving the upper lobes sparing the lung bases is characteristic of LCH. Similarly, the finding of characteristic tree-in-bud opacities in the appropriate clinical setting may be taken as diagnostic of small airway-bronchiolar infection. In distinction, the appearance of diffuse, poorly defined centrilobular nodules in the absence of a clinical history of established antigen exposure, infection, or a history of smoking generally requires open-lung biopsy for definitive evaluation. Given the wide diversity of potential causes for this appearance, the use of an HRCT scan algorithm should be considered an important, if not fundamental, component of clinical assessment in these cases.