For the purposes of this report, multinodular disease will be defined in a patient in which there are too many nodules to easily count on routine CT scan studies, with most of these nodules measuring < 1 cm in diameter. While the most common cause of multiple pulmonary nodules is metastatic disease, it is apparent that this definition encompasses a wide range of lung diseases, both benign and malignant. It is our contention that use of a dedicated diagnostic algorithm based on characteristic high-resolution CT (HRCT) scan features coupled with clinical findings can provide either a specific diagnosis or a markedly shortened list of differential diagnoses in a majority of patients presenting with diffuse lung nodules.
Due to its ability to evaluate the lung parenchyma in cross-section, eliminating the superimposition of densities, CT scanning offers a unique opportunity to evaluate lung nodules in exquisite detail. This includes first the ability to assess lesions by anatomic distribution, and second by morphology.
This includes the consideration of the following patterns: diffuse vs focal or clustered; central (peri-bronchovascular) vs peripheral (subpleural or peri-fissural); and upper vs lower lung distribution. Most importantly, nodules also need to be characterized by their relation to secondary lobular anatomy allowing a distinction between centrilobular nodules and those that predominantly involve the lobular periphery, including the interlobular septa (Fig 1).
For example, diseases such as sarcoidosis that localize within or adjacent to lymphatics predominate in those regions in which lymphatics are most extensive, specifically along the pleural and fissural surfaces, within the interlobular septae, and along the peribronchovascular axial interstitium (Fig 2). http://asthma-inhalers-online.com/buy-generic-ventolin-online.html
Figure 1. Secondary lobular anatomy. A side-by-side diagrammatic representation of two normal secondary pulmonary lobules. Secondary lobules represent fundamental anatomic units of the lung and are defined by centrilobular structures, including pulmonary arteries/arterioles and their accompanying bronchi/ bronchioles, and peripheral structures, including the pulmonary veins and lymphatics within the interlobular septae. As shown, most of these structures are < 1 mm in size and therefore, with the exception of the centrilobular arteries, lie below the resolution of even HRCT scans. Most importantly, note that centri-lobular structures do not extend to the pleural or interlobular septal surfaces. As will be illustrated, knowledge of basic lobular anatomy is the key to differentiating between different etiologies of diffuse pulmonary nodules.
Figure 2. Perilymphatic disease. A diagrammatic representation of the characteristic distribution of lung nodules in patients with perilymphatic disease. Note that nodules are preferentially sub-pleural, peribronchovascular within the axial interstitium, or along lobular septae. While this appearance is especially characteristic of nodular sarcoidosis, less commonly a similar pattern may also be seen in patients with silicosis or coal-workers pneumoconiosis.