Correct!
2. The thoracic CT shows multifocal ground-glass opacity associated with smooth interlobular septal thickening
Thoracic CT displayed in lung windows shows multifocal, bilateral, symmetric ground-glass opacity associated with fine linear and reticular abnormalities representing both intralobular interstitial thickening and smooth interlobular septal thickening. The smooth interlobular septal thickening is particularly evident in the upper lobes on images E-G, and in the lower lobes on images G-H. Also note the sharp, non-anatomic (i.e., not created by a fissure) demarcation between normal and abnormal lung, best illustrated on images C-E.
The pulmonary parenchyma affected by ground-glass opacity also shows numerous small linear opacities referred to as “intralobular lines” or “intralobular interstitial thickening.” Ground-glass opacity is defined as abnormal increased lung attenuation that does not obscure the underlying pulmonary vascular and bronchial margins. This definition contrasts with consolidation, which is defined on thoracic CT as a homogeneous increase in pulmonary parenchymal attenuation that results in obscuration of bronchial and vascular margins. Ground-glass opacity is present on the thoracic CT in this case, whereas consolidation is not. Within the areas of ground-glass opacity, numerous well-defined, smooth, “lines” are evident, creating a polygonal pattern; these lines reflect thickened interlobular septae and perilobular opacity [opacity within the airspaces in the periphery of the secondary pulmonary lobule, abutting the interlobular septae]. A small nodular pattern, often referred to as a “miliary” pattern, is not present. Finally, there is pronounced inhomogeneous lung attenuation- prominent areas of relatively “white” and “black” lung. As discussed above, the “whiter” areas- the areas of increased pulmonary parenchymal attenuation- reflect the presence of ground-glass opacity. The relatively “blacker” areas of lung merely reflect lung parenchyma spared by the pulmonary infiltration. Mosaic attenuation is a term used to indicate that the lung parenchyma shows areas of increased and decreased attenuation; it does not necessarily specify which pattern is abnormal. When ground-glass opacity is the cause of a mosaic pattern, as in this case, the areas of increased lung attenuation are abnormal. The areas of relatively decreased attenuation may simply reflect normal lung spared by the infiltrative process, or the areas of low attenuation may reflect decreased pulmonary perfusion- this latter situation is referred to as mosaic perfusion. Mosaic perfusion is typically the result of air trapping with associated hypoxic vasoconstriction, although it can also be caused by arterial obstruction, as may occur with pulmonary embolism. When air trapping is the cause of mosaic perfusion, often a number of secondary pulmonary lobules will show abnormally decreased attenuation [i.e., they will appear abnormally “black”]. This appearance is highly suggestive of small airway obstruction as a cause of a mosaic attenuation pattern and is often referred to as lobular low attenuation. While there are areas of relatively decreased attenuation on the thoracic CT in this case, these areas do not represent lobular low attenuation; rather, these areas are merely pulmonary parenchyma that is not affected by ground-glass opacity.
Regarding the assessment of the thoracic CT findings, which of the following is the term that is used to describe the lung parenchymal pattern?