Basic HRCT Patterns: Ground-glass Opacity

Definition: Ground-glass refers to the HRCT appearance of a hazy opacity that does not obscure the associated pulmonary vessels. This appearance results from parenchymal abnormalities that are below the spatial resolution of HRCT. Ground-glass opacity can be seen with alveolar wall inflammation or thickening, with partial air-space filling, or with some combination of the two.

How to Recognize Ground-glass Opacity

This image shows patchy ground-glass opacities throughout both lungs. Note that in the regions of ground-glass, one can see the vessels.

This patient had influenzal pneumonia.

Common causes (some of which are not routinely imaged by HRCT) include pulmonary edema; ARDS; viral, mycoplasmal, and pneumocystis pneumonias; hypersensitivity pneumonia; pulmonary hemorrhage; and other diffuse interstitial lung diseases.

Many air-space filling processes can also result in consolidation in advanced disease.

 

HRCT after Recovery

A scan performed after recovery from the pneumonia shows a marked decrease in ground-glass opacity. This image is taken at the same level as that shown above.

Some of the posterior increased density represents normal dependent lung.

 

Gross Appearance

This slice of lung from a patient who died of influenza pneumonia was heavier and redder than normal and uniformly firm throughout. Large airways were patent.

Although the appearance is often uniform at postmortem examination, diseases that have ground-glass opacity on HRCT begin as patchy opacities that become confluent. The terminal appearance resembles the red hepatization of bacterial lobar pneumonia, but the histologic picture differs from the neutrophilic alveolar filling of bacterial pneumonia.

 

 

Histologic Findings in Influenza Pneumonia

Alveolar walls are thickened by edema and RBCs. Alveolar epithelial cells degenerate, and coagulated proteinaceous exudate produces hyaline membranes. With time the membranes are replaced by hyperplastic type II cells, which then differentiate into type I cells when resolution occurs. Alveolar spaces contain a proteinaceous exudate and a few inflammatory cells. The alveolar wall thickening and alveolar exudate still mixed with air cause the ground-glass opacity on HRCT. Loss of air produces consolidation.

 

Find an example of hyaline membranes

Find 2 examples of type II cell hyperplasia along thickened alveolar walls.

Find an example of protein-rich edema fluid in an air space.

hyaltype2type2aedema

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Hyaline membranes

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Type II cell hyperplasia

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Protein-rich edema fluid

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