Radiographic findings: A plain radiograph was normal. A CT scan (A) shows an inspiratory view at the level of the aortic arch. Heterogeneous ground-glass opacities and ill-defined centrilobular nodules on this image are most prominent in the posterior part of the right lung. The circled area shows the nodules especially well. The arrows indicate examples of ground-glass opacity.

A

Later, CTs were made during inspiration (B) and expiration (C). Compare vessels and parenchyma in the expiratory CT with those in the inspiratory CT. Answer

B

Expiratory scan (C)

CD

Image D shows a slice of head-cheese, which bears some resemblance to the areas of mixed density (attenuation) seen in the expiratory CT of the right lower lobe [1].

Discussion: A mosaic pattern of inhomogeneous attenuation consisting of light and dark patches on HRCT may result from air trapping, vascular obstruction, infiltrative lung disease, or some combination. An expiratory CT can help to decide which category is present [2,3]. The degree of attenuation, the change in attenuation on expiration, and vessel sizes in areas of different attenuation are used in the assessment.

Diagram: The diagrams in panels A and B depict air trapping (dark areas) in otherwise normal lung (lighter areas). The attenuation of the normal lung increases during expiration (the light areas get lighter), but the areas of decreased attenuation do not (the dark areas stay dark). Note that the vessels in the normal lung have a larger size compared to the physiologically narrowed vessels in the low attenuation areas. This pattern represents the mosaic perfusion associated with air trapping.

Panel C depicts the complex HRCT appearance of abnormal alveolar infiltrates combined with air trapping. The dark areas are due to air trapping (with small vessels), the lightest areas are due to alveolar ground-glass infiltrates, and the light regions are normal lung. As noted above, this pattern has been likened to head-cheese [1].

References

1. Chung M, Edinburg K, Webb E, Webb W. Mixed infiltrative and obstructive disease on HRCT: differential diagnosis and functional correlates (abst). Thoracic Imaging 1998, The Society of Thoracic Radiology; 78.

2. Arakawa H, Webb W. Expiratory high-resolution CT scan. Radiol Clin N Am 1998; 36:189-209.

3. Arakawa H, Webb W, McCowin M, Katsou G, Lee K-N, Seitz R. Inhomogeneous lung attenuation at thin-section CT: diagnostic value of expiratory scans. Radiology 1998; 206:89-94.

Clinical summaryImage 1

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Answer: The parenchyma increases in density (attenuation) in a ground-glass pattern on the expiration view except in focal areas where it remains lucent. On the expiration image, the vessels in the lucent areas are smaller than the vessels surrounded by ground-glass. This pattern is typical of air-trapping, in which expiratory images show that the parenchyma without air-trapping increases in density, and the parenchyma with air-trapping remains lucent. The vessels associated with air-trapping are physiologically constricted and therefore appear smaller.

When abnormal, heterogeneous, ground-glass alveolar infiltrates are seen in lung parenchyma that also demonstrates variable degrees of air-trapping, the images show a pattern of multiple different densities, which has been likened to head-cheese (D) [1].

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