Increased density: calcifications and calculi
A marked increase in density is most commonly associated with tissue calcification or the formation of calculi.
In pulmonary alveolar microlithiasis (alveolar microlithiasis), small stones are detected only on overpenetrated (hard) radiographs. On CT, pronounced calcification is visible in approximately half of the cases, creating a pattern of dense parenchymal opacification (dense parenchymal opacification). Due to the high radiodensity of the lesions, the contours of the heart and diaphragm may merge with the surrounding tissue and become non-visualized (the "silhouette" sign).
In the pleura, high-density areas are formed by pleural plaques, which are focal fibrosis induced by asbestos exposure. On plain radiographs, calcified plaques in the posteroanterior projection produce irregular, dense shadows (the "holly leaf" sign — 'holly leaf' calcification). CT easily distinguishes them from adipose tissue precisely because of their high attenuation values. Artificial high-density inclusions are also left by talc following pleurodesis (forming the "sandwich" sign on CT).
Intermediate density: transudate, exudate, and inflammation
Fluid accumulation or cellular infiltration forms zones of local or diffuse opacification of varying intensity.
- Ground-glass opacity pattern (ground-glass opacification): reflects partial filling of alveoli or thickening of the interstitium, in which the vascular markings remain visible. This pattern is characteristic of microlithiasis, pulmonary alveolar edema, and also forms the "halo sign" (halo sign) around the area of necrosis in invasive pulmonary aspergillosis (invasive pulmonary aspergillosis).
- Consolidation: denser opacification of air spaces that obscures underlying vessels. It is observed as bilateral perihilar opacification ("bat-wing" appearance) in cardiogenic edema, as well as in fungal hemorrhagic pulmonary infarcts.
- Mosaic attenuation (mosaic attenuation): an alternation of areas of normal, increased, and decreased density on CT. This pattern, combined with bronchial wall thickening and mucous plugging, is typical of the radiological presentation of cystic fibrosis (cystic fibrosis).
- Micronodular pattern: miliary increase in density resulting, for example, from hemosiderin deposits in chronic pulmonary venous hypertension (often in the setting of mitral stenosis).
Decreased density: air trapping and destruction
A decrease in radiodensity indicates pathological air trapping (air traps) or destruction of lung tissue.
- Hyperinflation (increased aeration) acts as a background in bronchiectasis among patients with cystic fibrosis and is a classic sign of early-stage Pneumocystis pneumonia (Pneumocystis jiroveci).
- Areas of localized decreased density are represented by apical blebs (apical blebs), small subpleural cysts, and paraseptal emphysema (black subpleural band in microlithiasis).
- Cavitation (cavity formation): manifests as nodules with breakdown. A striking example is the air crescent sign in pulmonary aspergillosis, where a crescent-shaped layer of air forms inside a dense focus of necrosis. Cavitating nodules (cavitating nodule) are also seen in infectious exacerbations of cystic fibrosis.
Functional density on PET/CT
Fluorodeoxyglucose (FDG) PET/CT allows the assessment of tissue metabolic activity rather than physical density. Increased FDG uptake helps differentiate benign pleural changes from malignant ones (e.g., in mesothelioma).
However, the intensity of tracer uptake depends on the histologic subtype: sarcomatoid mesothelioma accumulates FDG more actively than epithelioid mesothelioma. A decrease in metabolic activity (reduced FDG uptake) after chemotherapy is a predictor of good treatment response and increased survival.
To memorize the evolution of densities in invasive aspergillosis, use the rule: "First the HALO (ground-glass opacity around the infarction due to hemorrhage), then the CRESCENT (air against the background of necrosis and cavitation)."
Class and exam questions
Why might the imaging appearance lead to an underestimation of pulmonary edema severity in extensive emphysema?
Due to prior alveolar destruction in emphysema, fluid cannot form a classic homogeneous airspace consolidation. Even in severe pulmonary venous hypertension, transudate remains in the interstitium, which looks less threatening on a radiograph.
How to differentiate pleural plaques from extrapleural fat on CT?
Pleural plaques have high tissue (fibrosis) or bone (upon calcification) density. In obese patients, they are clearly separated by a thin layer of fat, which has negative density values.
What can cause a false-positive result on pleural PET/CT?
Acute or chronic inflammatory processes. For example, talc pleurodesis causes intense FDG uptake in the pleura, which correlates with areas of high density on CT and can persist for several years.
What else to study on this topic
- Contrast-enhanced MRI and diffusion-weighted imaging (DWI) in the diagnosis of mesothelioma.
- Differential diagnosis of non-cardiogenic and cardiogenic pulmonary edema.
- CT criteria for diffuse involvement of the visceral and parietal pleura.
- Diagnostic imaging of pulmonary arterial hypertension (CT angiography and HRCT).
- Lymphocytic interstitial pneumonia as a manifestation of immunodeficiency.
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