Air-Crescent Sign

Definition

An air crescent is a collection of air in a crescentic shape that separates the wall of a cavity from an inner mass [1, 2] (Fig. 14.1).

Diseases Causing the Sign

The air-crescent sign is often considered characteristic of either

Aspergillus colonization of preexisting cavities (aspergilloma) (Figs. 14.1 and 14.2) or retraction of infracted lung in angioinvasive pulmonary aspergillosis. Other less common causes include tuberculosis, Rasmussen’s aneurysm (Fig. 14.3), ANCA-associated granulomatous vasculitis (former Wegner’s granulomatosis), complicated hydatid disease, hematoma, lung abscess, and necrotic lung cancer (Table 14.1).

Pneumocystis jirovecii infection. The air-crescent sign in invasive pulmonary aspergillosis is observed in up to 63 % of patients. The sign, occurring in about half of the patients with recovery from neutropenia, is caused by tissue ischemia and subsequent necrosis due to fungal angioinvasion [4].

Rasmussen’s aneurysm occurs in chronic cavitary tuberculosis with 5 % of prevalence. Hemoptysis is the usual presenting symptom, which may be massive and life-threatening. Generalized symptoms and signs, such as diffuse alveolar hemorrhage, acute glomerulonephritis, chronic refractory sinusitis or rhinorrhea, imaging findings of nodules or cavities, and multisystemic disease, precede typical imaging findings in pulmonary vasculitis [5]. Lung abscess may result from aspiration or systemic spread of infection (septic embolism). Common causes of lung abscess include anaerobic bacteria, Staphylococcus aureus, Pseudomonas aeruginosa, and Klebsiella pneumoniae.

Distribution

Air-crescent sign in aspergilloma and tuberculosis usually occurs in the upper lobe. Rasmussen’s aneurysms are usually distributed peripherally and beyond the branches of main pulmonary arteries [3]. Lung abscesses occur most commonly in the posterior segment of an upper lobe or the superior segment of a lower lobe. Hydatid cysts are usually located in the lower lobes.

Clinical Considerations

Normal host immunity and the long period, often years, required for the formation of aspergilloma, can aid in distinguishing this condition from invasive aspergillosis. The most common underlying causes of aspergilloma are tuberculosis and sarcoidosis. Other conditions that occasionally may be associated with aspergilloma include bronchogenic cyst, pulmonary sequestration, and pneumatoceles secondary to

Key Points for Differential Diagnosis

1.Whether a mural nodule within a cavitary lesion is contrast enhanced or not is one of the most important features in making a differential diagnosis between an intracavitary aspergilloma and a cavitary lung cancer [6]. The aspergilloma usually moves when the patient change position.

2.Air-crescent sign in hydatid disease occurs when there is communication between the cyst and the airways. Air may enter the space between the pericyst and exocyst and produce a thin crescent of air around the periphery of the cyst. When cyst has ruptured into the bronchial tree, the collapsed endo- cyst–exocyst may be contrasted with surrounding air, resulting in the water-lily sign [7]. The cysts have homogeneous water density on CT [8].

3.A focal dilatation of one of the pulmonary segmental arteries present in the vicinity of a tuberculous cavity is almost a pathognomonic finding of Rasmussen’s aneurysm.