Comet Tail Sign

Definition

The sign has been originally described on chest radiographs. It is formed by distorted blood vessel(s) and a focal area of atelectatic lung adjacent to pleural thickening. The vessel(s) are seen as curvilinear soft tissue density or densities, extending from the medial margin of the atelectatic lung to the pulmonary hilum. When the vessels are multiple thus there are multiple tails, the vessels are more likely parachute; then, parachute sign may be used (Fig. 5.1). The sign is due to contracted Þbrous scarring and shrinking pleural disease with rounded atelectasis. Likewise, on CT, swirling of bronchi and vessels extending from the hilum and converging on the atelectatic lung help produce comet tail appearance [1] (Figs. 5.1 and 5.2).

Diseases Causing the Sign

The CT comet tail sign is speciÞcally observed in rounded atelectasis.

Distribution

The comet tail sign, which is identiÞed in rounded atelectasis, is usually seen in the posterior aspect of the lower lung zones.

Clinical Considerations

Rounded atelectasis is seen in patients with asbestos exposure [2].

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Rounded Atelectasis

Pathology and Pathogenesis

Rounded atelectasis (RA) is a focal, pleural-based lesion resulting from pleural and subpleural scarring and atelectasis of the adjacent lung tissue. In cases reported in literatures, 60Ð70 % of patients with RA had been exposed to asbestos [3].

Symptoms and Signs

Most patients with RA are asymptomatic and the lesions are incidentally founded on chest radiographs, simulating a lung mass. Respiratory symptoms include dyspnea, cough, and chest pain in the order of frequency [4].

CT Findings

CT Þndings of RA are of a round or oval mass abutting a pleural surface, with swirling of the bronchi and vessels extending from the hilum and converging on the mass, producing a comet tail appearance, and with adjacent pleural thickening [5, 6] (Figs. 5.1 and 5.2). The central aspect of the mass usually has indistinct margins as a result of blurring by the entering vessels. Volume loss of the affected lobe is usually present. Air bronchograms are seen within the mass in about 60 % of cases. The atelectatic lung typically enhances signiÞcantly after injection of contrast material (Figs. 5.1 and 5.2). Synonyms include folded lung syndrome, helical atelectasis, Blesovsky syndrome, pleural pseudotumor, and pleuroma [7].

Fig. 5.1 Rounded atelectasis in a 66-year-old man complaining of shortness of breath and cough. (a, b) Consecutive thin-section (1.5-mm section thickness) CT scans obtained at ventricular level show the atelectatic right lower lobe (arrows) abutting pleura. Also note two vascular curvilinear soft tissue attenuations (arrowheads), extending from anterior margin of the atelectatic lung to pulmonary hilum; these two structures may help explain parachute sign (please refer to deÞnition of comet tail sign) on chest radiograph in this particular condition (rounded atelectasis). Also note right pleural effusion and thickening, posterior to

the atelectatic lung, and extrapleural subcostal fat accumulation. (c) Mediastinal window image demonstrates the enhancing atelectatic lung (arrow) abutting pleura. (d) Low-magniÞcation (×10) photomicrograph of pathologic specimen obtained from a different patient with rounded atelectasis depicts enfolded visceral pleura (arrow) curving into a rounded atelectatic lung that gestures wings (open arrows) of a vane. (e) High-magniÞcation (×100) photomicrograph discloses more clearly enfolded visceral pleura (arrow) and rounded atelectatic lung (open arrows)

Fig. 5.2 Rounded atelectasis simulating a lung mass in a 63-year-old man complaining of mild dyspnea and cough. (a) Chest radiograph shows right pleural effusion and a mass (arrow) in right middle lung zone superior to right minor Þssure (arrowheads). (b, c) Thin-section (1.5-mm section thickness) CT scans obtained at levels of the right main bronchus (b) and bronchus intermedius (c), respectively, show the atelectatic right upper lobe (arrows) abutting anterior pleura. Also note

vascular curvilinear soft tissue attenuations (arrowheads), extending from posterior margin of the atelectatic lung to pulmonary hilum. Also note right pleural effusion and thickening in posterior pleura (open arrows). (d) Mediastinal window of CT scan obtained at same level to

(c) demonstrates the atelectatic right upper lobe (arrow). Also note right pleural effusion and thickening