The aurora sign: an ultrasonographic sign suggesting parenchymal lung disease

The objective of this study was to clarify the cause and clinical significance of a large number of ring-down artefact (RA) observed on the dorsal side of the right hepatic lobe on abdominal ultrasound (US). 2000 abdominal US examinations were evaluated to investigate the frequency and number of RA behind the right lobe of the liver. In this study, RA observed by subcostal or intercostal US were described as the aurora sign when they were numerous. US findings were correlated with high resolution CT or three-dimensional CT of the right lung base. Experimental study was also performed to investigate the mechanism of the aurora sign. The results were as follows. (1) Aurora sign was noted in 43 patients. In 37 of these 43 patients (86.0%), chest CT or plain radiography revealed diffuse interstitial changes in the right lower lung field. Three-dimensional CT of the lung and the experimental model revealed that the aurora sign derived from the irregularity of air spaces immediately below the pleura associated with interstitial pulmonary disorders. (2) One or more bands of RA were observed in 907 of the 2000 patients (45.4%). Of 177 patients with fatty liver, RA was observed in 14 (7.9%), while of the 1823 patients with no fatty liver, RA was observed in 893 (49.0%). The frequency of RA was significantly different (p<0.001) between the patients with and without fatty liver. In conclusion, parenchymal lung disease should be suspected when the aurora sign is noted on the dorsal side of the right hepatic lobe. However, RAs are rare in patients with fatty liver.