Air bubble sign refers to a small air-containing space in the lung (Fig. 5a), which might be the pathological dilation of a physiological space or a cross section of the bronchiolectasis, or associated with the process of consolidation resorption. Shi et al termed this sign as round cystic change in their study [18], while Kong et al reported it as cavity sign [32]. However, given the displayed pictures in their studies and the definition of cyst and cavity [26], we believe it may be more appropriate to term this small bubble-like air-containing space as air bubble sign.
Fig. 5 a A 49-year-old male COVID-19 patient presenting fever with diarrhea for 3 days. CT scan shows a patchy GGO (red frame) with an air bubble sign (white arrow) in the apicoposterior segment of the upper left lobe. b A 76-year-old female COVID-19 patient presenting fever with cough for 4 days. CT scan shows an irregular nodule (white arrow) in the posterior segment of the right upper lobe. c A 46-year-old male COVID-19 patient presenting fever with dry cough for 5 days. CT scan shows a solid nodule surrounded by a ground glass halo in the lateral segment of the right middle lobe (red frame). d A 66-year-old woman confirmed with COVID-19 presenting fever and myalgia for 7 days. CT scan shows a reversed halo sign (red frame) in the posterior basal segment of the right lower lobe