Table 4.

Stages of COVID-19 infection based on CT images³³

Ultra-early	Refers to the stage without clinical manifestations, patients are often asymptomatic, but normally is seen 1-2 weeks after COVID-19 exposure. Main imaging manifestations are single or multifocal GGO, patchy consolidative opacities, pulmonary nodules encircled by GGO and air bronchograms.
Early	Refers to an early symptomatic presentation, 1-3 days after clinical manifestations. This is the most observed stage through radiological imaging (54%). Main imaging manifestations are single or double GGO combined with interlobular septal thickening. Pathological processes during this stage are dilatation and congestion of the alveolar septal capillary, interlobular interstitial edema and exudation of fluid in alveolar cavity.
Rapid progression	Refers to 3-7 days after clinical manifestations, where the pathological features of this stage are accumulation of exudates in the alveolar cavity, vascular expansion and exudation in the interstitium. These pathological features lead to the further aggravation of alveolar and interstitial edema. Fibrous exudation forms bonds between each alveolus through the interstitial space to form a fusion state. Main radiological manifestations are large, light consolidative opacities with air bronchograms.
Consolidation	Refers to the second week after initial symptomatic presentation. Main pathological features are fibrous exudation of the alveolar cavity and the reduction of capillary congestion. CT imaging can show multiple consolidations reducing in size and density, compared to before.
Dissipation	Refers to 2-3 weeks after onset of symptoms. CT imaging can show dispersed, patchy consolidative opacities, reticular opacities, bronchial wall thickening and interlobular septal thickening.

GGO, ground glass opacities.