Pan et al. investigated the time course of lung changes during recovery from COVID-19 pneumonia. The results showed that with the evolution of the disease, GGO would enlarge with a crazy paving pattern and partial consolidation. Lung involvement reached a maximum at about the 10th day after the onset, and the crazy-paving pattern was resolved entirely in the absorption stage about the 14th day after the disease onset, which means that the crazy-paving pattern may become an important indicator for evaluation of disease evolution.
Dynamic changes on chest CT
Chest CT can also evaluate the time course of COVID-19 and assess the evolution of disease severity. Chung et al. evaluated each of the five lung lobes and determined the degree of involvement and rated the severity of each lobe. The patients who had the highest score were admitted to the intensive care unit with multiple bilateral GGOs and subsegmental consolidation.
Critical stage disease may show further expansion of consolidations, with the whole lung showing increased opacity, a so-called “white lung”. In recovery stage patients, the lesion area is reduced and absorbed, or only residual fibrous lesions are seen. Less common findings include pleural effusion, stretch bronchiectasis, and thoracic lymphadenopathy.
The lesion range can be acinar, leaflet, lung segment or lung lobe. Lesions are distributed as bilateral asymmetrical subpleural wedge-shaped or fan-shaped, mostly in the caudal lesions of the lung and dorsal subpleural area. When interlobular septa and intralobular interstitium are thickened, this is displayed as subpleural reticulation according to a “crazy paving” pattern. The underlying pathological changes are pulmonary edema, protein exudation, thickening of intralobular interstitial, and multi-nucleated giant cells and macrophage infiltration in the alveolar cavity.
Disease progression mostly occurs in course of 7-10 days, with enlarged and increased density of GGO, and consolidated lessions with air bronchogram sign. GGOs from the early stage expand, and the density increases gradually. Consolidation shows increased lung parenchimal density, and blurred borders of the pulmonary blood vessels and air way walls due to the loss of air contrast.
Typical features on chest CT
Typical features on chest CT include ground-glass opacity with a cloud-like change on chest CT, not obscuring the accompanying pulmonary vascular structures in lungs. Corresponding pathological changes are exudation in the alveolar cavity with partial filling of the air cavity, mainly from serous and fibrinous exudates, alveolar septal vascular congestion and edema.
Typical features on chest X-Ray
Multiple small patchy shadows and interstitial changes in the lower zone of lungs are typical imaging features in common type patients. With the progression of the disease, it may manifest as a patchy distribution or multiple consolidations. In severe and critical type COVID-19 pneunomia, multifocal or diffuse consolidation can be seen in both lungs, showing as “white lung”. Chest X-ray is only suitable for primary hospitals without examination conditions and follow up of critically ill patients.
It is recommended to use volume CT with a maximum acquisition slice thickness of 5mm (>16 slice multi-detector CT) and a reconstruction slice thickness of 1.0-1.5mm. Multi-planar reformats (transverse, sagittal and coronal planes) are beneficial for the early detection of lesions in patients with negative nucleic acid tests.
The sensitivity and specificity chest X-ray for mild type patients are relatively low. It is not recommended for patients with early COVID-19 stage. Chest CT scans can demonstrate almost all abnormalities including early mild exudative lesions. Chest CT is the most valuable imaging tool for the clinical diagnosis of early stage COVID-19 pneumonia when patient’s symptoms are aspecific, especially in Wuhan with unsufficient PCR tests in the early pandemic period.
Recommendation for chest imaging techniques
Imaging, in particular chest X-ray and computed tomography (CT), is important in the detection of lung lesions, assessment of lesion size, density, and evolution. Chest X-ray examination is convenient and fast, and has been proven effective in diagnosing other coronaviruses, such as severe acute respiratory syndrome(SARS) and Middle East respiratory syndrome (MERS).