Ground-glass opacity (GGO): a review of the differential diagnosis in the era of COVID-19

Abstract

Thoracic imaging is fundamental in the diagnostic route of Coronavirus disease 2019 (COVID-19) especially in patients admitted to hospitals. In particular, chest computed tomography (CT) has a key role in identifying the typical features of the infection. Ground-glass opacities (GGO) are one of the main CT findings, but their presence is not specific for this viral pneumonia. In fact, GGO is a radiological sign of different pathologies with both acute and subacute/chronic clinical manifestations. In the evaluation of a subject with focal or diffuse GGO, the radiologist has to know the patient's medical history to obtain a valid diagnostic hypothesis. The authors describe the various CT appearance of GGO, related to the onset of symptoms, focusing also on the ancillary signs that can help radiologist to obtain a correct and prompt diagnosis.

Keywords: COVID-19; Computed tomography; Coronavirus; Differential diagnosis; Ground-glass opacity.

Fig. 1

Algorithmic approach to ground-glass opacities. It is important to exclude causes of false GGO (such as in expiratory scan or in case of mosaic perfusion). GGO ground-glass opacity, DAD diffuse alveolar damage, HP hypersensitivity pneumonitis, NSIP non-specific interstitial pneumonia, UIP usual interstitial pneumonia, DIP desquamative interstitial pneumonia, LIP lymphocytic interstitial pneumonia, OP organizing pneumonia, CMV cytomegalovirus, PJP pneumocystis jirovecii pneumonia, ARDS acute respiratory distress syndrome

Fig. 2

COVID-19 pneumonia. Four cases of GGO in COVID-19 lung involvement in the acute—subacute phase: alterations are mostly peripheral, bilateral, and basal (a–c). In d, there are halo signs of GGO around the consolidations. COVID-19 coronavirus disease 2019

Fig. 3

Infections. a is a case of early CMV infection with focal GGO in the left upper lobe. b is a case of EBV pneumonia with mild bilateral GGOs. Figure in c is a case of pneumonitis caused by Influenza A: HRCT pattern is very similar to COVID-19 pneumonia and differential diagnosis is tricky. Case in d is a PJP with typical crazy-paving appearance and multiple cysts within the lung. e, f are two cases of H1N1 pneumonia with diffuse lung involvement and patient in f then evolves in ARDS (note the diffuse bronchiectasis and bilateral lung involvement). CMV cytomegalovirus, EBV Epstein–Barr virus, COVID-19 coronavirus disease 2019, PJP pneumocystis jirovecii pneumonia, ARDS acute respiratory distress syndrome

Fig. 4

Hemorrhage. The first case in a is a bilateral alveolar hemorrhage with diffuse perihilar GGOs. Case in b is a focal parenchymal hemorrhage after cryobiopsy of lung lesion. Hemorrhage with GGO as halo sign in patient with ANCA + vasculitis (c). Finally, d is a case of “crack-lung”, diffuse hemorrhage after inhalation of cocaine. ANCA anti-neutrophil cytoplasmic antibody

Fig. 5

Pulmonary edema. a, b show two cases of cardiogenic edema: note the pleural effusions, the diffuse and bilateral GGOs with smooth thickening of interlobular septa. c is a case of acute edema caused by water aspiration, with diffuse centrilobular nodules surrounded by GGOs, with lung alterations involving mostly the lower posterior lobes

Fig. 6

Acute exacerbation of interstitial lung disease. a is an acute exacerbation of IPF, with a super-imposed bacterial infection. Patient in b shows diffuse lung involvement in GVHD after lung transplant in acute lymphatic leukemia. Patient in c has an exacerbation of NSIP pattern in antiphospholipid antibody syndrome. IPF idiopathic pulmonary fibrosis, GVHD graft versus-host disease, NSIP non-specific interstitial pneumonia

Fig. 7

Interstitial lung disease. NSIP pattern in rheumatoid arthritis with typical GGOs within reticular interstitial involvement, with focal areas of subpleural sparing (a). Another case of NSIP in SSc lung involvement (b). c is an example of ILA with tiny subpleural GGOs involving more than 5% of each lobe (arrows). Patient in d is a case of eosinophilic pneumonia in Churg–Strauss syndrome. NSIP non-specific interstitial pneumonia, SSc systemic sclerosis, ILA interstitial lung abnormalities

Fig. 8

Interstitial lung disease. a is an example of non-fibrosing HP (acute), with diffuse GGOs micronodules; instead, a subacute HP is characterized also by focal areas of air-trapping (b). The late stage is fibrosing HP (chronic) with traction bronchiectasis and bilateral basal involvement (c). d is a case of DIP, the end stage of RB-ILD in a heavy smoker patient, with bilateral and symmetric GGOs. HP hypersensitive pneumonitis, DIP desquamative interstitial lung disease, RB-ILD respiratory-bronchiolitis-related interstitial lung disease

Fig. 9

Drug toxicity and organizing pneumonia. a shows OP pattern after radiation therapy in breast cancer. b is another OP caused by immunotherapy (Nivolumab) in patient with renal cancer. c is a case of COP, with diffuse reversed halo signs. d is a case of interstitial pneumonia after immunotherapy (Pembrolizumab) for right lung cancer and e is another case of OP with atoll sign. OP organizing pneumonia, COP cryptogenetic organizing pneumonia

Fig. 10

Aspiration pneumonia. The two cases in a, b show similar GGO areas in lower left lobe, with a super-imposed crazy-paving pattern: the first case a is a chronic aspiration pneumonia; instead, b is an example of exogenous lipoid pneumonia in a young patient with chronic inhalation of oily drugs for constipation (confirmed with lung biopsy). The third case in c is another example of lipoid pneumonia in elderly, with a rounded consolidation in the right upper lobe and diffuse GGO with crazy-paving in the lower posterior lobes (please note also the dilated esophagus)

Fig. 11

Cancer. a A lung cancer in the upper right lobe surrounded by GGO halo sign. b An adenocarcinoma with lepidic growth that manifest as a pure GGO nodule. c An example of diffuse bronchial MALT with bilateral and broncho-vascular involvement. MALT mucose-associated lymphoid tissue

Fig. 12

Sarcoidosis. a, b Two cases of atypical sarcoidosis pattern with GGO appearance, both caused by diffuse coalescent granulomatous micronodules below HRCT power resolution