To study the extent of pulmonary involvement in COVID-19 with quantitative CT (QCT) and to assess the impact of disease burden on opacity visibility on chest radiographs.
Materials and Methods:
This retrospective study included 20 pairs of CT scans and same-day chest radiographs from 17 patients with COVID-19, along with 20 chest radiographs of controls. All pulmonary opacities were semi-automatically segmented on CT images, producing an anteroposterior projection image to match the corresponding frontal chest radiograph. The lung opacification mass (QCTmass) was defined as [(opacity attenuation value+1000 HU)/1000]*1.065(g/mL) * combined volume (cm3) of the individual opacities. Eight thoracic radiologists reviewed the 40 radiographs, and a receiver operating characteristics curve analysis was performed for the detection of lung opacities. Logistic regression analysis was done to identify factors affecting opacity visibility on chest radiographs.
The mean QCTmass per patient was 72.4±120.8 g (range, 0.7-420.7), and opacities occupied 3.2±5.8% (range, 0.1-19.8) and 13.9±18.0% (range, 0.5-57.8) of the lung area on the CT images and projected images, respectively. The radiographs had a median sensitivity of 25% and specificity of 90% among radiologists. Nineteen of 186 opacities were visible on chest radiographs, and a median area of 55.8% of the projected images was identifiable on radiographs. Logistic regression analysis showed that QCTmass (p<0.001) and combined opacity volume (p<0.001) significantly affected opacity visibility on radiographs.
QCTmass varied among COVID-19 patients. Chest radiographs had high specificity for detecting lung opacities in COVID-19, but a low sensitivity. QCTmass and combined opacity volume were significant determinants of opacity visibility on radiographs.
Quantitative opacity mass and 3D opacity volume on CT were quantifiable metrics affecting the visibility of COVID-19-related opacities on chest radiographs.
■ In COVID-19 patients, considerable variation was found in the QCTmass (72.4±120.8 g; range, 0.7–420.7 g) and relative 3D opacity extent on CT (3.2±5.8% of lung area; range, 0.1–19.8%).
■ Chest radiographs in patients under investigation for COVID-19 provided a sensitivity of 25% (5/20) and specificity of 90% (18/20) for COVID-19-related opacities.
■ The QCTmass (p<.001) and the 3D opacity volume on CT (p<.001) significantly affected the visibility of COVID-19-related opacities on radiographs.