Although CT findings of bilateral GGO or consolidation may prompt radiologists to diagnose a patient COVID-19[2, 3, 11, 12], there is a possibility of misdiagnosis based on imaging, because different diseases can show similar signs or findings. In adults, MP often manifests as non-specific interstitial changes (e.g., GGO and consolidation). Adult MP manifests as diffuse and/or multifocal ground-glass plaque lesions that can involve all lung lobes, which is similar to findings in viral interstitial pneumonia [4, 5, 6]. Therefore, it is important to distinguish between the two diseases. This is the first comparison between patients confirmed with mild COVID-19 and those suspected to have the disease but confirmed to have MP. We utilized quantitative image parameters that were automatically determined based on a deep learning algorithm to evaluate and compare longitudinal CT changes of COVID-19 and MP.
Compared with the first CT scan (<7 days of symptom onset), the volume and number of lesions increased on the second CT scan (7-14 days) in the COVID-19 group then decreased slowly, which is consistent with reports in the literature of imaging findings peaking ~13 days after symptom onset [13, 14, 15]. In the MP group, lesion number, volume, and involved lobes gradually decreased after the first CT examination (<7 days after symptom onset), and most lesions were absorbed by the third follow-up. In this study, nine MP cases were completely absorbed at the fourth follow-up, compared to just two COVID-19 patients with complete resolution at the 30-day follow-up. The residual signs at the final follow-up in both groups were mainly GGO, but more were observed in the COVID-19 group. The pathological absorption time was longer in COVID-19 compared to MP patients. The quantitative analysis using the Pneumonia-CT-LKM-PP model demonstrated that lung involvement with COVID-19 reached a peak 7-14 days after symptom onset, and this was the most prominent COVID-19 imaging change. The longitudinal change is conducive to distinguishing between COVID-19 and MP [14].
Patients with mild COVID-19 have a short interval between symptom onset and the first CT examination, and the patient’s lymphocyte count is still within the normal range, similar to the presentation of MP. Lymphocyte counts in both groups were normal and not significantly different from each other. In addition, the CRP level of the MP group was higher than that of the COVID-19 group, indicating that MP induced an obvious inflammatory response. This may be related to the enrollment criteria since COVID-19 patients had mild disease while the MP group did not.
The CT findings of 13 patients with MP showed GGO patterns distributed under the lung pleura with interlobular septum thickening. Combined with the history of fever and travel, they were suspected as having COVID-19 after the first CT examination. Although bronchial wall thickening has been reported in MP, this sign was not obvious in the MP group, and this intrinsic sign is also not specific. Combining quantitative CT changes and multiple nasopharyngeal rRT-PCR tests can ensure a clear diagnosis of COVID-19 pneumonia, but our experience is that when the viral pneumonia imaging manifestations appear, we should consider MP and perform specific IgM antibody detection if necessary.
Some limitations should be considered when interpreting our results. First, the sample size was small because we required three follow-up scans to longitudinally evaluate lesion changes. Second, this study was based on an open-source quantitative assessment model of pneumonia, which still requires the supervision of radiologists. Third, we did not analyze lung CT change patterns (e.g., GGO and consolidation) over time because it has been reported in the literature [3, 16, 17, 18, 19, 20, 21].
In conclusion, the Pneumonia-CT-LKM-PP model based on deep learning algorithms can objectively and quantitatively evaluate imaging changes in COVID-19 pneumonia. Lesion number, volume, and the lobes involved reached their peaks within 7-14 days after symptom onset. These characteristics of COVID-19 may be used to distinguish it from a diagnosis of MP and evaluate treatment effects and prognosis.