Patients
A retrospective review of patient data and imaging studies was approved by the institutional review board. No patient consent was required for this HIPAA-compliant study.
Between January 22 and February 8, 2020, 88 patients with COVID-19 were identified from six hospitals in Shaanxi Province in China, which is approximately 800 km northwest of Wuhan. The inclusion criteria were as follows: (1) patients with COVID-19 confirmed by at least one positive nucleic acid test performed at the Center for Disease Control and Prevention in Shaanxi Province in China and (2) patients who underwent baseline and at least one follow-up chest CT. According to the WHO, patients are divided into six types: uncomplicated illness, mild pneumonia, severe pneumonia, acute respiratory distress syndrome, sepsis, and septic shock [10]. All the patients were treated based on a standard diagnosis and treatment criteria for COVID-19 which includes initiation of antivirals, interferon, Chinese herbal medications, supplemental oxygen as needed, and hospitalization.

Image acquisition
CT scans of the chest were acquired on 16- to 64-multidector CT scanners (Philips Brilliant 16, Philips Healthcare; GE LightSpeed 16, GE Healthcare; GE VCT LightSpeed 64, GE Healthcare; Somatom Sensation 64, Siemens Healthcare; Somatom AS, Siemens Healthcare; Somatom Spirit, Siemens Healthcare; GE Optima 680, GE Healthcare). The CT parameters were as follows: 120 kVp, current intelligent control (auto mA) of 30–300 mA, and slice thickness reconstructions of 0.6–3.0 mm. All CT examinations were performed without intravenous contrast material.