The research was approved by Medical Ethical Committee (Approved Number. 2020037). Our institutional review board waived written informed consents for this retrospective study.
Patients and data source
All the patients in our study had been diagnosed of COVID-19 according to the guideline of 2019-nCoV (Fifth Trial Edition) issued by the National Health Commission of China [7]. A total of 127 patients (68 men and 59 women; mean age, 57.7 years; age range, 20-83 years) with confirmed SARS-CoV-2 were identified who had undergone at least two chest CT studies at Wuhan Leishenshan Hospital between Feb 12, 2020 and Apr 10, 2020 (see more details in Table 1). These patients underwent the first chest CT using the conventional manual positioning and centering method, and an AI-based positioning and centering method in the follow-up CT examination. Based on the different positioning methods, patients were categorized into the conventional manual positioning (MP) group and AI-based positioning (AP) group.
CT image acquisition and reconstruction
The imaging workflows for MP and AP groups are shown in Figure 1. A and B. The chest CT scanning was performed on a Revolution Maxima CT equipped with an AI-based automatic patient centering and anatomic positioning software (GE Healthcare, Waukesha USA) from the apex pulmonis to septum transversum. the AI-based positioning function uses the anatomical references and the scout range information to determine the landmark and the scan start and stop locations (Supplementary Fig 2). Both groups used the same scan protocol with the following parameters: tube voltage, 120 kVp; gantry rotation time, 0.4 second; pitch, 1.375:1; scan field-of-view (SFOV), 50cm; slice thickness, 5 mm; tube current (mA), automated tube current modulation (ATCM) to obtain a noise index of 11.57; All axial images were reconstructed using a standard reconstruction algorithm with the standard kernel; reconstruction display field-of-view (DFOV), 35-50 cm; reconstruction thickness, 1.25 mm.
Assessment of image quality
The image quality was analyzed by three radiologists (H.B.X., J.X.H., Y.D.G) at a standard pulmonary display window setting (window level, −700 and window width, 1500). Decisions were reached by consensus. The mean CT value and standard deviation (SDev) in Hounsfield Units (HU) of the aorta, trachea and erector spinae in the upper and middle thorax areas were measured by placing a 50 mm2 region-of-interest (ROI) on a homogeneous-appearing area of these structures, as is shown in Figure 5. A, B. Three consecutive images were measured in each ROI area for each study, and the average value was determined. The mean and SDev of CT values within pulmonary lesions were also measured according to the method described above. The pulmonary lesions mainly included ground glass opacification, consolidation opacification and interstitial thickening (Fig 2). The pulmonary segments are defined by referring to the branching patterns of bronchi [8-10]. If a lesion was located in the outer one third of the lung, it was defined as peripheral, otherwise, it was defined as central. The signal-to-noise ratio (SNR) of the lesions was calculated based on the formula: SNR = Mean CT values/SDev. The image noise was represented using the SDev value.
Examination and positioning time
Total examination time and positioning time were recorded by the CT technologist for each study. The total examination time was defined as the time from the patient entering the CT scanning room to walking out of the exam room after finishing the CT examination. The positioning time was defined as the time from the patient lying on CT examination bed to technologist finishing positioning and starting scanning.
Radiation dose
The volume CT dose index (CTDIvol in mGy) and dose length product (DLP in mGy-cm) were recorded from the dose report image by the CT technologist for each study. The effective dose (ED in mSv) of the patient was calculated based on the formula: ED = DLP × Cf, where the Cf represents the conversion factor for chest CT (Cf=0.014mSv/mGy-cm).
Off-center distance and positioning Accuracy
The patient off-center distance was measured using an axial CT image in the following steps: (ⅰ) select a transverse image containing manubrium and draw a horizontal line that passes through both armpits. (ⅱ) locate the center of the display field of view (DFOV) for the image. (ⅲ) record the vertical distance from the center of DFOV to the horizontal line. The vertical distance represented the patient off-center distance (Supplementary Fig 1. A). For the positioning accuracy, a complete coverage should contain the apex pulmonis to septum transversum. thus, if the images of apex pulmonis and septum transversum were fully covered, the patient positioning was considered successful, otherwise, it was defined incomplete or inaccurate.
Statistical analyses
Continuous variables were expressed as mean ± SD and compared using paired-sample t tests when the data were normally distributed; otherwise, the Wilcoxon signed-rank tests was used; The categorical variables were expressed as number (percentage %) and compared with McNemar's test. A two-tailed P value of less than 0.05 was considered statistically significant. All statistical analyses were conducted with IBM SPSS software (version 22.0).