2.1 Study population
This study was approved by the institutional review board of ---, and all patients provided written informed consent. Between November 2020 and August 2021, all consecutive patients with clinically suspected CHD were screened for inclusion in the study. The inclusion criteria were as follows: age ≥ 18 years, heart rate < 70 bpm and BMI ≥ 18 kg/m2. We excluded patients with pacemakers, severe respiratory artefacts, metal implants within the scanning range, allergies to contrast media or betaloc, or a history of cardiac tumours or cardiac surgery.
The study population consisted of 154 patients with CHD, classified according to BMI as described above and divided into four groups: multi-modality–normal BMI group (A1, n = 82); multi-modality–overweight group (B1, n = 17); conventional–normal BMI group (A2, n = 39); and conventional–overweight group (B2, n = 16).
2.2 MSCTA examination
MSCTA examinations were performed using a Toshiba 320-row dynamic CT system (Aquilion ONE; Canon Medical Systems). Prospective ECG gating was used to reduce the radiation dose . The scanning range extended 140 mm upwards starting from the lower edge of the heart. In groups A1 and B1, isocentric scans were performed with the patient lying on their back with the body shifted to the right. Before the MSCTCA scans, ultrasound examination was performed to mark the leftmost and rightmost edges of the heart on the body surface, and perpendicular lines were made along the leftmost and rightmost edges of the heart. The centreline of the two vertical lines was used as the vertical positioning line and the horizontal axillary centreline was used as the horizontal positioning line, with the maximum current during 60–80% of the RR interval. Groups A2 and B2 were scanned using the conventional method, with the patient in the natural supine position. The horizontal axillary centreline was again used as the horizontal positioning and the median line was used as the vertical positioning line, with the maximum current during 30–80% of the RR interval. The scanning parameters were as follows: groups A1 and B1, tube voltage = 80 kV, 80% tube current control to 80% using smart milliampere (done automatically by the Toshiba instrument according to body position); groups A2 and B2, tube voltage = 100 kV, tube current control using milliampere. Previous studies showed no difference in IQ between a tube voltage of 80 kV and injection rate of 4.0 ml/s versus a tube voltage of 100 kV and injection rate 5.0 ml/s [1,9,16]. Iodine contrast agent (Ultravist; 370 mg I/ml) was injected as a single bolus via the elbow vein in a volume of 30–50 ml at an injection rate of 4.0 ml/s, followed by 18 ml of saline in groups A1 and B1. In groups A2 and B2, the volume was 50–70 ml at an injection rate of 5.0 ml/s, again followed by 18 ml of saline. The bolus injection tracing method was used and the threshold was set to 280 HU (Table 2).
2.3 Preparation before scanning
All patients provided written informed consent to participate in the study. Heart rate and blood pressure were measured at rest, before the examination. Patients with a heart rate > 70 bpm were given betaloc (25–100 mg). Patients with a BMI ranging from 18 kg/m2 to 27kg/m2 were randomly assigned in a 2:1 ratio to groups A1 and A2, and patients with a BMI ≥ 27 kg/m2 were randomly assigned in a 1:1 ratio to groups B1 and B2. Before the examination, patients were given a 0.5 mg nitroglycerine tablet.
2.4 Radiation dose
The CT dose index volume (CTDIvol; mGy) and dose length product (DLP; mGy × cm) were automatically calculated for all CT protocols by the scanner software. DLP was then multiplied by a conversion coefficient, k, to determine the effective dose (ED = DLP × k) for each patient  (Table 1).
2.5 Image processing and analysis
All images were reconstructed with adaptive AIDR 3D. The scanning parameters were as follows: scan length, 140 mm; slice thickness, 0.5 mm; reconstruction field of view, 220 mm; and kernel, EU10 (Table 2).
2.6 Subjective and objective evaluation of image quality
The objective IQ was evaluated by two experienced cardiovascular radiologists (MQ Xiao, 17 years of experience, and PK Huang, 13 years of experience) who were blinded to the scan and reconstruction parameters. A circular region of interest (ROI) with a diameter of 1 cm was placed within the cortical part of the main bronchus. The coronary artery attenuation values of the proximal ROIs of the left main coronary artery (LMCA) and the right coronary artery (RCA) were measured. The ROI was as large as possible, and the vascular wall, vascular calcifications, and non-calcified plaques and artefacts were excluded. The average coronary artery attenuation was equal to the average value of the LMCA and proximal RCA [1, 17]. ROI measurement was performed on the adjacent myocardial fat. The ROI measurements were repeated three times at each location and averaged to ensure data consistency. The CT value, standard deviation (SD) of the main bronchus and average attenuation values of the coronary artery and pericardial fat were calculated by averaging the values obtained by the two observers. Noise was calculated as the SD of the CT value of the main bronchus. The signal-to-noise ratio (SNR) and contrast signal-to-noise ratio (CNR) were calculated as follows: SNR = average main bronchus CT value/image noise; CNR = (average attenuation of coronary artery − perivascular fat attenuation)/image noise .
Coronary artery segmentation was performed according to the 15-segment coronary artery segmentation method developed by the American Heart Association [1, 17]. Two radiologists with 18 years (MQ Xiao) and 13 years (PK Huang) of experience in cardiovascular medicine conducted independent evaluations of coronary arteries with diameter ≥ 1.5 mm. In the event of inter-rater disagreement, a consensus was reached through consultation. A 5-point Likert scale was used, as follows [18, 19]: 5 = excellent (sharp, smooth contours of the vascular wall and no streaking or radiating artefacts); 4 = good (slight irregularities of the contour and few streaks or radiating artefacts); 3 = fair (blurred and irregular contour of the vascular wall and numerous streaks or radiating artefacts); 2 = poor (deformation of the vascular wall and many artefacts); 1 = very poor (obvious deformation of the vascular wall and extensive artefacts). Images with IQ scores of 3–5 satisfied the requirements for diagnostic assessments (Fig. 2).
2.7 Statistical analysis
Statistical analyses were performed using SPSS 26.0 software (IBM Corp., Chicago, IL, USA). Continuous variables are expressed as the mean ± SD. The t test was used for pairwise comparisons between groups. The Chi-square test was used for analyses based on sex. The intraclass correlation coefficient (ICC) was calculated to determine the consistency of the subjective IQ scores. An ICC > 0.70 indicated adequate test–retest reliability . In all analyses, P < 0.05 was taken to indicate statistical significance.