This observational cross-sectional study was a mono-centric pilot study to a prospective trial registered with the Chinese Clinical Trial Registry (ChiCTR1900022527). A total of 457 consecutive patients with CAD were prospectively recruited in Beijing Anzhen Hospital between February 2018 and April 2021. Inclusion in this study required angiographically confirmed multivessel CAD, defined as having at least 2 of 3 epicardial vessels with a stenosis ≥70% or left the main stenosis ≥50%. Patients were excluded if: 1) a recent myocardial infarction (< 4 weeks), 2) history of malignancy, acute or chronic inflammatory and autoimmune disease, 3) history of cardiovascular surgery or cardiac transplantation. Finally, a total of 100 multivessel CAD patients were recruited in our current study. The study flow chart is shown in Figure 1. The project was approved by the Medicine Ethics Committee of Beijing Anzhen Hospital (2018055X) and adhered to the principles laid out in the Declaration of Helsinki. Baseline characteristics of study population are listed in Table 1.
Analysis of CAC burden and PCAT on CT
All cardias CT scans were conducted using electrocardiography-gated cardiac CT using a 128-slice multi-detector computed tomography scanner (Biograph mCT, Siemens Healthcare, Erlangen, Germany). The scan parameters were: 128 x 0.6 mm collimation; tube voltage, 120 kV; gantry rotation time, 330ms; and tube current, 770-850 mAs. Coronary calcium was quantified on both a per-patient and per-segment level by an experienced observer (WW) using volume analysis software (Cascoring Siemens Healthcare, mCT). The CAC score was derived using the Agatston method . To quantify the PCAT density, a CT attenuation threshold of -190 to -30 Hounsfield Units was used to isolate adipose tissue by Mimics Medical software (version 21.0; Materialise, Leuven, Belgium) , and the PCAT density was defined as the mean attenuation within such contamination-free volumes of interest and was measured in the reference region of the proximal left anterior descending (LAD), proximal left circumflex (LCX), and mid-right coronary artery (RCA) on axial CT images. For each coronary artery, five of regions of interest (ROIs, each ROI area = 3 mm2) were manually placed on the region of distance the outer coronary artery wall equal in width to the vessel diameter . The PCAT density of the LAD (PCATLAD), LCX (PCATLCX) and RCA (PCATRCA) was calculated by the average PCAT value from the value of five ROIs in LAD, LCX, and RCA, respectively. The PCAT density in each patient was calculated as the average PCAT value from three main coronary arteries (LAD, LCX, and RCA). PCAT density measurement by cardiac CT was performed by two experienced nuclear cardiologists (MJ and WW), who were blinded to the quantitative analysis data as well as 18F-NaF PET/CT image analysis.
Cardiac 18F-NaF PET/CT and image analysis
All patients were administered a target dose of 18F-NaF (3.7 MBq/kg) intravenously and subsequently rested in a quiet environment for a 120-min uptake period, an electrocardiogram-gated cardiac 18F-NaF PET/CT imaging (Biograph mCT, Siemens Medical Systems, Erlangen, Germany) was performed. A low-dose attenuation correction CT scan (120 kV, 50 mAs) was then acquired. The PET data were reconstructed using a point spread function + time of flight algorithm (time of flight + TrueX, Siemens Ultra-HD), with 5 iterations and 21 subsets. Due to the small size of the vulnerable plaques, an in-plane pixel size of 2 mm with a corresponding reconstructed image matrix size of 400×400 was used to achieve a high spatial resolution.
To evaluate the coronary 18F-NaF uptake, the maximum standardized uptake value (SUVmax) (a validated measure of tissue radiotracer uptake) of LAD, LCX and RCA were quantified from ROIs by delimiting three-dimensional regions, respectively. The tissue-to-background ratios (TBR) in the LAD (TBRLAD), LCX (TBRLCX), and RCA (TBRRCA) were then calculated by correction for background blood pool activity using the right atrium (mean SUV using cylindrical volumes-of-interest [radius: 10 mm; thickness: 5 mm] at the level of the RCA ostium). The TBR in the coronary artery (TBRCoronary) was calculated as the average TBR value from three main coronary arteries (LAD, LCX, and RCA) in each patient.
The aortic (ascending aorta, aortic arch, descending aorta) 18F-NaF uptake was determined by manually placing oval ROIs on the equatorial plane of these major arteries to avoid artifacts from the accumulation of 18F-NaF in the vertebral body . The SUVmax of 18F-NaF avid focus more than 1.6 times the mean SUV of the right atrium blood pool was considered an abnormal aorta lesion. The number of lesions and SUVmax of each lesion in the aorta were recorded and measured. The TBR in the aorta (TBRAorta) was calculated by the average of lesions SUVmax in the aorta corrected by the mean of SUV in the right atrium.
All statistical analyses were performed using SPSS software (version 25, SPSS, Inc., Chicago, IL). Continuous variables were tested for normality using Shapiro-Wilk test and were presented as mean ± standard deviation or median (interquartile range) dependent on the distribution. Patients were divided dichotomously by the median TBRCoronary value into group 1 (TBRCoronary ≥ 2.49, n = 50) and group 2 (TBRCoronary < 2.49, n = 50). Data were compared by using two-sample t-test or Mann-Whitney U tests. Categorical variables were summarized using frequencies and percentages and were compared by using a chi-squared test (with a Yates correction or a Fisher exact test for smaller sample sizes). Spearman’s correlation analyses and multiple linear regression analyses were used to assess the correlations between the coronary 18F-NaF uptake with the PCAT density, CAC burden, CAD risk factors, serum biomarkers, and aortic 18F-NaF uptake, respectively. Bland-Altman analyses were employed to assess the repeatability of the PCAT density and coronary 18F-NaF uptake (Additional file: Figure S1 and Figure S2). A 2-sided p-value < 0.05 was regarded as significant.