Abstract： Objective:To investigate the difference in the peri-coronary fat attenuation index (FAI) between using coronary calcium score (CCS) images and coronary CT angiography (CCTA) images, and to explore the feasibility and befitting threshold of FAI measured on CCS images.Methods:The clinical and imaging data of patients who underwent CCTA examination from August 2019 to August 2020 were retrospectively analyzed in the First Affiliated Hospital of Soochow University. According to the inclusion and exclusion criteria, there were 122 cases in non-calcified plaque group (144 coronary arteries) and 97 cases in none-plaque group (186 coronary arteries). The coronary arteries were delineated both on CCS and CCTA images with Perivascular Fat Analysis Tool; the regions of interest of peri-coronary adipose tissue were generated automatically after setting the threshold of fat tissue. Then the FAI value was calculated. The thresholds were set in four levels (-190--30, -185--25, -180--20 and -175--15 HU) for CCS images and one level (-190--30 HU) for CCTA images. The intra-class correlation coefficient (ICC) was used to evaluate the consistency of the measurements of FAI values on CCS and CCTA images between the two physicians. Paired t test was used to compare the differences of FAI values between CCS and CCTA images, and Pearson correlation coefficient was used to evaluate the correlation between CCS-FAI and CCTA-FAI. Results:(1) FAI values measured on CCS and CCTA images by 2 physicians showed good consistency; (2) At the threshold of -185--25 HU, there was no significant difference in FAI values between the CCS and CCTA images for non-calcified plaque group [(-84.15±5.99)HU vs. (-83.83±5.98)HU, t=0.79, P=0.429], as well as for the none-plaque group [(-83.41±5.75)HU vs.(-83.84±6.25)HU, t=-1.08, P=0.280]; (3) There were significant differences on FAI values between the CCS images and CCTA images at the threshold of -190--30、-180--20 and -175--15 HU (all P<0.05); (4) There were moderate correlations on FAI values between the CCS images and CCTA images under different thresholds both in non-calcified plaque group and none-plaque group. Conclusion:It is feasible to measure FAI on CCS images, and the befitting threshold is -185--25 HU.