ABSTRACT
Objective To investigate the difference of size-specific dose estimates ( SSDEs ) based on effective diameter ( def ) and water equivalent diameter ( dw ) in coronary computed tomography angiography ( CCTA) and explore the causes. Methods A total of 99 patients undergoing CCTA were enrolled in this retrospective study. SSDEs ( SSDEd ef and SSDEdw ) were calculated in two approaches using def and dw , respectively. Mean absolute relative difference ( MARD) was computed as an index to quantify the consistency of SSDEd ef and SSDEdw . Multivariate stepwise regression analysis was performed to study the factors influencing MARD. Results The values def and dw were positively correlated with body mass index (BMI) (r=0. 869, 0. 823, P<0. 05). The median (interquartile range) of SSDEdef and SSDEdw were 12. 34 ( 11. 75, 12. 98) mGy, 13. 78 ( 13. 02, 15. 04) mGy, respectively. SSDEdef was lower by 10. 45％ than SSDEdw( Z=-8. 186, P<0. 05) . Both SSDEdef and SSDEdw were negatively correlated with BMI and dw(r=-0. 765, -0. 680, -0. 701, -0. 840, P<0. 05). MARD of SSDEdef and SSDEdw was generally at 11. 39％. No statistical significance was found in the correlation of MARD with BMI ( r=0. 031, P>0. 05) , however, positive correlation was shown between MARD and def ( r=0. 251, P<0. 05) , but negative correlation for MARD and dw(r=-0. 379, P<0. 05). With respect to the factors influencing MARD, four variables were included into the regression equation. MARD was positively correlated with the area of both air-filled lungs ( Arealow ) and soft tissues ( Areasoft ) (β=0. 634, 0. 102, P<0. 05) , and negatively correlated with the area of bone, enhanced cardiac chambers and aorta ( Areahigh ) and the CT value of air-filled lungs ( SIlow ) (β=-0. 234,-0. 343, P<0. 05) . Conclusions SSDEdef was approximately 10. 45％ lower than SSDEdw , which was predominantly influenced by the area of air-filled lungs due to the characteristics of low X-ray attenuation in CCTA.