Comparative Study Of Volume Computed Tomography Dose Index With Size-specific Dose Estimate In Evaluating Chest CT Radiation Dose

Objective CT has been widely used in clinic,and the radiation dose received by the subjects is also increasing greatly.The radiation dose problem has been paid more and more attention.The objective of the study is to compare the difference of volume computed tomography dose index(CTDIvol)and size-specific dose estimate(SSDE)in evaluating chest CT radiation dose,and to better control the radiation dose of subjects in CT examinations.Methods This retrospective study was approved by ethics committee of the hospital and exempt from informed consent.Chest CT scan were performed on a total number of 167 subjects with Canon Aquilion One Vision 640-slice volume computed tomography(formerly Toshiba Medical in Japan).All the images met the diagnostic requirements.We measured the length of the lateral dimension(LAT)and the anterior-posterior dimension(AP)of the two nipple planes on the transected images of each subject,and record the volume computed tomography dose index(CTDIvol)of each subject to calculate the effective diameter(ED)、conversion factor(fsize),and size-specific dose estimate(SSDE).According to the reference standard of body mass index(BMI)in China,all subjects were divided into three groups:underweight group(group A),BMI<18.5 kg/m2;normal weight group(group B),18.5≤BMI≤23.9 kg/m2;overweight group(group C),BMI ≥ 24 kg/m2.Group B was divided into two groups according to its BMI median:lighter group of normal weights(group B 1),18.5 ≤ BMI<20.2 kg/m2;heavier group of normal weights(group B2),20.2 ≤BMI ≤23.9 kg/m2;Group C was divided into three groups according to the reference standard of body mass index in China:mild obesity group(C1 group),24≤BMI≤26.9 kg/m2;obesity group(C2 group),27≤BMI≤29.9 kg/m2;severe obesity group(C3 group),BMI≥30 kg/m2.The lateral diameter(LAT),the anterior and posterior diameter(AP),effective diameter(ED)and conversion factor(fsize)of different body mass index groups were analyzed,and the differences between CTDIvol and SSDE in evaluation radiation dose of chest CT were compared.Results(1)Comparison of subjects in BMI standard group:① There were significant differences in lateral diameter(LAT),anterior and posterior diameter(AP),effective diameter(ED)and conversion factor(fsize)among groups A,B and C(F=35.282,50.788,67.104,68.251,P<0.05);②There was significant difference between CTDIvol and SSDE in 167 subjects(t=75.489,P<0.05).③There were significant differences between CTDIvol and SSDE in three groups(t=44.271,56.414,32.443,P<0.05).The SSDE of the three groups increased 57.40%,45.16%and 33.90%respectively compared with CTDIvol,and the SSDE of group A with the lowest body mass index increased the most compared with CTDIvol.(2)Comparison of subjects in subgroup B:①There were significant differences in LAT、AP、ED and fsize between B1 and B2 groups(t=-2.439,-1.203,-2.508,2.559,P<0,05);②The difference between CTDIvol and SSDE in 90 subjects was statistically significant(t=56.414,P<0.05);③There were significant differences between CTDIvol and SSDE in the two groups(t=31.101,50.945,P<0.05);SSDE of B1 groups and B2 groups increased by 49.79%and 44.31%respectively compared with CTDIvol,and SSDE in the group of B1 with smaller body mass index increased the most than CTDIvol.(3)Comparison of subjects in subgroup C:① There were significant differences in LAT,AP,ED and fsize among three groups(F=11.100,8.371,16.254,16.095,P<0.05);②The difference between CTDIvol and SSDE in group C was statistically significant(t=32.443,P<0.05);③The differences between CTDIvol and SSDE in the three groups were statistically significant(t=37.052,16.846,7.233,P<0.05);SSDE in three groups(C1,C2,C3)increased by 37.00%,31.00%and 23.00%respectively compared with CTDIvol,among which,the group of C1 with relatively smaller body mass index increased the most.Conclusion First,size-specific dose estimate(SSDE)adjusts the volume computed tomography dose index(CTDIvol)according to the size,so that the radiation dose of the patient can be estimated individually.Second,in the chest CT scan,within the body mass index range of this study(16 ≤ BMI ≤ 37.6 kg/m2),the radiation dose estimated by ssde is higher than CTDIvol,so CTDIvol may underestimate the radiation dose of the patient.Third,the comparison of radiation doses among subjects of different body mass index groups in this study suggests that the radiation dose underestimated by CTDIvol may be higher for those with lower BMI.