| Part 1:phantom study on the influence of different scanning centers on CT scan radiation dose of fixed tube voltage and tube currentObjective:To investigate the effects of different scanning centers of head and neck on the the absorbed dose on the surface of the sensor during helical scanning of the head and neck with fixed tube current(mA)and fixed tube voltage(kV)).Methods: The head and neck of anthropomorphic Phantom were scanned by using the default values of tube voltage and tube current for clinical devices.In the head and neck of anthropomorphic Phantom,15 different CT table location(i.e.different scanning centers)were selected from the center of the outer ear hole(represented by 0),which were 1cm,2cm,3cm,5cm,6cm,7cm,8cm forward and 1cm,2cm,3cm,4cm,5cm,6cm backward.Three localization image scans were performed for each scanning center,followed by three spiral scans(in order to ensure the accuracy of TLD measurement,the absorbed dose on the surface of the corresponding sensor is taken as the average value of three times).The area of interest(ROI)was selected at the level of the junction point of the intracranial middle cerebral artery in the head and neck model,and then the signal-to-noise ratio(SNR)was measured and recorded compared with the noise ratio(CNR).Thermoluminescence dosimeter(TLD)was used to measure the absorbed dose level of the eye lens,thyroid gland,mammary gland and other organs after each scan.The volume CT dose index(CTDIvol)of each scanwas recorded,and the data of each group were analyzed by a completely randomized analysis of variance(anova).P<0.05 was considered statistically significant.Results: Among the 15 different scanning centers of head and neck phantom,the scanning center was 6cm forward from the outer ear hole along the auditory canthus line,and the cumulative absorbed dose of eye lens was the highest(right eye: 150.667±4.933 mGy,left eye: 156.000±6.557mGy).When the scanning center was 6cm backward from the outer ear hole along the auditory canthus line,the cumulative absorbed dose of eye lens was the lowest(right eye: 90.067±6.144 mGy,left eye: 90.000±19.995mGy).The highest cumulative absorbed dose of thyroid radiation(187.667 ± 6.028 mGy)was 6cm forward of the outer ear hole along the canthal line,and the lowest cumulative dose of thyroid radiation(128.000 ± 2.646 mGy)was 6cm backward of the outer ear hole along the canthal line.The highest cumulative absorbed dose(9.543 ± 0.578 mGy)of the right breast was 2cm forward of the epicanthus line in the center of the outer ear hole,the lowest(9.067 ± 0.737 mGy)was 7cm forward of the epicanthus line in the center of the outer ear hole.The highest cumulative absorbed dose(10.573 ± 0.566 mGy)of the left breast was in the center of the outer ear hole,and the lowest(9.387 ± 0.602 mGy)6cm backward of the epicanthus line in the center of the outer ear hole.The highest cumulative absorbed dose of gonadal radiation(0.133 ± 0.058 mGy)was 4cm forward of the line of auditory canthus in the center of outer ear hole and and the lowest(0.083 ± 0.049 mGy)was 5cm backward of the line of auditory canthusin the center of outer ear hole.The difference of body surface absorbed dose of sensitive organs such as lens and thyroid in bilateral eyes was statistically significant(P < 0.001),There was no significant difference in the absorbed dose of bilateral mammary glands and gonads(P > 0.05).The differences of blood vessel noise,soft tissue CT value and blood vessel CT value were statistically significant(P < 0.05),The differences of vascular noise value,soft tissue CT value and vascular CT value were statistically significant(P < 0.05),There was no significant difference in soft tissue noise(P > 0.05)There was no significant difference in SNR and CNR among the regions of interest(P >0.05).Conclusion:The choice of CT scanning center has great influence on the radiation dose of CT scanning site,especially on the absorbed dose of radiation sensitive tissues and organs.Part 2:phantom study on using the automatic tube current technique and iterative reconstruction algorithm in CT scan radiation dose of head and neckObjective: to investigate the effects of automatic tube current modulation and iterative reconstruction on the absorbed dose and image quality of radiation-sensitive organs in simulating head and neck CTA scanning mode on phantom.Methods: using the prospective research method,CTA scan was performed on the head and neck of the simulated human body.According to different scanning methods,the subjects were divided into 5 groups,and each group was scanned lateral location image and simulated CTA in non-timing mode for 5 times;In group A,conventional dose scanning scheme was adopted,namely fixed tube voltage and electric flow(120kv,220 mas,Clearview dual domain iterative reconstruction technology,reconstruction level of 30%),Group B(140k V),group C(120kv),group D(100k V)and group E(80k V)are all low-dose scanning technologies(using CTA sequence non timing scanning mode,automatic tube current modulation technology,o-dose 30% level Clearview dual domain iterative reconstruction technology),And according to the signal-to-noise ratio gearfurther divided into B1(s/N gear 1.3),B2(s /N gear 1.0),B3(s/N gear 0.7),C1(s/N gear 1.3),C2(s/N gear 1.0),C3(s/N gear0.7),D1(s/N gear 1.0),D2(s/N gear 0.7)and E0(s/N gear 0.7),Calculating the radiation dose of each group(CTDIvol,DLP)and measuring the absorbed dose of each sensitive organ area(binocular lens,thyroid,breast,gonad),Recording and comparing separately the radiation dose of CTDIvol,DLP and absorbed dose of sensitive organ area of human body model,CT value,image noise(SD)and CNR of the region of interest(simulated cavernous sinus of internal carotid artery and M1 segment of middle cerebral artery)were measured on the head and neck images.Comparison of measurement data among groups use variance analysis of complete random design,dunnett-t test was used for further comparison between the two groups(P < 0.05).Results: Phantom study showed that,the radiation dose data automatically generated by the system after 10 groups of scanning are comparable,using automatic tube current modulation technology to simulate head and neck CTA scan can effectively reduce the absorbed dose of sensitive organs,the CTDIvol and DLP values of B1(140k V,1.3 SNR gear)group are the highest,they were 21.9m Gy and 908.4m Gy·cm,respectively,the cumulative dose length increased by 47.3% compared with group A,The values of CTDIvol and DLP in E0 group were the lowest,which were 6.1mgy and 250.2m Gy·cm,respectively,the cumulative value of dose length was 59.4% lower than that in A group.Among them,the values of CTDIvol and DLP in B3,C3,D2 and E0 groups were relatively low,about 57.7% lower than the average level in group A,the values of CTDIvol and DLP in groups B2,C2 and D1 decreased by 12.7% compared with group A,and the values of CTDIvol and DLP in groups B1 and C1 were significantly higher than those in group A;The radiation dose of each sensitive organ in 10 groups was comparable,The difference of body surface absorbed dose of sensitive organs such as right lens,left lens,thyroid,bilateral mammary gland and gonad was statistically significant(P< 0.001);The differences of CT value,noise,SNR and CNR of each segment were statistically significant(P < 0.001).Conclusion: 64 slice spiral CT head and neck vascular imaging adopts automatic tube current modulation technology combined with iterative reconstruction technology,with the signal-to-noise ratio 0.7,tube voltage 140 k V scanning,which can effectively reduce the radiation dose of the subject,ensure the optimal reduction of absorbed dose of sensitive organs such as lens and thyroid gland,and can get lower noise image.Although the CT value and SNR of blood vessels are relatively lower,but Image quality can meet the needs of clinical diagnosis. |
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