Computational Models For Estimating Patient Doses From Diagnostic X-Ray Examinations

Purpose With the rapid development of medical care,new radiological technologiesand new equipments have made medical X-ray procedures much more popular.Therefore,more and more patients are involved in the X-ray diagnostic examinations,which lead to the collective dose keeping increasing.As a basis of radiation protection,the radiation doses from various X-ray diagnostic procedures has become animportant research topic in the fields of radiation medicine,radiation protection,medical physics and other subjects.The conventional methodology based on fieldmeasurements was too high a price for evaluating the collective dose from X-raydiagnosis,and thus it is difficult to be widely spread.Based on the comprehensiveinvestigation and analysis of the related studies,the aim of this study was to exploreand establish some corresponding computational models based on the equipments andexposure parameters of the main types of X-ray diagnostic procedures,and to providea new methodology for convenient assessment of the collective dose from medicalexposure.Methods According to the characteristics of different X-ray diagnostic procedures,the main concern of this study are the conventional X-ray radiography,digital X-rayradiography and X-ray computed tomography,as they are more widely used or rapidlyincreasing and higher dose contributors.(1) For the conventional X-ray radiography,the air kerma of X-ray output was experimentally quantified with the factors of tubevoltage and tube current and exposure time product (mAs) first,then the correction ofthe total filtration and generator type was made,having calculated the back-scatteringfactors (BSF) under different exposure conditions by using Monte Carlo method,amathematical model for estimating patient dose including the entrance surface dose(ESD) and effective dose was built and a software was programmed.(2) For thedigital X-ray photography,the relationship between the ESD and the exposure indexwas theoretically qualified first,the parameters in the qualified formula wereexperimentally determined,having considered the influence of tube voltage andenergy response of the detector (Imaging Plate) on the parameters mentioned above,a mathematical model was established for estimating the ESD from the exposure index(3) For X-ray computed tomography,based on the most advanced voxel phantomsand CT models,a organ doses database of for each slice scan in reference conditionswas built first,having corrected the influence of scanner type,tube voltage,mAs,pitch factor and collimator width on the radiation dose,a computational model forcalculating both organ doses and effective doses was built.Furthermore,anothermethod was also established for calculating the effective dose by multiplying the doselength product (DLP) and the corresponding dose conversion factors,and softwarewas also programmed according to this method in this study.The DLP was evaluatedbased on the database of various technical conditions and parameters of CT scanning.In this study,experimental verifications of the computational models were mainlycarried out through the in-phantom measurements with the radiophotoluminescentglass dosimeters (RPLGD).The ESD was directly given,and the organ doses wereestimated from the measured value of RPLGD.New tissue weighting factors wereused to calculate the effective dose with the measured organ doses.Results (1) A new mathematical model was built for estimating the ESD andeffective dose from conventional X-ray radiography by using technique parameters,including the tube voltage,mAs,total filtration,focal skin distance,generator typeand exposure field.Experimental verifications showed that the deviation between thecalculated ESD and the measured ones ranged from -12.1% to 4.5% and the deviationof the effective dose were in -29.2%～31.4%.(2) A mathematical model wasestablished for calculating the ESD with the exposure index of digital X-rayphotography.The experimental verifications showed that the deviation between thecalculated ESD and the measured ones ranged from -22.1% to 19.2%.(3) Twomethods were established for estimating the patient dose in X-ray computedtomography.For the method based on Monte Carlo simulation,experimentalverifications showed that the deviation between the calculated organ doses and thein-phantom measured ones were in a ranged of-17.7%～32.2%,and the deviation ofthe effective dose ranged from -9.8% to 5.5%.For the method based on the DLPmodel calculation,experimental verifications showed that the deviation of effectivedose ranged from -10.8%～13.3%.Conclusion Compared with the generally acceptable error in the field of radiationprotection dosimetry,all computational models built in the study are basicallysatisfied for estimating patient doses in the diagnostic X-ray examinations.The above-mentioned computational models,built with the working conditions andtechnical parameters in the three categories of X-ray diagnostic equipment,provideconvenient ways for large-scale surveys of patient dose level and for assessment ofthe collective dose from X-ray diagnosis.They have a high value for extendingapplication,and they also possess great theoretical significance for the developmentof relevant disciplines.