A Study Of The Practicality Of The In Vivo Electron Paramagnetic Resonance Tooth Dosimetry

After an unexpected radiation accident,there is a critical need to make rapid and accurate dose assessments of involved individuals for the clinical diagnosis and accident rescue.Electron paramagnetic resonance?EPR?technique can be used to estimate the absorbed dose of tooth by detecting the radicals induced by the ionizing radiation.Conventional EPR dosimetry involves the isolation of tooth enamel for measurements,which has been applied to perform retrospective radiation dosimetry.The in vivo EPR dosimetry can detect the absorbed dose by in vivo measurements without the limitation of invasive sample extraction,which is recognized as an ideal approach for rapid initial triage in the field after unexpected radiation accidents.In the previous work,our group has achieved to acquire the radiation-induced EPR signal by the X-band in vivo EPR measurements.But the accuracy of dose reconstructions is limited by some factors in the special condition of in vivo measurement.These factors mainly include the dielectric losses caused by water on the tooth surface,the volume and geometry variations in teeth and the interference from the human physiological activities.In this study,we investigated the effect of these affected factors,made corrections and established solutions that were tested by experiments to solve these limitations.Furthermore,by writing software that specially used for the measurement control and dose estimation with in vivo EPR measurements,a comprehensive correction was made to deal with the main affected factors.Finally,this work improved the accuracy of dose reconstructions effectively and made progress on the application development of X-band in vivo EPR dosimetry.The main contents and results in this study are as following:1.For the influence of water on the tooth surface,its effect was studied by finite element analysis and experiments with water films of various thicknesses on teeth.As results,the detection sensitivity of the cavity decreased with increasing thickness of the water layer.The uncertainty caused by the interference of tooth surface water can be diminished by normalization of the radiation-induced signal to the signal of a reference sample,which contains Mn²⁺/CaO and permanently present in the cavity inner.The relative standard deviation of the radiation-induced signal of the tooth was reduced from30.8%to 17.9%after the normalization.Furthermore,a physical method using the oral rubber dam was proposed for keeping the tooth relatively dry by preventing the saliva flow to the tooth of interest.These work diminished the influence of the dielectric losses caused by the tooth surface water.2.For the influence of the volume and geometry variations in teeth,considered the application in the field of radiation accident,we proposed a normalization method that was based on the tooth volume and geometry and the sensitivity distribution in the active detection area of cavity to solve this issue.A convenient and precise reconstruction method of the volume and geometry of measured tooth was established by the digital image processing with images of the tooth impression slices.The sensitivity distribution in the active detection area was measured basing on experiments with point sample.Therefore,by the weighted calculation with volume elements,a normalization process that could calibrate the evaluated dose effectively was carried out by concerning the influences not only from tooth volume and geometry but also from the non-uniform distribution of sensitivity.The effect and practicability of the method were evaluated by isolated tooth samples.The results showed that the standard deviation could be reduced about 54.8%at the most after the normalization,which was better than that only by the tooth volume.The correlation coefficient of the dose-response curve could be improved from 0.731 to 0.986.The measurement error caused by this factor has been effectively reduced.3.For the influence of human physiological activities,a special fixture was designed and manufactured to fix cavity,modulation coils and the center position of magnetic field;the stability of the measurement process was improved by optimizing the measurement posture of the subject and the position of the sweeping device,which reduced the vibration of the device and the subject in the in vivo measurement;the baseline correction and spectral screening for the spectrum from the multiple and rapid scans were used to solve the spectral invalidation caused by the baseline drift and excessive interference signal.4.Based on the effects and improved solutions of these affected factors,software that specially used for the in vivo EPR measurement was written.First,this measurement software achieved the basic functions such as magnetic field scanning control and spectrum acquisition,storage and display that required for the in vivo measurement.Then,it could analyze the affected factors to make the normalization calculations and the correction of the spectral data by the method proposed.The results show that the software can effectively combine measurements control and data calculation in the in vivo EPR measurement,which reduced the uncertainty caused by the manual data process.5.An experiment for the dose estimation by EPR tooth in vivo measurements was made with a volunteer,which preliminarily verified the feasibility and effectiveness of the results in this study.Compared with the EPR in vitro measurements,the intensity of the radiation-induced signal decreased by approximately 40%.However,with the solutions proposed in this study,the in vivo measurement process still could be maintained effectively.At present,the dose limit of the in vivo EPR measurement was about 2 Gy.At present,in vivo EPR tooth dosimetry is a novel and developed technique for EPR tooth dosimetry.As for the affected factors in the in vivo measurements,there is no generally accepted study about its effects and solutions.In this work,we have studied effects and features of these affected factors and proposed related correction method and improved solutions.The limitations of these main affected factors on the dose reconstruction with in vivo EPR tooth dosimetry are preliminarily solved,which improve the practicality of the in vivo EPR tooth dosimetry.