Development Of Electron Paramagnetic Resonance Cavity For In Vivo Tooth Dosimetry

The rescue and treatment urgently depends on quickly triages according to the victims’ absorbed doses when nuclear and radiation events occur. The enmergency dosimetry for early triage of mass people is also a key problem to be solved in radiation medicine study. The Electron paramagnetic resonance is now considered as one of the ideal methods to accomplish in vivo and quickly dosimetry. To achieve in vivo EPR dose measurement the development of microwave cavity that can work in oarl is the most pivotal issue.Conventional EPR tooth measurement is only available in retrospective dosimetry applications. Its sampling process is invasive and it can’t access the dose quickly in the early period after an accident, which limits it’s widely applied. Microwave cavity is the pivotal component of in vivo EPR spectrometer. The design of the cavity affects the implement scheme of in vivo measurement and detection sensitivity. In this work, X-band microwave cavities of special structure which had detection apertures on their walls were developed. Microwave magnetic field could leak into the aperture from the cavity; the scan magnetic field and modulated magnetic field excited by modulation coils could apply into the aperture at the same time; the microwave cavity would be inserted into patient’s mouth, and then the incisor cusp of the patient could bite into the aperture, therefore the EPR conditions were fulfilled and the spectra with radiation induced signal (RIS) could be acquired. These cavities were able to perform in vivo tooth measurements and consequently access the irradiated doses of the tooth.The major contents and results of the study contain:1. Three kinds of X-band EPR cavities in different modes for in vivo tooth dosimetry were designed. TE101 rectangular mode, TE111 and TM010 cylindrical mode cavities were regarded rational for in vivo purpose. All the above modes had such an area near the cavity metal wall where the microwave magnetic field energy concentrated while only very weak electric field energy distributed in the same area. When a detection aperture was opened in this area, intense microwave magnetic field would leak into the aperture without significant microwave power loss. Numerical calculation and finite element simulation were carried out in the process of designing.2. Cavities of different modes and structures were manufactured. Serial cavities of different structures and modes were manufactured according to the design. The concerned factors included microwave mode, cavity volume, geometry of the aperture, coupling between cavity and microwave bridge, modulation magnetic field application. The final optimized cavity structures were:The dimension of TE101 cavity was 22.8×10.1×21.4 mm, and its aperture was in the center of the narrow side wall. The dimension of TE111 cavity was 22.0(27.0) mm in diameter and 28.2(21.2) mm in length, and its aperture was in the center of the cylindrical wall or on boundary of the cylindrical wall and the circle plane wall of the cavity. The dimension of TMOio cavity was 24.0 mm in diameter and 12.0 mm in length, and its aperture was in the center of the narrow side wall.3. Feature experiments and comparison of different cavities. The physical parameters measurements, DPPH measurement and intact tooth measurement experiments were carried out on the developed cavities. The cavity dimension, resonant frequency, resonant quality factor Q and coupling condition were surveyed. The microwave power and modulation feature of the cavities were acquired by DPPH measurements. The feasibility of in vivo tooth measurement was estimated by intact tooth measurements. The resonant frequency of the cavities was between 9.50～9.60 GHz. The loaded resonant quality factor Q was between 2500～3500. The signal of sample could be acquired when the microwave power was between 0.1～200 mW and the modulation amplitude was between 0.1～1mT. The comparison of three cavities of different modes implied that:(1)TE111 mode cylindrical cavity was higher in quality factor Q, more convenient for incisor’s biting and easier for modulation magnetic field application. But it was large in volume, and had a degenerate mode at the same frequency. (2) The microwave modes of TM010 cylindrical cavity and TE101 rectangular cavity were innovative for EPR spectroscopy, and TMoio cylindrical mode was simple, stable. But the structure of TM010 cylindrical cavity was complex inducing relatively difficulty of manufacture process. (3) The structure of TE101 rectangular cavity was very simple. It was easier to be operated in human mouth because its volume was smaller. The leaking efficiency of microwave was higher, which improved the detection sensitivity. But the background noise signal increased when the microwave power was higher.4. Intact tooth measurements. The human teeth were separately irradiated by doses of 0-8 Gy. The teeth were fixed with gum model when measured. The signal noise ratio (SNR) of the spectra was analyzed to acquire the curve of SNR against doses. The SNR was increased linearly as the radiation dose increased followed by a calibration process of the volume and quality of each tooth. The linear correlation coefficient was 0.9884. The detection limit dose of intact tooth measurement was 1 Gy within 30 seconds scan, which was sufficient for triage situations.5. The feasibility of in vivo dosimetry was verified by Animal and human in vivo experiments. The rats and monkey was partially irradiated around mouth. Radiation induced signal from 5-20 Gy irradiated teeth of rats and 2 Gy irradiated tooth of monkey was acquired. Human tooth signals were also obtained by in vivo measurement. The experiments implied that:(1) The cavity and its coupling could performe well when there was water in mouth. (2) The background noise increased to a small extent by the activities of heartbeat and the breath and could be reduced by shorten scan time and accumulation scaning. (3) The in vivo measurement could perform under slight vibration of human body. The SNR decreased by less than 20% for in vivo measurement comparing to in vitro measurement.Conclusion. In vivo EPR tooth dosimetry method was established based on the design of X-band microwave resonant cavity. Three kinds of cavities of different modes and apertures were designed by numerical calculation and simulations. The microwave modes were TE101 rectangular, TE111 cylindrical and TM010 cylindrical cavity. The modulation excitation coils were separated from the cavity, which simplified the cavity structure and improved the modulation efficiency and cavity resonant quality factor Q. TM010 cylindrical cavity and TE101 rectangular cavity were innovative design in EPR spectroscopy and no reports were found before. They had the advantage of stable in microwave mode and sensitive in detection. TE101 rectangular cavity was simple and small therefore its aperture and volume could be adjusted according to the needs of in vivo measurements. The characteristics of cavities were evaluated by physical parameters measurements and experiments of DPPH and intact teeth measurements. The feasibility of in vivo dosimetry was evaluated by experiments of animal and human in vivo measurement. The in vivo dosimetry was available and radiation induced signal could be acquired by special designed cavity even there was water in mouth and the vibration caused by breath, heartbeat and other conditions. The preliminary results suggested that the performance features of developed cavities had basically fulfiled the demonds of EPR in vivo dosimetry and early dose evaluation of people involved in nuclear and radiation events.