Research On Several Key Problems Of Neutron Stimulated Emission Computed Tomography

Neutron Stimulated Emission Computed Tomography(NSECT),a newly developed molecular imaging technique,can measure the trace element concentrations,similar to magnetic resonance imaging(MRI),which has been proven successful for a limited set of isotopes and the response of these isotopes can be strongly influenced by their molecular binding.Using neutrons to stimulate characteristic gamma emission from atomic nuclei in the body,spatial projections of the emitted energy spectra allow tomographic image reconstruction of the elemental concentrations.NSECT can be pictured as a modification of conventional emission computed tomography(ECT) where the gamma emissions are not from naturally radioactive isotopes(as is conventional),but instead are from stables that have been stimulated tO emit characteristic gamma photons through inelastic scattering of an external neutron beam.Earlier studies demonstrated a significant difference in trace element concentrations between benign and malignant tissue for several cancers. Through NSECT,the spatial distribution of elemental concentration in the body can provide the molecular process,which can be used to identify cancer by its change in elemental concentration long before it has begun to cause the anatomical changes.NSECT has several unique advantages:1) Any isotopes,stable or radioactive,can be imaged(with the exception of hydrogen or helium).2) Neutrons are highly penetrating particles and they can image structures deep within the body that cannot be reached using most other probes.3) NSECT can obtain true 3D maps of chemical isotopes,by using detector geometries that yield 3D information.However,it is difficult to obtain energy and position information at one time due to high energy of the characteristic gamma photons,which causes the existing technology cannot tomography well.In addition,there are a serial of other problems for attention,such as the large background noise,the neutron irradiation effect,et al.The key point of the NSECT technology is the design of the imaging detector. Due to the special nature of neutron interaction with the material,the complexity of the neutron transport process,and the characteristic gamma photons with emitting angles throughout the space,the Monte-Carlo simulation methods have been widely adopted in practical researches.In view of several key issues of NSECT technology, especially the contrary problem of spatial and energy resolution in detecting high-energy gamma photons,this thesis introduces a great deal of work using Geant4 simulation toolkit.Main results are as follows:1) After analyzing the characteristic of neutron source for NSECT,we proposed a conception of optimal incident neutron energy,according to the truth that the incident neutron energy differences will produce different nuclear exciting reaction and the probability of nuclear reaction.In Geant4 simulations,the energy range of the characteristic photons has been obtained through analyzing the energy spectrum achieved by stimulating the elements using incident neutrons with optimal energies. The results indicate that optimal energy for different elements is varied and the energies of characteristic gamma photons of all the elements are mainly in the range of 0.1～7 MeV.There are some similar characteristics for photons in this energy range, such as large cross-section of Compton Scattering.All these results will provide some guidance in our future work,such as achieving optimal detector material and installing configuration.2) The possibility of traditional array detector to obtain both spatial and energy resolution of high-energy gamma photon has been presented through simulation.As the results show,when using signals form a single BGO crystal it is impossible to achieve good energy resolution for those high-energy photons.When summing additional signals from adjacent crystals,the energy spectrum becomes much better. Using an energy window with energy resolution of 90 keV,an average intrinsic spatial resolution of 3.938 mm FWHM is obtained.3) For its advantages in the field of detection and imaging,the potential possibility of plastic scintillating fiber applied in the NSECT technology has been discussed.As the characteristic under high-energy gamma radiation show,plastic scintillating fiber has limited application in high-energy photon detection,because of the severe energy-leakage and cross-talk.That is,the fiber array is impossible to directly contribute in NSECT technology;however,it should provide a meaningful role in guiding the detector installation.For example,it is possible to carry out a certain degree of n /γidentification through the energy deposition characteristics in the scintillating fiber for photons with different energy range.When interacted in the scintillating fiber,there exist different characteristics for electrons and photons.According to this difference and also the Compton-Scattered electron spectrum,We proposed the idea of measuring high-energyγphotons using big-diameter scintillating fiber.There is a large cross-section of Compton Scattering in the energy range,and the energy deposition of high-energy photons in scintillating fiber is mainly contributed by the Compton-Scattered electron.This characteristic will play the role in developing a similar Compton-Camera for high-energy photon detection in future.4) Through Geant4 simulation,the characteristics of recoil proton in plastic scintillating fibers irradiated by fast neutrons have been presented.The energy and angular distributions of recoil proton have been analyzed and optimal radius of scintillating fiber has been found out with different incident neutron energy.As the results show,the energy of recoil proton varies from zero to incident neutron energy. The recoil proton energies vary inversely as angles,that is,the protons along the direction of incident neutrons have big energies and that along the fiber radius have relative small energies.All these results should lay the foundation for fast neutron absorption imaging based on plastic scintillating fiber.5) The varieties of different parameters have been studied in single fiber and fiber array system.The possibility of improving detection efficiency and sensitivity by loading a synchronous neutron absorption imaging system in the NSECT detection installing has also been discussed.The detection efficiency is higher than that using traditional fluorescence screen,because of the scintillating material mainly composed of hydrogen and oxygen,which have large cross-sections for fast neutrons. Furthermore,the efficiency can be improved by increasing the fiber length,which basically does not change the spatial resolution of the image.We also investigated the approach of coating fiber with metal layers,which indeed would play a positive role in restraining cross-talk and improving spatial resolution.However,to increase the thickness of the metal layer,the overall resolution would be further reduced.It is consistent with the previous conclusion for high-energy X-ray imaging.