Study On The Effect Of Positron’s Diffusion Behavior On Image Distortion By Geant4 Codes

When positrons are emitted from nuclides in PET systems,a series of physical procedures,such as inelastic collisions and diffusion,happens with electrons,phonons,and other elementary particles.Thus,the position of the positron annihilation point is away from the nuclide position,and the distance is farther for nuclides which emit higher energy positrons.This may produce serious distortion of the image.Moreover,the actual pathological tumor size cannot be observed by nuclear medicine instruments(i.e.,PET,PET/MRI).Therefore,the Monte Carlo method Geant4 is used to study the difference between distribution of positrons(size of tumor developed in MRI)and radionuclide distribution(pathologically true tumor size)in terms of source size,magnetic field strength,and source type.In addition,the effects of positron distribution on imaging in several other applications,such as the case of tumors at the border of tissues and malignant tumors(whose density is different from the surrounding medium)are investigated.Furthermore,the possibility that the detector resolution reflected in imaging is explored.The main contents and conclusions of the paper are as follows:1.Study the differences in radionuclide distribution and positron distribution.When using high-energy positron nuclide to detect small tumors,correction algorithms are indispensable.2.Study the difference between the distribution of positrons and the distribution of nuclide under magnetic field.The compression effect of the magnetic field on the distribution of positrons is not unrestrained.When the source is increased to a certain scale,the magnetic field has no compression effect on the distribution of positrons.The compression effect of the magnetic field on the positron distribution corresponding to the nuclides emitting low-energy positrons is not significant.The value of the magnetic field strength when the magnetic field effect is saturated is related to the type of nuclide but not to the source scale.This value is the same for sources with different scales.3.Study the source at the boundary(tumor occurred between tissues)of two mediums and the situation of different density inside and outside source(malignant tumor).When the density of tissues on both sides of the boundary is significantly different and the source is within a certain distance from the boundary,a pseudo-tumor area appears in the adjacent tissue.When the source is close to the boundary and its size is smaller than a certain scale,a pseudo source area also appears in the adjacent tissue.4.Study the effect of the concentration in normal tissues on imaging.Weak sources and small-size sources(i.e.,small early tumors)are not easily displayed on two-dimensional distribution maps.