Study On Single-sided Nuclear Magnetic Resonance Superficial Imaging System And Skin Imaging Experiments

The most widely used method for burn depth assessment in current clinical is that the doctors assess the burn depth rely on the experience with the naked eye,this method relies on a subjective evaluation and is inaccuracy.No suitable test equipment is available to diagnosis the rehabilitation of skin tissue after burns.The existing medical imaging diagnostic techniques such as magnetic resonance imaging(MRI),computed tomography(CT)imaging and so on are not used for burn depth assessment because of their respective limitations.Therefore,a diagnostic equipment is urgently need for quantitative assessment of burn depth and to accurately making treatment and rehabilitation plan in clinical.The single sided nuclear magnetic resonance(NMR)technology is a new kind of nuclear magnetic resonance(NMR)technology.The equipment is of small size and low cost.Its region of interest(ROI)is located at the external area of magnets and the detecting way is open.Its application has been expanded to other field from the oil industry field.The single sided MRI system with a constant magnetic field gradient in the direction of depth and produces high resolution,it is very suitable for the detection of the skin with multilayer structure.In addition,the imaging of single sided MRI can provide more information of skin,it is useful for the estimation of burn depth and recovery degree.However,due to the specific application of single sided MRI magnet system for skin burn depth assessment,some special design is need for single sided magnets with a suitable constant gradient,planar RF coil,planar gradient coil and the RF shielding,and the appropriate ways to detect the characteristic information of burns.This paper propose some design methods for the single sided magnet,RF coil,gradient coil and RF shielding based on the electromagnetic field theory.Some measurement,performance testing,model experiment and the application research on skin burn depth estimation are also introduced.The content of paper includes the following several aspects:1)Design of the single sided MRI magnet system.Two kinds of single sided magnet are designed in this paper.The constant magnetic field gradient of the first single sided magnet is low,therefore,the whole thickness of skin can be excited in one shot,but the initial magnetic field uniformity is too low.Therefore,a hybrid shimming method using mobile magnets and iron pieces is proposed based on a linear integer programming algorithm for the shimming of the initial magnet structure.The effect of the hybrid shimming method is proved by the actual shimming.The uniformity of magnetic field is also poor due to the limited length of magnet for the second single sided magnet.Therefore,a method with three bar magnet split into six bar magnet is used to improve the uniformity,and the validity of this method is proved the through the actual measurement.Considering about the poor Signal to Noise radio(SNR)of single sided MRI system caused by inhomogeneous magnetic field,the calculating method of RF magnetic field is derived in this paper according to the electromagnetic field theory,then a novel inverse method is proposed to design the planar radio frequency(RF)coil under the constraints including the field(B1)and the main magnetic field(B0)being orthogonal in vector direction and being related in the amplitude in the ROI.The validity of this method is proved the through the actual SNR testing experiment.Considering the the instability of main magnetic field and radio frequency offset maybe caused for heat conduction of gradient coil,a Fourier analysis method to quickly calculate temperature distribution of plannar gradient coil is proposed to optimize the parameters of gradient coil,according to the electromagnetic field theory,a calculating method of the target gradient magnetic field is derived and the plannar gradient coils are designed,finally the target field method is validated by the measured gradient magnetic field.2)The performance test experiment and the physical model experiment of single sided MRI magnet system.Performance testing experiment includes: the relative SNR and the dead time test experiment of RF coils,the effect test of RF shielding;the measurement of the transverse relaxation time(T2).The performance test experiments are used verified the excellent performance of the single sided MRI magnet system.The physical model experiment mainly includes: the one-dimensional layered experiment.The experiment to test the layered filter method based on FFT method,one dimensional signal measuring experiments,and 2D imaging experiment.The feasibility of layered filter method and 2D imaging are proved.3)Themeasurement of one-dimensional magnetic resonance signal and study of 2D imaging experimental for skin.A series of applied research is conducted based on the designed platform of this single sided MRI magnet system.The normal skin of the palm and forearm of men and female is measured to understand the characteristics of magnetic resonance signal of the skin on the hierarchical structure;The burned pigskin is measured to analysis the distribution of relaxation time(T2)spectrum on different depth from the magnetic resonance signal of burned pigskin with different burn depth;The burned pigskin of different burn depth is 2D imaging.The obvious difference of signal strength is observed between the parts of different burn depth and the normal controls part.Therefore,the single sided MRI magnet system designed in this paper is verified to assess the burn depth of the skin.In this paper,a comprehensive study on single sided MRI magnet system is carried out through the research work of the design theory of single sided MRI magnet system,the model experiment and practical application.Those work laid a foundation for the extended application of the single sided nuclear magnetic resonance(NMR)technology.