Measurement Method Of Magnetic Induction Tomography For Detecting Brain Edema

The technique of brain magnetic induction tomography is a new modality ofmedical imaging and one frontier direction in international biomedical engineering field.The study of the technique, which is contactless,noninvasive and imaging monitoring,would appears meaningful for solving the problem in the exsiting techniques ofdetecting brain edema. In this dissertation a introduction of the classification,pathological and physiological mechanism and detecting method of brain edema, andthe electromagnetic characteristics of biological tissues is given first. Then themeasurement system, simulation method and key techniques of brain magneticinduction tomography are reviewed. The work is focused on the investigation of singlechannel MIT measurement system for detecting brain edema from theory andsimulation to experiment system and can be summarized as follows:1. A prototype MIT measurement system built by us, which was a single channelmeasurement MIT system for detecting brain edema, is introduced, including itsdesign principle, its composing parts and measurement mode. The measurement resultsof this system , when the location and conductivity of objects detected was changed, isshown and its performance is evaluated. The measure to improve the performance of thesystem is mentioned after the shortcoming of the system is found through analyzing theexperiment results.2. Based on the theory of static electromagnetic field the electromagnetic relationsbetween the conductivity of objects detected and the current and phase of the detectingcoil are deduced for the measurement of the single channel MIT system. The changes ofthe current and phase of detecting coil for different brain tissues, like skull,cerebro-spinal fluid, gray matter, white matter and edema, are estimated. The calculatedresults are the base of the system design.3. According to the theory of time harmonic electromagnetic field, the analyticalsolution to electromagnetic problem of the measurement of the single channel MITsystem is given for studing the electromagnetic characteristics of the measurement ofthe system with time harmonic electromagnetic field. The head is simplified to a two-layers conducting sphere and the system is approximated to an axial symmetricelectromagnetic model consisting of excitation and detecting coils and the conductingsphere. Using magnetic vector potential the boundary value problem of the timeharmonic electromagnetic field is established and solved. The study can provide ananalytical method for analyzing and calculating the sinusoidally time-varyingelectromagnetic fields in our system and a theory base for design and improvement ofthe MIT system.4. To further optimize and improve upon our MIT experiment system,based on thetheory of time harmonic electromagnetic field, the simulation study of the idealphysical model of our system of MIT is carried out with finite difference time domain(FDTD) method. According to antenna theory, an excitation coil similar to a helix ismodeled as a stack of electric and magnetic sources. The current phase difference of thesingle channel measurement between the excitation and detecting coils is analysed withdifference ideal spherical models of the human brain including skull, cerebro-spinalfluid, brain tissues and an edema. The phase differences are calculated for differentbrain tissues models when using our coaxial coil system at 10 MHz. The simulationstudy can provided a new simulation method of MIT and more reliable reference data5. According to the calculation method and FDTD simulation model consisting ofthree layers brain tissues and excitation and detecting coils, as are mentioned above,current phase changes of the detecting coil are estimated as space location, size andconductivity of edema are changed, to obtain the sensitivity of the detecting coil tochanges of edema and determine the detectability to size and conductivity of edema.This results can suggest the capability of MIT system and are a great help to theconstruction of optimal imaging algorithm of MIT system.6. Based on our single channel MIT system which was built before, the choice ofthe excitation frequency is discussed to detect intracellular and extracellular brainedema according to the conductivity and permittivity change of biological tissues andthe security of biological effects of electromagnetic field when the frequency increasesor decreases. The reference signal is obtained directly from excitation source and differsfrom the built system. A crystal oscillator with high frequency stability is used as thegenerator of 10 MHz sine wave and a passive phase detector with high linearity isadopted to get higher precision of phase detection. A new design of single channelMIT system with higher performance is proposed...