Study On The Detection Of Permittivity And Conductivity Of Brain Based On Microwave

In neurological settings, intracerebral hemorrhage(ICH) and cerebral edema are commonly happened critical condition, and the delayed therapeutic cure may result in seriously consequence. To date, the conventional recommended techniques for brain detection, such as CT, magnetic resonant imaging, etc, are featured in high resolution. However, these techniques require expensive equipment, specialized personnel, substantial time, and even require the patients to be transported to the radiology suite, which may lead to uncomfortable and risk for patients. As a result, in clinic it is a demand for a noninvasive, low cost, easy to operate, portable device, which is able to set on the ambulance for prehospital detection and for continuous monitor.At present, the promising technology in the field for noninvasive detection of brain, are near infrared spectroscopy(NIRS), electrical impedance tomography(EIT) and microwave imaging. But it is difficult for NIRS to penetrate deeply inside the brain; the electricity is hard to penetrate the skull lead to low sensitivity; due to multi-reflect and scatter may occur in the brain, the microwave imaging requires a set of high sensitive antenna array to acquire transmitted microwave signal, and the complex algorithm and substantial computational power and time, may hinder the development of microwave imaging. These defects are why these technology are not applied in clinical application.Although microwave imaging is restricted by the defects, such as algorithm and computational power, microwave is specialized in high frequency resolution, high sensitivity, and high precision. The detection technique based on microwave is able to evaluate permittivity and conductivity of brain. Many researches demonstrate that the permittivity of hematoma is higher than normal tissue, which means high contrast between hematoma and brain, the conductivity of brain edema is clearly different from the normal brain tissue, which is theoretical fundament for using microwave to detect ICH and brain edema. Besides, permittivity and conductivity of biological tissue contain important physiological and pathological information, so detection of dielectric properties expand human horizon for brain diseases.Given the advantages and disadvantages of microwave, this thesis proposes a novel brain detection method based on microwave, which detects the overall permittivity and conductivity, with the support of the national natural science foundation in key project(grant No. 11532004). And the method avoids the complex reconstruction algorithm and substantial computational time, take the advantages of high sensitivity and high precision. The research steps as follows:(1) Design coaxial cable coupled to open-ended circular waveguide as sensor, large size waveguide can cover the brain, all the reflected and scatter signals can be acquired, which may increase the result precision.(2) Using the software HFSS to do finite element simulation, optimize the structure and size of inner cable in the waveguide. The simulation results can demonstrate the feasible of detecting the permittivity and conductivity of brain using this method, and it also gives some instruction for further research.(3) In order to evaluate the sensitivity and precision of this device, we proposed a new ICH and brain edema physical model, which can simulate the permittivity and conductivity of the brain based on 3D print technique.(4) After derivate the theoretical equations to calculate the relative permittivity, and utilize the proposed device to detect the physical model. Evaluate the sensitivity and precision of the proposed method for detecting permittivity, through comparison of experimental results and theoretical ones.(5) Derivate the theoretical equations for compute the conductivity of brain, then employ the proposed device to detect the physical model. Evaluate the sensitivity and precision of the proposed method for detecting conductivity of brain, through comparison of experimental results and nominal ones.The main research findings of this thesis are as follows.First, a novel detecting device based on microwave is proposed to evaluate the permittivity and conductivity of brain. At present, the conventional methods to detect the permittivity and conductivity of brain are transmitting method and reflecting method, but the transmitting method only analyze the object qualitatively based on the change in amplitude and phase; and the reflecting method only detect the local part of tissue under the skin, is not able to evaluate the overall dielectric properties of brain. The experimental device is consist of network analyzer, coaxial cable and open-ended circular waveguide. The space between the waveguide and brain is filled with a matching medium, which is able to minimize the reflection on the surface of brain. Microwave couples into circular waveguide through coaxial cable, and we can explore the interaction between brain tissue and electromagnetic field. Using software HFSS to do finite element simulation, make sure the proposed method is able to detect the change in permittivity of brain, and optimize the structure and size of inner cable inside the waveguide in light of the simulated result.Second, a novel physical model based on 3D print technique, which is able to simulate the dielectric properties of ICH and brain edema. At present, the conventional physical models all are homogenous, which is difficult to simulate the complex structure and geometry of brain. And the animal models, which have the same size as human brain, are disadvantage of expensive, hard to make, unable to manipulate the parameters of models. This thesis proposed a physical model by 3D print technique, which is able to simulate the features of brain as multilayer and inhomogenous, and also can simulate the geometry and structure of grey matter, white matter and hematoma or edema. The advantages of the proposed physical model are convenient to make, low cost, and can manipulate the parameters and structures of different tissue accurately. Moreover, this physical model can be used in the electromagnetic research field of detecting brain. This thesis adopt this physical model for estimating the sensitivity and precision of the device.Third, two permittivity calculating algorithms are proposed related to the proposed device. The first method is on the frequency domain, it is easy to observed that with the increase in the volume of hematoma, the resonant frequency shift toward the left, from the S11 amplitude waveform versus frequency. And we can get the relative permittivity of brain by the resonant frequency. The second method is on the time domain, the S21 waveforms versus time of flight demonstrate that the time between the first two wave crest changes, which means the group velocity varies, with the change in permittivity of brain. Due to the distance the microwave propagate in the waveguide for all different models, the relative permittivity of brain can be estimated by the relationship between permittivity and group velocity.Fourth, two conductivity calculating algorithms are proposed related to the proposed device. The first method is on the frequency domain, with the edema growth, the conductivity will increases, the electric loss will also increase. From the S21 amplitude waveform versus frequency, it is observed that with the increase of conductivity, the amplitude of resonant peak will decrease, and the peak will become smooth. The conductivity can be computed by the relationship between loaded quality factor and conductivity of brain. The second method is on the time domain, derivate the relationship between conductivity and amplitude of second crest peak.At last, utilize the proposed device and algorithm to detect the ICH and edema models, and it is demonstrated that experimental results match theoretical values through comparison. The error of the permittivity calculated by the frequency domain algorithm is less than 0.645%, the error of conductivity calculated by the time domain algorithm is less than 1.821%. Moreover, the proposed method is capability of detecting 1 ml change in volume of hematoma, which shows this method has a high precision. The experimental results testify the high sensitivity and precision of the proposed device.