Study Of Electrical Impedance Tomography Technology For The Monitoring Of High Intensity Focused Ultrasound Therapy

High Intensity Focused Ultrasound(HIFU)is a technology for tumor treatment.Clinical ultrasound is focused on the nidus,and the tumor ablation can be achieved for heat effect and cavitation effect.Then macrophages in organism clear away the dead cells to achieve the tumor ablation without hemorrhage.There is relationship between cell death,ultrasonic power,treatment time and tissue condition.So,a reliable non-destructive monitoring tool is the necessity to monitor treatment effect and guide the focal region.Nowadays,Computerized Tomography(CT)and Magnetic Resonance Image(MRI)are the common tools in the medical field.But either of the techniques has a radiological hazard or needs a cumbersome facility,which makes them not proper tools for real-time monitoring HIFU surgery at the bedside.Electrical Impedance Tomography(EIT)technology is a detecting technique based on electromagnetic field theory.The conductivity and permittivity inside the target can be obtained by measuring the transfer impedance on the boundary and reconstructing the image with a proper algorithm.Some physiological information can be obtained by the relationship between the electrical parameters and pathological/physiological statuses.EIT is a visual non-destructive technology,which has the advantages of non-radiation,non-invasion,high speed,low cost,and relative small facility.The heating process leads to changes of tissue property during thermotherapy.And EIT is sensitive to changes of tissue impedance caused by changes of tissue property.Therefore,EIT has potential to be a monitoring tool for HIFU therapy.The independent measurements of EIT are less than the common tools like CT or MRI.For some medical applications,the necessary spatial resolution cannot be achieved.The natural ill-posed condition of EIT may lead to failure of reconstruction because of small noise and assembling error of the electrode array.To solve these problems,a broadband high-performance data acquisition system and a good reconstruction algorithm are the key issues for medical applications of EIT.This thesis is a research on the possibilities of EIT as a monitoring tool for HIFU therapy.First,there is a brief review of the HIFU monitoring and existing problems.Then,the principles and the problems of low resolution for HIFU monitoring are discussed.A method to improve the resolution of the reconstructed image has been presented,and a broadband high-performance distributed prototype system has been designed for biological information detection on the bedside.Finally,some heating experiments of in-vitro tissues have been studied to study the relationship between tissue resistance and temperature.The feasibility of EIT on HIFU monitoring has been verified.The research was financially supported by the National Natural Science Foundation of China(Grant No.61371017).The major contributions of this thesis include:The thesis has presented the algorithm on EIT image reconstruction.And a method called Restricted Region of Interest(RROI)has been proposed to improve the image resolution for those applications that has only parts of the conductivity changes in the sensitivity field.The method is used to extract the region of interest(ROI)to enhance the information of the conductivity change and improve the resolution and precision in the ROI.Besides,it can decrease the ill-posed condition and the dimension of sensitivity matrix by deleting the unchanged elements in the sensitivity field.Therefore,the reconstruction needs less calculation time,and the computational efficiency will be increased.The author has established a high performance,wide spectrum,multi-channel parallel EIT data acquisition system(DAS)for different purposes of medical process monitoring.The thesis has presented the analysis of the influences of error sources and parasitic capacitance for signal measurement.And the innovation and optimized design of the modules has been explained thoroughly,and a prototype system has been accomplished.The resistor networks and tank experiments have been tested to verify the performances within the spectrum 1kHz – 1MHz.With user-configurable amplification factor of signal condition circuit,the system has flexibility for different targets.The performance of high-frequency range has been improved by the distributed structure design.The independent controllers on front-ends offer the expandable ability of measurement channels.The performance of the system has been verified by resistor networks and tank experiments.The feasibility and safety on medical purpose have been tested on volunteer breathing experiments.The experiments of thermotherapy process monitoring have been performed.The author has built a comprehensive experimental platform.With the developed comprehensive control graphical user interface(GUI)by MATLAB,the platform can control the heating source,data acquisition instrument,and DAS synchronously,and display the result on real-time.Some heating experiments of in-vitro pig tissues and fresh mice livers have been studied to study the relationship between bioimpedance and temperature with the developed system.The correlativity of tissue resistance and temperature has been verified and the sharp change caused by lethal temperature of the tissues has been observed.This feature may be an evidence to test the treatment effect of HIFU.Compared with MRI and CT,EIT has the advantages of relatively low price,safety,and fast reconstruction speed.It is hopeful as an auxiliary monitoring tool for HIFU thermotherapy.In the end,the innovative achievements of the thesis have been proposed.Problems and suggestion have been given to improve the following research.