| Background and objectives: Cerebral ischemia is a severe clinical disease in which a certain region of the brain is deprived of oxygen and nutrient-rich blood and will cause severe neurological symptoms.Acute cerebral ischemia with the largest proportion in cerebral ischemia is featured with acute onset,complex pathogenesis,and high incidence of disability and mortality.In recent years,brain stroke has become the first major cause of death among all diseases,the mortality in China is the highest in the world and is rising at an annual rate of 9%.Brain stroke has brought severe economic burdens to entire society and numerous families.Brain stroke formed by cerebral ischemia is the first major types of stroke,accounts for about 80% of stroke.Hence the early detection and real-time monitoring is the key to earlier diagnosis and treatment of cerebral ischemic stroke.The most effective way to reduce the high mortality and high disability of cerebral ischemia is to achieve early diagnosis and early treatment.Currently,the detection of brain ischemia mainly relies on large and expensive imaging equipment such as CT,MRI and PET.But these large bulky and expensive equipments are unable to satisfy the requirements of early diagnosis and continuous monitoring,and cannot meet the clinical needs to some extent.Hence there is a need to develop a non-contact,miniaturization,low cost equipment which can realize the rapid detection and continuous monitoring of brain ischemia.Thankfully,the device based on Magnetic Induction Phase Shift(MIPS)detection method has the potentials to meet these requirements and to be developed into a new detection means of cerebrovascular disease.However,the MIPS detection sensitivity is rather low due to the very low electrical conductivity of biological tissues and the very limited frequencies of excitation signal,so we proposed Magnetic Induction Phase Shift Spectroscopy(MIPSS)detection method.MIPSS detection method is to expand the frequency of the excitation signal to a certain frequency wideband,and then we can make the judgment of the pathological physiology status of brain tissues by analyzing the relationship between the frequency of wideband and MIPS.In this paper,we established a detection system based on MIPSS technology to receive the MIPS information of frequency bands and to provide information of the detection and diagnosis for acute cerebral ischemia.The work of this paper mainly includes the following two parts.Part one: We developed an acute cerebral ischemia MIPSS detection system based on radio frequency network analyzer(RFNA)according to the research needs.MIPSS detection system mainly includes RFNA,sensor coil,intracranial pressure monitoring,multi-channel physiological signal acquisition instrument,transmission lines,fixing device,etc.We used Agilent E5061 B model's RFNA as the signal source,the data acquisition and processing unit,the display unit of detecting system,to provide the excitation signal and internal phase discrimination.Designing and winding two different structure sensor coils according to the head size of experimental animals,one was a parallel coaxial single incentive-receive coil(single coil-coil),and the other one was a circular single coil.The two kinds of structures sensor coil we designed were both able to achieve the detected signal,but the electromagnetic field distribution and S parameter curve of two sensors were significantly different.In the process of measuring,we analyzed transmission parameter S21 and reflection parameter S11 when used parallel coaxial single coil-coil sensor,and analyzed reflection parameter S11 when used the circular single coil sensor.We designed the two coils for acute cerebral ischemia MIPSS detection system to compare the differences of experimental results caused by sensor coil,and to provide guidance for sensor optimization in future study.Intracranial pressure(ICP)data acquired by intracranial pressure monitoring was used to the subsequently MIPSS data analysis.Multi-channel physiological signal acquisition instrument was used to collect the ECG,heart rate and EEG of experimental animals and to verify the validity of the acute cerebral ischemia animal model.Transmission line was mainly used to transmit the excitation signal,transmission signal and reflection signal.Fixing device was used to fix the head and body of experimental animals into the same position to ensure the consistency and stability of the experiment.The MIPSS detection system with four independent measurement channels could not only measure the transmission characteristics but also measure the reflection characteristics of the object under tested,and the data format could be divided into the amplitude,phase,impedance chart,real part,imaginary part.The frequency range of detection system was 100KHz-3GHz,scan time was 25ms(1000 point,IF bandwidth of 10KHz)and the phase detecting precision was 0.01°which could completely meet the needs of MIPSS detection experiments of acute cerebral ischemia.Part two: Based on our MIPSS detection system an experimental study was conducted in rabbit on acute cerebral ischemia.We measured 25 rabbits with parallel coaxial single coil-coil,and 15 rabbits with the circular single coil.The experimental animals were both divided into experimental group and control group.The experimental group was divided into unilateral carotid artery ligation group(unilateral group)and bilateral common carotid artery ligation group(bilateral group).In the experiments the measuring time was about 1 hour and the measuring frequency range was 100 KHz to 500 MHz.MIPSS data were analyzed within the selected optimal frequency range,which had higher sensitivity and better stability of MIPSS and was defined as a feature band(FB).The frequency with the largest amplitude(absolute value)of S parameters was defined as characteristic frequency(CF)in FB.We considered the concept of the cut-off frequency of the filter when we selected FB,so the cut-off frequency of FB was the frequency when the amplitude was equal to the largest amplitude(at CF)of 0.707 times.In this paper,the MIPSS were all calculated with time-difference method,namely the phase differences were obtained before and after different time periods under the same condition and backgrounds.We used the ICP data and the EEG data in the experimental process as a reference data,so as to explain the measurement MIPSS results on histopathology level and to verify the validity of acute cerebral ischemia model of animals.Conclusions:1.MIPSS detection system based on RFNA had the ability to realize rapid data acquisition,high accuracy phase discrimination and to extend the frequency range to GHz which could totally meet the needs of our experiment study.2.Detecting acute cerebral ischemia model of rabbits by MIPSS detection system,the results showed the MIPSS detection sensitivity had high frequency-dependent properties.It should be noted that the MIPS at CF had the highest detection sensitivity and the greatest stability,and the MIPS data at CF had a greatest change with ischemia time and ischemic degrees.The MIPS data at the CF gradually increased with the ischemia time in an approximate linear relationship.And the MIPS data at CF had significant statistical difference between different ischemic degrees(control group,unilateral group,bilateral group),and increased with the ischemic degrees.3.The ICP data,EEG data showed the closely consistency with the measurement MIPSS data,which demonstrated the validity of MIPSS results and illustrated the feasibility of MIPSS detection system for measuring acute cerebral ischemia of rabbits.It also proved that the MIPSS detection system had great potential for clinical detection of cerebral ischemia. |
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