Research On Phase Difference Detection In Magnetic Induction Tomography

Magnetic induction tomography(MIT)imaging technology based on the principle of eddy current testing to obtain the distribution information of object's conductivity is received extensive attention in medical detection field.It can be used for non-contact measurement of tissue edema.The MIT technique detects the electrical conductivity of an object by measuring the phase change of the magnetic field.However,due to the small electrical conductivity of biological tissue eddy current effect is very weak.It's difficult to achieve the resulting in high precision phase measurement.At present,analog phase difference detection methods can be divided into time sequence method and multiplier circuit method,but the temperature drift of analog devices will lead to larger phase offset.In addition to the above methods,there are digital quadrature method,digital correlation method and other digital phase detection method.The digital phase detection method is relatively flexible,but lack of real-time and high cost.According to the technical characteristics of MIT,the lower the accuracy of phase difference detection system is,the higher the imaging quality is.But at present,most of the phase difference detection systems are usually not accurate enough.How to measure the phase difference of high frequency magnetic field with high efficiency and precision is the key problem of MIT technology.In the first part of this paper,we studied several methods of phase difference detection methods which can be used in MIT technology.It analyses their respective features through theory and experiment testing.The phase detection method includes: multiplication phase detection method,digital quadrature transformation method,digital correlation method,FFT spectrum analysis method,AD8302 method.In the course of the study,the multi-channel DDS signal generator is used to generate the two channel 10 MHz sine signal with phase difference.And there are two different routes in the phase discrimination experiment: The digital phase detection method is used to detect the phase difference of the signal;For the phase detection method,the two channel signal is directly input to the analog phase detector for phase detection.By comparing the results of each phase detection method,it is found that the Fourier spectrum analysis method has a better phase detection accuracy and a wide range of phase detection(accuracy: ±0.2 degrees,range: 0-360 degrees).This paper also analyzes and realizes the method of phase difference measurement device based on phase-locked loop(PLL).The experiments show that the phase detection accuracy can reach 0.2 degrees(10MHz),because of the dead time effect of current pump in experimental phase range.At the end of this paper,the active phase shifting phase demodulation method based on the combination of DDS signal source,high and low precision phase detector is proposed.And we employ PLL and AD8302 to realize the design of the active phase shifting phase detection system.Through the experimental test method of active phase shift phase and phase error distribution curves,phase and other phase comparison: active phase shift method not only can avoid the nonlinear region phase discriminator and low precision,but also can improve the phase range and the phase accuracy of traditional methods in the learning phase(accuracy: ±0.2 degrees,range: 0-180 degrees).The method in this paper has some reference value and application prospect.