Research On Control And Measurement Method Of Variable Interval Spin Echo Sequence For Magnetic Resonance Sounding

With the rapid development of economy and society,China has formed enormous dependence on the freshwater resources,which leads to the more and more prominent problem of water resource is,especially freshwater shortage and frequent water disasters.Magnetic Resonance Sounding?MRS?method has the advantage of directly and quantitatively ground water detection,which is widely used in the fields of groundwater assessment and disaster water detection.At present,MRS takes advantage of a single pulse sequence to measure the average relaxation time T₂^*However,when the magnetic field is uneven,T₂^*is much smaller than T₂.In this paper,aiming at the problem that the single pulse sequence can't measure T₂,we propose the transverse relaxation time detection method based on the variable interval spin echo sequence,and develop the control and measurement system for the new sequence of T₂ measurement.Based on the principle of MRS,this paper expounds the influence of the geomagnetic field inhomogeneity on the transverse relaxation process of hydrogen nuclei,and gives the relationship of T₂^*? T₂.In order to achieve high precision and high efficiency in the detection of MRS,we research on the echo time and the echo number.According to the characteristic of the exponential decay of MRS signal,the variable interval echo time is designed in the form of logarithm based on the minimum echo time temin,the maximum echo time temax and the number of spin echo.Numerical simulation experiments of different values temax and N are conducted,the simulation results show that the optimal number is 7 to 10,and the detection accuracy is higher when temax is close to or greater than T₂.Aiming to realize variable interval spin echo sequence detection,on the basis of the existing MRS instrument,the control software of variable interval spin echo sequence is designed.The PC control software based on Lab VIEW includes control parameter setting,instrument operation monitoring and other functions.The control module based on ARM+CPLD framework,can produce high precision instrument control signal.Through testing the control and measurement system of variable interval spin echo sequence for MRS,the results show that:the transmitter current frequency is equal to the Larmor frequency setting value,and the error is 0.06%,the transmission time error range is within 1.33%,the relationship between the duration of the transmitter current is ?_p180=2?₉₉₀,which satisfies the principle of the spin echo signal generation,the excitation current have the same phase,which can ensure that the polarity of the MRS signals are same,so that the MRS signal can be detected accurately.The control signal of the high-voltage relay has a 10 ms switching time,which can effectively avoid the impact of the inductive high voltage on the receiving system.Aiming to protect the safety of H Bridge,the dead time between two pairs of bridge control signals is 10.1?s,and the set value error is 0.5 ?s.The stability and reliability of the control software are verified by the test of the whole instrument.Finally,the feasibility of the proposed method is verified by the physical simulation experiment.