Electromagnetic Tomography System for Biological Electrical Impedance Imaging
|Course||Control Science and Engineering|
|Keywords||Biological Electrical Impedance Electromagnetic Tomography High Frequency Excitation Data Acquisition In-phase and Quadrature (I/Q)Demodulation|
Electromagnetic Tomography technique is a new process tomography techniquebased on law of electromagnetic induction. Exciting primary magnetic field isproduced by injecting alternating current into excitation coils which are distributed inthe boundary of the object field. The conductive or magnetic materials to be studied,which are affected by primary field, induce new secondary magnetic field. Bymeasuring signal in the receiver coils, the conductivity and permeability informationcan be acquired, and then the spatial distribution images of the object space can bereconstructed. It has advantages of non-contact, low cost and non-hazard, and it issensitive to the center position of the object space. Magnetic coupling allows eddycurrents easily to be induced in samples by high-resistivity. It becomes an attractivetechnique for the biomedical applications compared to traditional contactelectrode-based electrical impedance tomography.In this paper, current research situation and exciting problem of the biologicalelectrical impedance imaging and electromagnetic tomography are presented in details.In view of the present low frequency EMT systems cannot be used for thelow-conductivity biological tissue, a two-channel prototype EMT system wasdesigned to operate from500KHz to10MHz. The main work of this paper can beconcluded as follow:Firstly, the screened cable was used to wind a set of balanced coils for bothtransmitting and receiving operation. The screen was grounded at one point, whichcan avoid magnetically induced currents in the screen. This approach providedeffective screening and ease of assembly. A balanced coil configuration can eliminatethe capacitive coupling between the excitation and receiver coils effectively andimprove the measurement sensitivity of the system.Secondly, because the high frequency signal is very easy to introduce noise errorand produce phase shift, a second-order active feedback measurement circuit isintroduced in the detection channel. It can use the low gain bandwidth productop-amp to achieve high phase accuracy and eliminate phase error. Thirdly, the system utilizes NI PXI-5105board to collect the high frequencysignal directly, which simplifies the complexity of hardware circuit design. By usingLabVIEW, the signal can be processed and monitored online easily. At last, the dataare transmitted to the upper computer and the in-phase and the quadraturedemodulation technique is used to acquire the amplitude and phase information of thedetection signal.Finally, based on the two-channel EMT system, the salt solution of differentconductivity is used as detection object to conduct a series of relevant experiments.The relationship between the conductivity and the amplitude and the phase of thedetection signal has been studied. And then the relationship between the phase of thedetection signal and the excitation frequency, the space position of the object and thedistribution of the receiver coils has been discussed. The weak high frequency signalacquisition and data processing have been completed. It makes solid foundation forthe future work of developing multi-channel high frequency EMT system.