| As a key technology of surgical navigation system,electromagnetic tracking has the characteristics of no vision occlusion and easy integration with surgical instruments,and has been gradually applied in surgery and interventional therapy.The way it works is that alternating excitation fields generated by a transmitting coil,like a human fingerprint,correspond to position in space.Electromagnetic navigation can be realized by calculating the correlation between the measurement value of sensor and the distribution of excitation magnetic field.However,according to Faraday’s law of electromagnetic induction,when a metal surgical instrument is placed in a changing excitation magnetic field,eddy current will be generated inside the conductor,and the eddy current caused by magnetic interference will cause positioning errors.Aiming at this problem,the compensation method of eddy current magnetic interference is studied in this paper,and a set of anti-interference electromagnetic tracking system is developed to improve the electromagnetic positioning accuracy.The main work is as follows:(1)Aiming at the eddy current magnetic field caused by metal surgical instruments,an eddy current interference compensation algorithm was proposed to improve the electromagnetic tracking accuracy.The eddy current magnetic field generated by metal is modeled under the condition of alternating excitation magnetic field.Theoretical studies show that there is a proportional relationship between the amplitude of eddy current magnetic field and excitation magnetic field when the excitation frequency is fixed.An eddy current magnetic interference compensation algorithm was proposed based on this characteristic: the ratio of eddy current magnetic field to excitation magnetic field was obtained by preoperative calibration,and the eddy current magnetic field was calculated by using this value and magnetic field measurement value during operation,and the interference compensation was carried out.(2)Developed a set of high-precision,fast and efficient electromagnetic tracking hardware system.The system consists of transmitting,receiving and core control modules.The transmitting module is composed of digital-to-analog converter,matrix switch and coil array,among which matrix switch realizes channel multiplexing function and improves system efficiency.The receiving module is composed of a 5-DOF magnetic sensor and A/D converter,and a phase-locked amplifier is added to improve the SNR of the receiving module.The core control module is composed of FPGA and upper mechanism,and highspeed data transmission is carried out through Ethernet,which effectively improves the positioning speed of the system.(3)FPGA logic control program is designed and electromagnetic tracking software is developed.In order to ensure the efficient and accurate transmitting and receiving of magnetic field,and the realization and visualization of tracking algorithm,the following designs are made: FPGA uses DDS to strictly control time-sharing transmitting to generate stable magnetic field,and transmits the receiving magnetic field to the top computer through signal arbitration and FIFO data backpressure design,which effectively avoids network congestion;The upper computer performs pose calculation and result display through multithread queue design,which improves the response speed and stability of the program.(4)Simulation and surgical simulation tests were carried out to verify the compensation effect of the algorithm.The magnetic field values after compensation were substituted into the designed electromagnetic tracking model,and the positions of sensors in surgical instruments were calculated using Levenberg-Marquardt algorithm.In the simulation test,the electromagnetic positioning error under eddy current interference is 2～3 mm,and the positioning error decreases by about 0.5 mm after the compensation algorithm,which confirms the theoretical feasibility of the compensation algorithm.In the operation simulation test,the positioning accuracy is improved by about 12.5% after eddy current magnetic interference compensation,which verifies that the developed electromagnetic tracking system has good anti-interference characteristics. |
