In this paper, an electromagnetic tracking method based on fast determination of the maximum magnetic flux density vector (MMFDV) represented by two azimuth angles is proposed. The system designed to validate this method includes two magnetic sources and a 3-axis magnetic sensor. Each source consists of three mutual orthogonal coils. The main characteristic of this method is that the direction of MMFDV generated by a magnetic source can be determined as long as each coil of the source is excited once. Then the sensor can be located with a noniterative geometrical algorithm in real time. This method is easy to be implemented and can realize tracking in full coordinate space. Simulation and experimental results demonstrate a good performance of this method. It can be used as a part of computer-assisted navigation system in minimally invasive surgery after proper optimization.

A magnetic sensor attaching on the moving object will have a maximum intensity measurement when the rotating magnetic field is pointing at it. Before solving the position and orientation, the magnetic field could be rotated in two orthogonal planes to find the maximal pointing direction. In order to realize a real-time tracking, efficient searching strategies for this maximum-oriented magnetic field rotation must be provided. This paper sets up a novel rotation searching efficiency model to quantitatively evaluate the relationship between the rotation searching time and the rotation axis in the two-round rotation based magnetic field measurement. Based on this model, two searching strategies are proposed. Simulation results suggest that both searching strategies can realize full space tracking without any latency larger than 30 ms. Corresponding system platform is under development. (C) 2015 Elsevier Ltd. All rights reserved.

目的 针对传统电磁跟踪方法存在的依赖磁场模型具体磁感应强度值、迭代算法繁琐的问题,提出一种采用非迭代几何算法的电控旋转磁场跟踪方法,以避免模型误差、简化定位算法.方法 采用三轴正交磁场源,通过控制磁场源三轴激励电流强度实现合成最大磁感应强度矢量在空间中遍历旋转,根据矢量指向三轴磁传感器时的旋转角利用非迭代几何算法进行定位,对该方法进行了仿真研究.结果 仿真半径为0.3m时,平均位置误差为0.32 cm,方差为0.02,定位时间为0.54 s.结论 基于电控旋转磁场与非迭代几何算法的电磁跟踪方法可以较准确地定位传感器,具有定位速度快、性能稳定的潜在优势,值得进一步优化提高性能以满足介入式微创手术临床实用要求.

This paper presents an electromagnetic tracking method based on two electrically-controlled magnetic sources. Each source consists of three mutual orthogonal coils. The method can skillfully determine the rotation angles of the maximum magnetic flux density vector (MMFDV) generated by a magnetic source according to the Biot-Savart Law. Then the position of the sensor can be calculated with a noniterative geometrical algorithm in real time. Simulation results demonstrate a good performance of this method.

在电磁跟踪系统中,磁场源的优化设计不仅可简化系统结构,还可提高系统性能.设计了一种电控旋转磁场源,并对该磁场源实现电磁跟踪的可行性进行了仿真研究.首先提出了一种立方体磁芯缠绕三轴正交方形线圈,通过控制3个正交线圈激励电流强度,实现合成磁感应强度矢量旋转的磁场源方案;之后,采用有限元分析法,通过仿真软件ANSYS对其空间磁场分布情况及激励电流对磁场分布的影响进行了仿真研究.仿真结果表明:可以通过控制三轴线圈的电流强度,控制立方体三轴正交线圈合成的总磁感应强度最大值的方向,实现磁场的“旋转”.该磁场源方案符合电扫描实现旋转磁场跟踪的要求,为下一步的实际系统设计提供了依据.

Magnetic tracking technology using tri-axial magnetic source is widely used in medical positioning and tracking equipment. Calibration of the system prior to the measurement is required in order to achieve better accuracy. In normal circumstances, optical positioning system is used to calibrate the magnetic positioning system. In this paper, a new calibration method of magnetic system is presented by using magnetic field and micro-motion platform. It is possible to solve the sight constraints of the optical system. The simulation result shows that this method has high calibration accuracy.

When it comes to the navigation tool for minimally invasive surgery, electromagnetic tracking has prominent advantages and features over the traditional endoscope and X-ray imaging. The electromagnetic tracking system using two DC pulsed independent rotating coils to generate two changing magnetic fields alternately can track the position and orientation of the magnetic sensor based on an non-iterative algorithm. However, owing to the restrain of the step size and the distortion of the magnetic field, the coils cannot be pointing at the sensor accurately in the real system. Thus the problem of low resolution area is raised. In this paper, an optimized position algorithm, averaging position algorithm, is proposed to solve the problem of the non-coplanar two coils and the low resolution area. The simulation results demonstrate that the overall static position tracking accuracy was improved by 97.1%, from 11.722 cm to 0.336 cm, when the pointing angle inaccuracy is 0.346°. Further real system research based on the optimized position algorithm should be conducted to offer better navigation performance for the minimally invasive surgery.

目的 提出一种不依赖磁场分布模型且采用简单几何方法求解的双磁棒旋转搜索定位/跟踪方法.方法 设计基于数字信号处理器(digital signal processor,DSP)的双磁棒旋转搜索定位/跟踪系统,对双磁棒旋转搜索定位/跟踪方法的可行性进行初步验证.结果 平均位置误差为0.954,平均姿态误差4.86°.结论 双磁棒旋转搜索定位/跟踪系统有很好的稳定性,搜索速度和定位精度还需进一步提高.

In this paper, an electromagnetic tracking method that uses two rotating orthogonal coils is proposed. Two cross-shaped coils can rotate together and be driven in sequence to generate a magnetic field. One of the rotating orthogonal coils is used to track the position of the three-axis sensor and the other is to help solve the orientation of the sensor. As rotation can provide a geometrical relationship between the magnetic source and the sensor, the method does not require the generated magnetic field to imitate the ideal dipole in calculation. A fast noniterative algorithm and simple 1-D mapping along the axis of one coil are used to realize six degree-of-freedom (6DOF) tracking. Thus, the complexity of the magnetic tracking method and the corresponding system can be greatly reduced. Simulation results show that the method has good tracking accuracy and speed, which can be further improved by using an adaptive step-size searching strategy. So, it is concluded that the method can be used as an effective target monitoring method in many application domains such as minimally invasive therapy.