Magnetic Tracking for Medical Applications
This thesis explores the implementation of an electromagnetic positioning system to track medical instruments used in minimally invasive surgeries. The end application is for catheter cardiac ablation. Cardiac ablation is a low-risk procedure that can correct arrhythmia. In the procedure, a diagnostic mapping catheter is inserted into the heart to identify locations causing incorrect heartbeat, and an ablation catheter applies radiofrequency (RF) thermal energy, which burns tissue that emits abnormal heart rhythm. Current techniques which determine the mapping catheter’s tip position while a patient is undergoing heart surgery are usually invasive, often inaccurate, and require some forms of imaging.
Most existing electromagnetic (EM) tracking systems track a tiny sensing coil on the catheter tip by placing planar magnetic transmitters in reference locations around a patient. However, the tracking speed of these systems is extremely limited apart from deficiency in positioning accuracy due to poor sensitivity of the small sensor. In this study, we develop a unique real-time tracking system which can track the position and orientation of a medical catheter tip inside a human heart. A configuration of a small transmitting coil on the catheter tip with multiple larger receiving coils placed at reference locations is investigated.
We propose a novel tracking system based on a single uniaxial transmitter (1.5 mm diameter) placed on a medical catheter tip and two triaxial receivers placed in reference locations. The electromagnetic field generated by the uniaxial transmitter is controlled by an operational amplifier LC tank driver with a unique active feedback sensing system in the form of a digital phased lock loop (DPLL), which generates a low noise low distortion AC signal for the LC circuit. Such control is vital because the small transmitting coil has a relatively large DC resistance, resulting in copious amounts of heat. This unique transmitter driver active feedback system is optimized to ensure a stable magnetic field transmitted with minimal noise and distortion.
Precise and efficient calibration and compensation techniques are developed for the proposed system. The calibration techniques include mutual coupling correction, which rectifies one of the main limitations of a triaxial coil-based implementation. In addition, a novel divergence mitigation method for the position algorithm is developed in the form of a software-based reference sensing coil distance offset. This is advantageous compared to a hardware-based solution, which involves adding more coils to the system, in turn, leading to decreased tracking speed and higher risk of interference among coils. Because of its simplicity, the proposed EM tracking system also has the advantage of supporting a wide dynamic range, multiple catheters, and can be applied to other medical systems in need of real-time positioning.
This EM tracking system is demonstrated on a test bench in a research lab and in a pre-clinical environment with a 3D-printed heart inside a phantom. The tested system features a fast update rate of 200 Hz and an average position error of 1.6 mm, which indicates that the proposed system can successfully track a catheter RF tip with millimeter precision.
This dissertation presents the proposed EM tracking system. First, the motivation of this research and a review of existing tracking methodologies used in the medical field are presented. Then, the hardware design of individual modules and magnetic positioning firmware are described, which is followed by discussions of full system integration and calibration, as well as system test results. A summary, highlighting novelties of the tracking system, and discussion of future research directions are included in the final chapter.
- Doctor of Philosophy
- Electrical and Computer Engineering
- West Lafayette