Design of an Electromagnetic Coil Array for Wireless Endoscope Capsule Localization

Abstract

Wireless capsule endoscopy is a technique that visualizes mucosa of gastrointestinal tract. The first wireless capsule endoscope was developed in 2001, and since then, the tech-nology has been in constant development. With its reduced amount of discomfort, ability to visualize the whole gastrointestinal tract and many interesting future prospects, capsule en-doscopy is challenging the conventional procedure, in which a camera module at the end of a tube is inserted to the gastrointestinal tract. In general, the method can be used to diagnose many diseases, such as cancers, Celiac disease and Crohn’s disease. In order to achieve all the future prospects of the technology, for example, active steering of the capsule, biopsy and drug-delivery with the capsule, challenge of detecting the cap-sule’s position and orientation inside human needs to be overcome. Therefore, the localiza-tion challenge is considered in this thesis. The aim is to select an optimal method for localiza-tion based on current research, and to model and develop a prototype that could be utilized in capsule localization. The designed sensor array is evaluated with the help of finite element method -based modelling, sensitivity analysis and practical experiments. Based on the studied literature, an active magnetic field strength -based localization tech-nique was selected for further analysis. According to the literature, the method provides high accuracy and could also be utilized in other purposes, for example, wireless charging and ac-tive locomotion of the capsule. In addition, the method does not suffer from attenuation of the fields within a human body. Therefore, theoretical basis of the magnetic field strength -based localization was presented, and four electromagnetic coil arrays were designed for sensitivity analysis. Contrary to many developed arrays seen in the literature review, all the designed systems were planar, in order to develop a system that could be fitted, for instance, inside a hospital bed. Based on the sensitivity analysis, an array with relatively large sensitivity and optimal number of measurement channels was selected for practical study. In addition, pos-sible markers that could be fitted inside a regular sized endoscopic capsule were numerically modelled. It was found that a resonated solenoid marker with ferrite core causes the largest voltage compared to other modelled targets, and therefore it was constructed for experi-ments. The whole array was built and equipped with electronics and measurement devices to be able to perform testing. Two channels of the array were selected for example measurements, and the results were analysed and compared with modelled values and sensitivity patterns. The system was tested at multiple different heights and positions, and the effect of changing the marker’s orientation with respect to the array was analysed. It was found that the system gives a reasonable response when the marker is oriented along the excitation magnetic field. With this orientation, it was possible to measure significant voltages caused by the marker even at distance of 25 cm from the array. However, when the marker was oriented so that the excitation field could not properly excite it, the measured voltages got smaller. In addition, at certain orientation, the measured voltages did not seem reliable because voltage caused by the marker could not been discriminated from the noise of the system. The study indicates that the method is suitable for localization of resonated solenoid sample with ferrite core, but further improvements are needed to make the system work at each position and orientation of the marker. In addition, an inversion algorithm that estimates marker’s position and orien-tation based on the measured voltage needs to be integrated to the system.