Magnetic Localization

A major advantage of using magnetic fields for localization is their ability to function in harsh conditions. Magnetic fields are invariant to temperature, pressure, radiation and other environmental factors. Compared to other non-contact sensing systems such as optical and vision sensors, magnetic sensors do not require ‘a line of sight’, permitting sensing across multiple non-ferromagnetic mediums. Despite major advancement in miniaturization and magnetic sensing technology where modern sensors possess small physical footprints and high sensitivity and bandwidth, the deployment of magnetic sensors for positional sensing is under exploited.

Magnetic localization can be categorized into two approaches that involve harnessing active or passive fields. Localization utilizing artificially generated electromagnetic fields can be either pulsed or static and possess superior measurement ranges and better immunity to background geomagnetic noise. However, active fields require power, which can be provided internally using a battery or externally via constricting tethering wires which are less desirable for applications such as medical devices that require compact and lightweight footprints. Permanent magnets provide static magnetic fields with zero power and can be embedded directly into the target for non-obtrusive localization and tracking, which has immediate and direct applications in medical/surgical applications as shown below. Both approaches, however, like all sensing principles and systems, require a correspondence between the measured field and instantaneous position/orientation.

The main difficulties in cultivating position-field correspondence are the complexities of analytical field models and absence of bijectivity (both injective and surjective or encompassing one-to-one and onto correspondence) between field measurements and position/orientation. In a non-bijective relationship, multiple positions/orientations share a common field measurement value. It is clear that without bijection, associating an arbitrary field measurement with a unique position is difficult. Another critical issue is the manner and speed of extracting position from field measurements to satisfy the stringent requirements of feedback control. While theoretical field models for the prediction of fields in space are available, they are often highly complex and not in a tractable form for direct inverse computation operations, requiring computationally heavy non-linear optimization methods which are unsuitable for real-time operations.


Enhancing Efficacy during Nasogastric Intubation

Nasogastric (NG) intubation is one of the most commonly performed clinical procedures. Real-time localization and tracking of the NG tube passage at the larynx region into the esophagus is crucial for safety, but is lacking in current practice. By harnessing passive magnetic localization technology, the efficacy of the clinical NG intubation process can be seamlessly significantly improved. By embedding a small permanent magnet at the insertion tip of the NG tube, a wearable system containing embedded sensors around the neck can determine the absolute position of the NG tube inside the body in real-time to assist in insertion. In order to validate the feasibility of such a system in detecting erroneous tube placement, typical reference intubation trajectories are first analyzed using anatomically correct models and localization accuracy of the system are evaluated using a precise robotic platform. It is found that the root-mean-squared tracking accuracy provided by the system is within 5.3 mm for both the esophagus (correct) and trachea (incorrect) intubation pathways. Experiments were also designed and performed to demonstrate that the systemis capable of tracking the NG tube accurately in biological environments even in presence of stationary ferromagnetic objects (such as clinical instruments). With minimal physical modification to the NG tube and clinical process, this system allows accurate and efficient localization and confirmation of correct NG tube placement without supplemental radiographic methods which is considered the current clinical standard.

  1. Z. Sun, S. Foong, L. Maréchal, U.-X. Tan, T. H. Teo, and A. Shabbir, “A Non-invasive Real-time Localization System for Enhanced Efficacy in Nasogastric Intubation,” Annals of Biomedical Engineering (2015) 43: 2941. doi:10.1007/s10439-015-1361-0
  2. Z. Sun, L. Maréchal and S. Foong, “Passive Magnetic-based Localization for Precise Untethered Medical Instrument Tracking,” Computer Methods and Programs in Biomedicine, (In Press)
  3. Z. Sun, S. Foong, L. Maréchal, T. H. Teo, U.-X. Tan and A. Shabbir, “Design and analysis of a compliant non-invasive real-time localization system for nasogastric intubation,” 2014 IEEE/ASME International Conference on Advanced Intelligent Mechatronics, Besacon, 2014, pp. 1091-1096. doi: 10.1109/AIM.2014.6878226
  4. Z. Sun, S. Foong, L. Maréchal, T. H. Teo, U. X. Tan and A. Shabbir, “Using heterogeneous sensory measurements in a compliant magnetic localization system for medical intervention,” 2015 IEEE International Conference on Advanced Intelligent Mechatronics (AIM), Busan, 2015, pp. 133-138. doi: 10.1109/AIM.2015.7222521
  5. Z. Sun, K. C. T. Soh, S. Udomsawaengsup, A. Shabbir and S. Foong, “Modular design of a real-time passive magnetic localization system for enhanced safety in nasogastric intubation,” 2016 6th IEEE International Conference on Biomedical Robotics and Biomechatronics (BioRob), Singapore, 2016, pp. 329-334. doi: 10.1109/BIOROB.2016.7523647
  6. Z. Sun, S. Foong, L. Maréchal, T. H. Teo, U. X. Tan and A. Shabbir, “Real-time sensor fault detection and compensation in a passive magnetic field-based localization system,” 2016 IEEE International Conference on Advanced Intelligent Mechatronics (AIM), Banff, AB, 2016, pp. 1040-1046. doi: 10.1109/AIM.2016.7576907



Device Design Optimization for Accurate Localization for Ventriculostomy

The accuracy of many freehand medical procedures can be improved with assistance from real-time localization. Magnetic localization systems based on harnessing passive permanent magnets (PMs) are of great interest to track objects inside the body because they do not require a powered source and provide non-contact sensing without the need for line-of-sight. While the effect of the number of sensors on the localization accuracy in such systems has been reported, the spatial design of the sensing assembly is an open problem. A systematic approach to determine an optimal spatial sensor configuration for localizing a PM during a medical procedure has been developed and evaluated. Two alternative approaches were explored and compared through numerical simulations and experimental investigation: one based on traditional grid configuration and the other derived using genetic algorithms (GAs). Results strongly suggest that optimizing the spatial arrangement has a larger influence on localization performance than increasing the number of sensors in the assembly. It was found that among all the optimization schemes, the approach utilizing GA produced sensor designs with the smallest localization errors.

Through numerical simulations and experimental investigation as shown in the figures below, sensor layout design has a larger influence on localization accuracy and is more efficient than the conventional approach of employing additional sensors. An optimized arrangement of sensors has been shown to be highly advantageous in reducing localization errors in multi-sensor systems, and in this case, permitting high-accuracy tracking of instruments inside the brain. Traditional grid-based arrangement of sensors may be suitable when the localization path is unknown or irregular but may not be the optimal approach for specific applications such as repetitive catheter insertion trajectories. Moreover, having a rectangular grid pattern restricts the number of sensors in the array. For example, an array of 13 sensors cannot be directly constrained into a regular grid assembly. A GA approach relaxes this requirement and allows a greater flexibility in design and also capitalizes on emerging 3D printing technologies, which allow bespoke shapes and customized patterns to be fabricated.

  1. L. Maréchal, S. Foong, S. Ding, K. L. Wood, V. Patil and R. Gupta, “Design Optimization of a Magnetic Field-Based Localization Device for Enhanced Ventriculostomy.” ASME Journal of Medical Devices, 2016, 10(1):011006-011006-9. doi:10.1115/1.4032614
  2. Z. Sun, L. Maréchal and S. Foong, “Passive Magnetic-based Localization for Precise Untethered Medical Instrument Tracking,” Computer Methods and Programs in Biomedicine, (In Press)
  3. L. Maréchal; S. Foong; S. Ding; D. Madhavan; K. L. Wood; R. Gupta; V. Patil; C. J. Walsh, “Optimal spatial design of non-invasive magnetic field-based localization systems,” 2014 IEEE International Conference on Robotics and Automation (ICRA), Hong Kong, 2014, pp. 3510-3516. doi: 10.1109/ICRA.2014.6907365
  4. L. Maréchal, S. Foong, Z. Sun and K. L. Wood, “Design optimization of the sensor spatial arrangement in a direct magnetic field-based localization system for medical applications,” 2015 37th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC), Milan, 2015, pp. 897-900. doi: 10.1109/EMBC.2015.7318507