C. Taylor, Magnetomicrometry : Tissue length tracking via implanted magnetic beads (2020). https://dspace.mit.edu/handle/1721.1/130210

Abstract

Target tracking is necessary across a wide range of disciplines and scales, such as in monitoring tissues and cells, beam bending, fluid dynamics, human-computer interaction, and traffic. Due to these widespread applications, advances in target tracking drive cascades of new medical, social, and scientific capabilities. In particular, this dissertation advances magnetomicrometry, a technology that tracks visually-obscured magnetic beads implanted within biological tissue to monitor in-vivo tissue length and speed within freely moving animals and humans.

There are many methods to track visually-obscured objects, but magnetic-target tracking has the advantages of being low-cost, portable, and safe. However, current magnet tracking technologies are slow, precluding high-speed real-time magnetic-target tracking. This is due to the mathematics of magnet tracking, whereby magnet positions are traditionally determined via numerical optimization, suffering from instability and significant delays. This dissertation develops the mathematics for an improved method to track one or more magnets with high speed and accuracy and validates this method by demonstrating real-time muscle length tracking.

We develop a high-speed, real-time, multiple-magnetic-target tracking method using the analytic gradient of the magnetic field prediction error. We extend this method to compensate for magnetic disturbances in real time using a simpler, more portable strategy than currently-published disturbance compensation methods. Validating our method in a physical system against state-of-the-art motion capture, we demonstrate increased maximum bandwidths of 336%, 525%, 635%, and 773% for the simultaneous tracking of 1, 2, 3, and 4 magnets, respectively, with tracking accuracy comparable to state-of-the-art magnet tracking.

Using pairs of implanted magnetic beads to wirelessly track muscle length and speed, we apply a mechanical frequency sweep to an in-vivo turkey gastrocnemius muscle and find submillimeter agreement between magnetic-bead-derived real-time muscle length measurements and stereo X-ray videofluoroscopy. We further collect longitudinal data using computed tomography and find a minimum magnetic-bead separation distance in muscle of approximately two centimeters.

It is our resolve that magnetomicrometry will lay the groundwork for peripheral nervous system control of wearable robots via real-time tracking of muscle lengths and speeds, as well as for the in-vivo tracking of biological tissues to elucidate biomechanical principles of animal and human movement.