Localization and real-time tracking of sensors and surgical devices in vivo with high precision are required during many surgical procedures and medical diagnostic techniques. For instance, in orthopedic surgeries, long bone fractures are fixed by putting a metal rod into the bone and holding the two together using screws. It is crucial to know the precise location of the screw-holes before drilling into the fractured bone to put the screws in place. Currently, multiple X-ray images are taken to accurately locate the screw-holes. Another important application is robotic surgery that requires highly precise movement of surgical tools inside the body. The gold-standard technique to achieve precise alignment and positioning of surgical tools and implants during these procedures is X-ray fluoroscopy, which produces real-time images on a screen by using continuous X-ray beams. The total duration of fluoroscopy can vary from 1-15 min per patient and is highly dependent on the skills of the surgeon, thus causing high levels of radiation exposure to the patient, surgeon and staff.

As part of our NSF-funded research at Caltech, we have worked on developing a radiation-free system for high-precision surgical alignment, navigation and tracking, using magnetic-field gradient-based localization of microscale devices. Fig. 1a shows the application of our work in intramedullary nailing, a class of high-precision orthopedic surgery, which requires insertion of a Titanium metal rod into the medullary canal of a fractured bone, followed by locking screws. Our system is designed such that a small electronic device (shown in green) can be attached right next to the screw-hole at a known position on the rod. Another identical device can be installed on the surgical drill. A planar electromagnetic assembly consisting of magnetic field gradient generating coils for X, Y and Z, is placed beneath the surgical bed. The electromagnets produce monotonically varying magnetic fields, resulting in gradients that encode each spatial point uniquely. Fig. 1b shows the magnetic field gradient in the X-direction. The two devices can simultaneously measure and communicate the magnetic field at their respective locations to an external receiver, which maps the field-data to spatial coordinates and displays the relative locations of the devices on a computer screen in real time (Fig. 1a). This can enable the surgical team to maneuver to screw-hole locations without using any X-ray fluoroscopy.

We designed the microdevices D1 and D2 (Fig. 1b) to be completely wireless (for hermetic sealing when used as an implant) and battery-less (to eliminate lifetime and bio-compatibility issues) by custom-designing an ASIC Chip in CMOS technology. We also designed the planar electromagnetic coils (can slide beneath the patient’s bed to ensure no inhibition to the surgeon’s movement) for creating the 3D magnetic field gradients in a high and scalable FOV. The requirement of planarity and absence of permanent magnets (to avoid interference with metals and ensure safety) made the coil design process challenging, which we overcame by employing the gradient in the magnitude of the field and used a combination of fields to correct for the non-linearity. The complete system was tested in vitro to successfully localize the microdevices with an accuracy of 100mm in 3D, which is better than the typical imaging resolution of 200-500μm obtained by clinically used X-ray imagers and CT systems.

Another focus of our NSF-funded research has been on continuous and real-time monitoring of ingestible microdevices in the Gastrointestinal (GI) tract. Achieving high spatiotemporal accuracy during localization of such microdevices is of substantial clinical benefit for accurate diagnosis, treatment, and management of GI motility disorders, quantitative assessment of GI transit-time, anatomically targeted sensing and therapy, localized drug delivery, medication adherence monitoring, selective electrical stimulation, disease localization for surgery and minimally invasive GI procedures. The existing gold-standard solutions include invasive procedures such as endoscopic observation and manometry, or procedures requiring potentially harmful X-ray radiation such as scintigraphy. These techniques also require repeated evaluation in a hospital setting, which is another drawback.

To alleviate the above challenges, we have developed a platform for localizing and tracking wireless microdevices inside the GI tract with mm-scale spatial resolution in real time and in non-clinical settings, without using X-ray radiation. We designed highly miniaturized and wireless devices (different from above) to sense and transmit their local magnetic field to an external receiver. Using the 3D magnetic field gradients that encode each spatial point uniquely in the FOV, the real-time position of the devices can be found as they move through the GI tract. We have successfully tested our system in vivo in porcine models and these results are currently under publication. Additional capabilities can be readily added to these devices, enabling them to measure and report pH, temperature, pressure, and other biologically relevant markers, thus providing a spatiotemporal map for comprehensive patient diagnosis.

Last Modified: 08/12/2021
Modified by: Azita Emami-Neyestanak