Electromagnetic Tracking: Detecting and Mitigating Distortion Advances Surgical Navigation

Thin, light, modular antennas can be easily assembled to generate large sensing volumes. Radiotranslucent construction makes it compatible with intraoperative fluoroscopy and CT imaging. (Credit: Radwave/TT Electronics)

Advancing minimally invasive surgery and its usefulness in diagnosis and treatment in more healthcare situations is a direct challenge for medical device OEMs. Transforming these image-guided and interventional procedures requires precise surgical navigation that empowers physicians when there is no line of sight. Electromagnetic (EM) tracking systems — once hindered by the presence of metals and other equipment in a surgical setting — are gaining ground on this front.

Distortion detection and mitigation caused by equipment in a surgical setting, coupled with advanced EM sensors, improve accuracy and precision to enable a flexible platform approach for a greater number of devices and procedures. This combination of characteristics represents a breakthrough advancement for devices requiring highly accurate surgical navigation. Solving inherent distortion challenges and enabling a broadly viable EM solution empowers physicians to navigate more areas of the human body with greater accuracy and precision.

Overcoming Obstacles

EM surgical navigation is currently deployed in a variety of interventional and surgical domains, including electrophysiology, ear nose and throat (ENT), pulmonology, neurology, and other emerging procedures. These systems are used wherever the surgical procedures are performed — operating rooms or interventional suites in hospitals, outpatient centers, ambulatory surgical centers, etc.

A control unit enables seamless tracking of multiple sensors and tools at the same time with flexible port configurations and high sampling rates. (Credit: Radwave/TT Electronics)

And yet, the Achilles heel of EM tracking is inaccuracy due to distortion and interference with other equipment in a surgical setting. When metal is present in a sensing volume, the solution’s sensors may either not be tracked or will be tracked inaccurately. Physicians and other clinicians won’t know that their surgical navigation tool position is incorrect, or they just won’t get the location information they need — a silent failure of the tracking system.

How prevalent is this issue? In today’s minimally invasive surgical (MIS) and robotic procedures, there are many more types of imaging equipment and robotic arms in any procedure room; their presence speaks to EM interference. For example, surgical tables and patient beds often contain conductive materials that cause distortions. However, since these are static structures, the resulting interference can be “mapped” prior to the procedure and taken into consideration to correct any distortion of sensor location.

At the same time, the procedure space also houses metal structures that move around, in, and out of the surgical field — these cannot be mapped before the procedure. In spinal surgery, for example, metal structures in the procedure space may include the fluoroscopic C-Arm, metal robot parts, and various surgical and fixation tools.

Small 5DOF and 6DOF sensors can be easily integrated into a variety of interventional and minimally invasive devices. (Credit: Radwave/TT Electronics)

The issue of frequency interference compounds these challenges. Older EM systems may utilize wide EM frequency bands, creating interference with medical equipment like an electrocardiogram, other biopotential signals, and the image from the fluoroscopic C-Arm. In these cases, the legacy EM system cannot be used simultaneously or in proximity to certain other pieces of surgical equipment or tools. In this less-than-ideal scenario, clinicians would be forced to minimize active equipment in the surgical suite as a trade-off to gain surgical navigation beyond the physician’s line of sight. For broader and more seamless deployment across more lines of clinical applications, modern EM systems must detect the presence of EM interference with all sensor types and mitigate any resulting distortion in real time.

Addressing EM Challenges

A number of off-the-shelf EM systems have been available for some time, although many were developed before wide adoption of minimally invasive and robotic surgeries. This often limits the tracking capabilities of legacy systems to only fixed and limited navigation volumes. Instruments cannot be tracked throughout the entire procedure space, which in turn may expose patients and clinicians to additional harmful x-ray radiation.

These systems also use bulky EM field generators incompatible with commonly used imaging equipment (for procedures like CT, fluoroscopy, CBCT). And with limited or no detection or correction of EM-induced distortions, inaccuracies are commonplace. This is especially true in environments where additional equipment such as fluoroscopes, robots, and other metal objects are often used. Further, customization options are limited; legacy EM tracking systems typically only work with sensors provided by the navigation system manufacturer and rely on low location sampling rates. Historically, only a small number of sensors could be tracked and at less-than-ideal sampling rates; and as the number of sensors increase, the sampling rates decrease.

Antenna Breakthroughs

In an optimized EM tracking platform now available, high confidence is delivered in as many as 24 sensors, each offering location information previously unavailable at this high sampling rate. Because the platform is paired with a full range of sensors featuring both five and six degrees of freedom (5DOF and 6DOF), sampling rates remain high when all 24 sensors are tracked. The platform’s antenna is placed underneath the patient and generates a magnetic field.

As more minimally invasive surgery and robotic surgery approaches are applied to different procedures, there will be a growing need for advanced object tracking technology that can locate instruments and tools inside the body when there is no clear line of sight. Electromagnetics can be the technology of choice, but it has to overcome its current limitations. To solve the shortcomings of current electromagnetic systems, Radwave and TT Electronics have collaborated in the design and development of a modern electromagnetic system from the ground up. (Credit: Radwave/TT Electronics)

Where previous antenna designs would introduce image artifacts when a C-arm was brought into the sensing field, for example, to obtain alternative lateral or oblique image angles, the Radwave® antenna offers new clinical advantages. As a flat, translucent panel, its pioneering design can reduce radiation to the patient and does not reduce the fluoroscopy image quality. The field generated is the sensing volume, and sensors placed within that volume can be tracked within the surgical tool to submillimeter accuracy. Because it creates a substantial sensing volume, navigation can be achieved from the patient’s thigh to the heart. Fluoroscopy images are of consistent quality no matter the angle of the C-arm. Fluoroscopy can be reduced or eliminated with greater confidence in accurate EM tracking.

Reducing Limitations from Distortion

Critically, the platform’s distortion mitigation module is a novel advancement. When no sensors are in the field, the antenna recognizes the presence of distortion, the direction of its source, and its magnitude. Consider a real-world setting today, where a variety of reference sensors may be placed on the exterior of a patient’s body. Sensors on the front and side may deliver high confidence, while those at the patient’s back may provide lower confidence based on distortion present. The Radwave platform featuring TT Electronics sensors globally identifies the distortion and enables the execution of various mitigations. All information is present from the full array of sensors within the sensing volume, allowing the physician to confidently perform the procedure.

The platform can track many different types of sensors that vary in size and shape, including both 5DOF and 6DOF sensors. Sensors from TT Electronics are precalibrated for compatibility with the Radwave electromagnetic tracking platform. This includes a variety of sensor designs that can be applied to devices used within cardiology and electrophysiology, vascular, interventional radiology and oncology, neurology, ENT, orthopedics, pulmonology, and other procedures. For example, in some of these procedures, a catheter or a needle may be used, and different sensors can be applied to these devices to enable accurate navigation during the procedure. When the device’s integrated sensor is introduced into the EM field and paired with an EM tracking system, it pinpoints the device’s location throughout the procedure.

For device developers, time to market is reduced with plug-and-play sensors precalibrated to perform with the EM platform. Sensor performance is validated to handle the tracking platform’s distortion detection and mitigation capabilities that enhance the device’s precision. The platform is easy to integrate into medical devices via a simple, encrypted API powered by an open-source software developer kit. Further, the platform is highly customizable, offering a sensing volume that can grow or shrink to match the requirements of a specific surgical procedure.

Surpassing Line of Sight

The primary challenge of EM tracking has been maintaining its accuracy and precision while deploying a system across a range of surgical needs and environments. Some clinical and surgical suites are decades old, with modern equipment in rooms featuring stainless steel and ferrous and nonferrous metals. EM tracking can be the viable navigation technology of choice by detecting and mitigating the distortion caused by these environments.

In procedures that use fluoroscopy, catheters, and other surgical tools, the precision enabled by distortion mitigation adds real value — increasing clinical confidence when there is no line of sight. As a result, minimally invasive techniques can be applied safely and confidently to more and more surgical navigation procedures.

This article was written by Garrett Plank, Business Development Director, TT Electronics, Woking, UK; and Andrew Brown, CEO, Radwave Technologies Inc., St. Paul, MN. For more information about TT Electronics, visit here . For more information about Radwave, visit here .