As its name suggests, electromagnetic (EM) motion tracking monitors the movement of an object using the phenomenon of electromagnetism.

How EM Motion Tracking Works

In motion tracking, the object is not only tracked, but its motion data is also captured for analyzing it according to a particular application. For instance, a digital avatar can copy gestures and movements.

Key Components: Sensors and Transmitters

The EM object tracking system consists of two main parts: sensors and transmitters. An antenna, referred to as a transmitter, is generally fixed at its place and is responsible for the generation of a magnetic field. Field generators are mostly used to produce at least three spatial dimensions of the magnetic fields. It is imperative that the geometry of the field is known. Sensors are usually systems integrated into the object to be monitored (for instance, the object to be followed). In the three spatial dimensions, such sensors obtain the strength of the magnetic field to identify the orientation and location of the object.

Doctors are studying the results captured by EM tracker.

Field Generators: Types and Their Range

Typically, inductors are arranged inside the field generator (FG) to produce the required magnetic field. A significant feature of every field generator is the monitoring range that defines the space around the generator in which the objects can be monitored. For example, an FG with inductors arranged in the shape of a tetrahedron may offer a monitoring range of up to 0.5 m around itself. In several instances, custom field generators bring benefits such as greater robustness against disturbances, or better positioning conditions in a given environment. Therefore, vendors introduced different kinds of autonomous FGs while some were designed particularly for rehabilitation or medical products.

Regular FGs from several vendors are available and are therefore the most popular. Flat field generators were designed to place them directly under the resting patient, providing the ability to protect magnetic fields from disturbances from underneath, and also to conceal them in the patient’s table. Mobile FGs are compact and thus can be directly placed in the region of investigation, which is an advantage since tracking accuracy is strong near FGs, but it is also necessary since the tracking range is quite low. However, Polhemus sells long-range FGs covering areas up to a few meters.

Navigating Challenges in EM Motion Tracking

In EM motion tracking devices, there are also various possible errors. Physics rules, design limitations and industrial or environmental noise imperfection can all result in inaccurate location tracking. Although certain errors can only be addressed on the side of the manufacturer, numerous others can be managed by careful experimental design and implementation.

EM Tracking in Medicine: Applications and Advancements

There are many potential uses for EM detection in healthcare and many of them also have commercial systems. When instrument visualization becomes ineffective, EM tracking is preferred for the localization of objects in medical systems. Several researchers describe the computer-assisted intervention methods based on EM in the fields of catheter treatment, percutaneous punctures, endoscopic procedures and open surgery for minimally invasive involvements. In some instances, though, EM monitoring systems have also been used for open surgeries or other therapeutic diagnoses.

Percutaneous procedures can be performed on various parts of the body for examination (biopsy) or therapy (ablation). Computer-aided approaches are generally carried out under ultrasonogram (US) or computerized tomography (CT) rules, although MRI, positron emission tomography (PET), or fluoroscopy instructions, and their combinations are also feasible. Nevertheless, in the field of percutaneous punctures, the line of vision is often available, thus optical tracking based on a camera should be considered as a strong and precise alternative for the patient and for instrument localization. Therefore, based on the needs of a particular application, a trade-off between specific monitoring devices should be taken into account.

Modern hospital services include catheter interventions that are feasible within the human circulatory system. These typically use a guidewire to direct the catheter in a vessel tree. The guidewire diameters are in the range of 1 mm or less, making it difficult to include sensors. Nonetheless, the majority of research on EM-tracked catheter applications do not monitor guidewires, but only monitor the catheters that provide large diameters for easy incorporation of sensors. The majority of applications deal with cardiac catheterization for ablation or cardiac mapping. Some other uses of EM-based motion tracking in catheter treatments are in vena cava catheterizations, neurovascular and aortic. With catheters, EM monitoring is sometimes the only available technique to locate devices when sight line is not accessible. The need for a wired connection to the sensor, however, presents a concern for these types of procedures as it typically inhibits the catheter and may not allow guidewire usage. However, research is going on to integrate EM sensors with guidewires.

Endoscopes have a much wider diameter than catheters, which makes it easier to use EM monitoring. Most applications have been discovered on the computer-assisted bronchoscopy in which the bronchoscope can be steered by finding the tip and capturing preoperative details, such as CT for a patient, via the bronchial tree. Laparoscopic treatments have also been conducted, such as radio frequency ablation (RFA) or lung brachytherapy. For otolaryngologic (ENT) procedures, endoscopes were found using EM monitoring to implement navigation with US/CT instructions. Colonoscopies were monitored with an embedded sensor to envision the instrument's path during the operation. Therefore, the monitoring of endoscopes during an operation seems to be a fair application for EM motion tracking and is practical with the current technologies.

For some specific applications, EM motion monitoring has also been used for open surgery. For example, during intramedullary nailing, which is a specific bone fracture procedure that implants the nail in the bone when the line of vision is blocked, there is an excellent opportunity for using computer assistance. Likewise, in some other applications, such as epilepsy treatment where depth electrodes need to be placed and line-of-vision gets blocked, EM motion tracking seems to be the only device tracking method. EM tracking along with optical tracking has also been employed in open liver surgery. Therefore, EM motion tracking holds an appeal in special cases of open surgery when the line of vision is obscured.