Using Electromagnetic Tracking Sensors in Medical Devices with Computer-Assisted Navigation
Welly Chou • Director of R&D, TT Electronics
In the constantly advancing realm of medical technology, the focus on precision, reliability, and efficiency remains paramount. Every detail, from the smallest component to the most complex system, plays a critical role in ensuring devices function as intended in lifesaving procedures. Among these details, one crucial yet often overlooked aspect is the choice of sensor technology, particularly in devices utilizing computer-assisted navigation and electromagnetic tracking systems. Two of the most commonly used sensors in this field are air coil sensors and solid core sensors.
Both types offer unique benefits that can significantly enhance the performance and capabilities of medical devices, and understanding the distinct advantages of each is essential for optimizing their use in various medical applications.
Understanding the Basics
To appreciate the unique advantages of both air coil sensors and solid core sensors, it is necessary to understand their fundamental differences. These sensors function similarly in terms of detecting electromagnetic fields, but the materials and structures they employ set them apart.
Solid core sensors, as their name suggests, use a solid magnetic core material (typically a ferrite or other magnetic material) surrounded by a wire coil. This design enhances the sensor’s ability to detect and transmit signals, as the magnetic material intensifies the strength of the electromagnetic field inside the coil. This feature allows solid core sensors to be incredibly sensitive to magnetic fields, making them particularly useful in situations where precise tracking is essential. However, their solid core construction also makes them more rigid and potentially bulkier compared to air coil sensors.
On the other hand, air coil sensors replace the magnetic material with air (or a vacuum) in the center of the coil. This seemingly minor distinction leads to significant differences in performance and application. The absence of a magnetic core in air coil sensors reduces the overall weight of the sensor and allows for greater flexibility in design, particularly when space is limited, or when a more pliable sensor is required. In certain medical devices, especially those that need to maintain a highly flexible structure, this can be a crucial advantage. Additionally, air coil sensors are less prone to issues such as magnetic saturation, which can occur in environments with strong external magnetic fields.
Sensor and Device Integration
One of the most critical considerations when choosing between air coil and solid core sensors is how they integrate with the medical device itself. Medical devices, particularly those used in minimally invasive surgeries, often have complex shapes and structures, which can pose challenges for sensor placement.
Air coil sensors are typically implemented around the outer circumference of medical devices. Their hollow nature is especially beneficial for use in steerable sheath or multi-lumen medical devices as it allows for the passage of treatment tools or other components through the center of the sensor. This design enables the sensor to be incorporated without obstructing the functionality of the device, providing both tracking and flexibility.
Solid core sensors, by contrast, do not allow any passage through their center, which means they must be integrated in a different way—typically along the sidewall or embedded in the device. When placed in the sidewall, solid core sensors can introduce a minor asymmetry to the device profile. However, this does not necessarily limit their effectiveness. The solid core construction allows for various configurations, and the sensors can be integrated at different points based on the size and tracking needs of the device. Despite the potential for asymmetry, their compactness and ease of installation often make solid core sensors the preferred option in devices where internal space is at a premium.
Precision and Sensitivity
In medical applications, precision and sensitivity are key factors. Whether tracking the position of a surgical tool or guiding a catheter through a patient’s anatomy, the sensor’s ability to accurately detect and respond to electromagnetic fields is critical.
Air coil sensors offer a unique advantage in this regard because their magnetic center aligns with the center of the device in which they are implemented. This feature simplifies the calibration process, as the sensor’s readings can be directly correlated with the centerline of the medical device, minimizing the need for complex calculations to account for off set. This can be particularly useful in devices where exact positioning is essential, such as in brain surgery or cardiac procedures.
Conversely, solid core sensors may introduce some challenges in this area. Depending on their placement within the device, the sensor’s magnetic center may not align with the tool’s physical center, creating a need for additional calculations to determine the exact position of the device in space. However, this potential drawback is off set by the other unique benefits of solid core sensors, including their enhanced sensitivity due to the magnetic core material. In applications where sensitivity is the highest priority, such as in deep tissue navigation, the solid core sensor’s superior magnetic response can outweigh the complexities of calibration.
Another important factor in both sensor types is the consistency and repeatability of sensor placement during device manufacturing. Ensuring sensors are consistently placed in the same location and orientation within the device is critical for maintaining the accuracy of electromagnetic tracking systems.
Effects of External Magnetic Fields
One of the primary challenges of using electromagnetic tracking systems in medical devices is the potential for interference from external magnetic fields. Hospitals and surgical centers are environments rich with electromagnetic noise, from MRI machines to monitors and other electronic equipment. This interference can potentially affect the performance of both air coil and solid core sensors, but the two sensor types respond differently to these external influences.
Solid core sensors, with their magnetic core, are more susceptible to a phenomenon known as magnetic saturation. In the presence of a strong external magnetic field, the core material can become saturated, meaning it can no longer effectively respond to changes in the surrounding electromagnetic environment. This can lead to poor tracking performance or even complete failure of the sensor to detect its position accurately.
Air coil sensors, by contrast, are much less prone to magnetic saturation due to their lack of a magnetic core. This makes them a better choice in environments where external magnetic fields are likely to be strong or variable. For example, in procedures performed near MRI machines or in operating rooms with a high density of electronic equipment, air coil sensors may provide more reliable tracking performance by avoiding the saturation effects that can plague solid core sensors.
Enhanced Flexibility and Adaptability
As medical devices become more advanced and specialized, the need for flexible, adaptable sensor technology continues to grow. Both air coil and solid core sensors offer excellent options for enhancing the flexibility of medical device design, though they do so in different ways.
Air coil sensors, with their hollow center and lightweight construction, offer inherent flexibility. They can be incorporated into devices of varying sizes, including those with very small French sizes, without compromising the functionality of the device. This makes them ideal for use in devices that require both precise tracking and a high degree of flexibility, such as catheters or steerable sheaths.
Solid core sensors, while more rigid than air coil sensors, offer their own form of adaptability. Their compact size allows them to be integrated into devices with limited space, and their robust construction makes them highly durable. In applications where the device must withstand mechanical stress or where the sensor needs to be placed in a confined area, solid core sensors can provide a more suitable solution.
Conclusion
The choice between air coil and solid core sensors in medical devices is not a simple one, as each sensor type offers distinct advantages depending on the application. Air coil sensors, with their flexibility and resistance to magnetic saturation, are well-suited for environments where external magnetic fields are a concern or where device flexibility is critical. Solid core sensors, with their enhanced sensitivity and compact design, excel in applications where precision tracking is paramount and space is limited.
As medical technology continues to evolve, the role of sensor technology in improving patient outcomes will only become more significant. Understanding the unique benefits of air coil and solid core sensors is key to selecting the right technology for each specific medical application, ultimately contributing to the advancement of medical innovation and the enhancement of patient care.
