Electromagnetic (EM) tracking has revolutionised minimally invasive surgery. Unlike optical tracking, it does not require a direct line of sight, allowing clinicians to track catheter tips inside the heart or instruments deep within the brain anatomy. However, every MedTech engineer and surgeon faces the same persistent adversary: the operating theatre environment itself.
The modern Operating Theatre (OT) is essentially a magnetic minefield, crowded with C-arms, surgical tables, and stainless steel instruments. Since electromagnetic tracking relies on stable magnetic fields to determine position and orientation, the introduction of conductive or ferromagnetic metals creates distortion.
If you are evaluating tracking technologies for a new medical device, you likely have one burning question: Can electromagnetic tracking sensors actually maintain sub-millimetre accuracy in a real-world theatre?
To be perfectly frank, raw EM physics will always struggle with metal. However, modern system architecture has evolved to manage, map, and mitigate this interference. This article breaks down exactly how today's systems handle the metallic noise.
To understand the solution, we must first separate the two types of interference occurring in the theatre: Ferromagnetic and Conductive.
Ferromagnetic materials, such as certain steel grades found in operating tables, permanently warp the magnetic field lines generated by the Field Generator (FG). When field lines bend, the sensor reports a position that is geometrically incorrect. This results in static error—the instrument appears to be several millimetres away from its actual location.
Conductive metals, such as aluminium or copper, do not magnetise, but they conduct electricity. When an alternating magnetic field hits a conductive surface, it induces Eddy currents. These currents generate secondary magnetic fields that oppose the original field, typically resulting in dynamic errors or "jitter".
Leading medical device manufacturers apply a combination of hardware design, advanced software, and multi-modal sensing to maintain reliable electromagnetic (EM) tracking performance, even in complex clinical settings.
A foundational element of accurate EM tracking is thoughtful field generator design.
Planar Field Generators: Modern systems use flat generators placed directly under the patient. By keeping the tracking volume tight and localised, the system reduces interaction with metal objects outside the immediate surgical field.
Internal Shielding: FGs contain shielding layers designed to significantly attenuate interferences coming from below the table, effectively ignoring the metal mass of the surgical bed.
Advances in signal processing and modelling algorithms can now "map" the distortion. Some systems take a baseline map of the room's magnetic signature. If a static metal object distorts the field, the algorithm recognises the warp and mathematically corrects the sensor's reported position.
Additionally, DC magnetic fields—which do not generate Eddy currents in conductive metals—can be utilised to minimise interferences.
Using 6 Degrees of Freedom (6DOF) electromagnetic sensors (tracking X, Y, Z, Pitch, Yaw, and Roll) provides more data points for error correction. If the system detects that the internal coil geometry has mathematically "broken" due to distortion, it flags the data as unreliable, ensuring patient safety through rigorous integrity checks.
For complex environments, engineers increasingly adopt hybrid tracking strategies. This involves pairing electromagnetic tracking with other modalities such as impedance-based tracking or optical references. If the EM system detects a sudden spike in distortion (perhaps a C-arm has moved too close), the system momentarily leans on the secondary modality to maintain visualisation until the interference clears.
Despite these advancements, electromagnetic tracking is not magic. Certain constraints remain:
The C-Arm Intensifier: Bringing a sensor directly into the bore of an older image intensifier will likely cause signal loss.
Electrosurgical Tools: Active cauterisation tools emit massive electromagnetic noise. Software "gating" is often used to freeze the display position while the cautery is active.
Metallic distortion is no longer a dealbreaker for electromagnetic tracking, but it remains a constraint that dictates system design. By utilising localised field generators and sophisticated distortion mapping, MedTech engineers can deliver sub-millimetre accuracy even in the most crowded operating theatres.
Ready to integrate high-precision tracking into your next medical device? Contact our engineering team to discuss your specific sensor requirements.
Yes. Carbon fibre tables are the gold standard because they are radiolucent and non-conductive. Standard stainless steel tables cause significant distortion and usually require spacers or specific FG shielding to function correctly.
Generally, yes. However, the Field Generator is usually positioned so that the tracking volume avoids the direct metal mass of the C-arm detector.
Static errors are constant positional shifts caused by stationary ferromagnetic metal. Dynamic errors are fluctuating "jitters" caused by moving metal or Eddy currents.