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Navigated targeting of liver lesions: pitfalls of electromagnetic tracking

One of the major challenges related to percutaneous radiofrequency ablation (RFA) of liver tumors is the exact placement of the instrument within the lesion. Previous studies have shown the benefit of computer-assisted needle insertion based on optical tracking of both the instrument and internal fiducials used for registration. However, the concept has not been accepted for clinical use. This may in part be attributed to the line-of-sight constraint imposed by optical tracking systems which results in the use of needles thick enough to avoid bending. Electromagnetic (EM) tracking systems allow the localization of medical instruments without line-of-sight requirements, but are known to be less robust to the influence of metallic and/or ferromagnetic objects. In this paper, we apply a previously introduced fiducial-based system for navigated needle insertion with an EM tracking system and assess the overall targeting error using a static phantom in two different settings: in a non-metallic environment (REF) and on a CT stretcher (CT). While accurate needle insertion could be achieved in the reference environment (REF: 2.6±0.7 mm), targeting errors dropped drastically in the presence of metal (CT: 10.4±6.1 mm). For accurate and robust computer-assisted needle insertion, EM field distortions should thus either be avoided by assuring a suitable environment or by using methods for shielding or error compensation.