Design and application of an assessment protocol for electromagnetic tracking systems

Abstract

This paper defines a simple protocol for competitive and quantified evaluation of electromagnetic tracking systems such as the NDI Aurora (A) and Ascension microBIRD with dipole transmitter (B). It establishes new methods and a new phantom design which assesses the reproducibility and allows comparability with different tracking systems in a consistent environment. A machined base plate was designed and manufactured in which a 50 mm grid of holes was precisely drilled for position measurements. In the center a circle of 32 equispaced holes enables the accurate measurement of rotation. The sensors can be clamped in a small mount which fits into pairs of grid holes on the base plate. Relative positional/orientational errors are found by subtracting the known distances/ rotations between the machined locations from the differences of the mean observed positions/ rotation. To measure the influence of metallic objects we inserted rods made of steel (SST 303, SST 416), aluminum, and bronze into the sensitive volume between sensor and emitter. We calculated the fiducial registration error and fiducial location error with a standard stylus calibration for both tracking systems and assessed two different methods of stylus calibration. The positional jitter amounted to 0.14 mm(A) and 0.08 mm(B). A relative positional error of 0.96 mm +/- 0.68 mm, range -0.06 mm; 2.23 mm(A) and 1.14 mm +/- 0.78 mm, range -3.72 mm; 1.57 mm(B) for a given distance of 50 mm was found. The relative rotation error was found to be 0.51 degrees (A)/0.04 degrees (B). The most relevant distortion caused by metallic objects results from SST 416. The maximum error 4.2 mm(A)/ > or = 100 mm(B) occurs when the rod is close to the sensor(20 mm). While (B) is more sensitive with respect to metallic objects, (A) is less accurate concerning orientation measurements. (B) showed a systematic error when distances are calculated.

Navigated ultrasound in laparoscopic surgery

“…EM trackers in the OR are subjected to distortion from several sources, and the impact of the level of interference may vary between the different trackers. A number of papers deal with distortions to the EM tracking systems from metals (Hummel et al, 2005;Kirsch et al, 2006;Nafis et al, 2006), surgical instruments (Hummel et al, 2002;Schicho et al, 2005), ultrasound probes (Hastenteufel et al, 2006;Hummel et al, 2002;Schicho et al, 2005), OR tables (Hummel et al, 2005;Nafis et al, 2008) and OR environments (Wilson et al, 2007). In summary, these papers also show that the EM trackers robustness regarding distortion sources have improved significantly over the latest years.…”

Newly Developed Electromagnetic Tracked Flexible Neuroendoscope -Technical Note-

“…Use of electromagnetic tracking systems for medical applications has grown sharply in recent years, partly due to the miniaturization of tracking sensor coils, 1,8,9,22) allowing greater freedom of surgical intervention for the surgeon and facilitating the tracking of flexible endoscopes. Surgical navigation systems for flexible endoscopes must clearly show the position of the scope tip.…”

Clinical motion tracking and motion analysis during ophthalmic surgery using electromagnetic tracking system

“…In order to maintain optimum tracking accuracy a non-metallic table was used. The DC EM tracking system is theoretically susceptible to metals with high magnetic permeability [5], but in our application such effects were minimized. In order to have a visual supervision over the experiments a 1.3 megapixel camera was added to the tracking system in an appropriate location to cover the surgical area (see Figure 3).…”