Standardized evaluation method for electromagnetic tracking systems

The major aim of this work was to define a protocol for evaluation of electromagnetic tracking systems (EMTS). Using this protocol we compared two commercial EMTS: the Ascension micro-BIRD (B) and NDI Aurora (A). To enable reproducibility and comparability of the assessments a machined base plate was designed, in which a 50 mm grid of holes is precision drilled for position measurements. A circle of 32 equispaced holes in the center enables the assessment of rotation. A small mount which fits into pairs of grid holes on the base plate is used to mount the sensor in a defined and rigid way. 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 SST 303, SST 416, aluminum, and bronze) into the sensitive volume between sensor and emitter. Additionally the dynamic behavior was tested by using an optical sensor mounted on a spacer in a distance of 150 mm to the EXITS sensors. We found a relative positional error of 0.96mm +/- 0.68mm, range -0.06mm;2.23mm (A) and 1.14mm +/- 0.78mm, range -3.72mm-1.57mm(B) for a give distance of 50 mm. The positional jitter amounted to 0.14 mm(A)/ 0.20mm (B). The relative rotation error was found to be 1.81 degrees(A) / 0.63 degrees(B). For the dynamic behavior we calculated an error of 1.63mm(A)/1.93mm(B). The most relevant distortion caused by metallic objects results from SST 416. The maximum error 4.2mm(A)/41.9mm(B) occurs when the rod is close to the sensor(20mm).