The purpose of this study was to examine the effects of different sensor orientation on the positional accuracy of an AC
electromagnetic tracking system, the second generation NDI Aurora, within a CT scanner environment. A three-axis
positioning robot was used to move three electromagnetically tracked needles above the CT table throughout a 30cm by
30cm by 30cm volume sampled in 2.5cm steps. All three needle tips were held within 2mm of each other, with the
needle axes orthogonally located in the +x, +y, and +z directions of the Aurora coordinate system. The corresponding
position data was captured from the Aurora for each needle and was registered to the positioning system data using a
rigid body transformation minimizing the least squares L2-norm. For all three needle orientations the largest errors were
observed farthest from the field generator and closest to the CT table. However, the 3D distortion error patterns were
different for each needle, demonstrating that the sensor orientation has an effect on the positional measurement of the
sensor. This suggests that the effectiveness of using arrays of reference sensors to model and correct for metal distortions
may depend strongly on the orientation of the reference sensors in relation to the orientation of the tracked device. In an
ideal situation, the reference sensors should be oriented in the same direction as the tracked needle.
This paper proposes an assessment protocol that incorporates both hardware and analysis methods for evaluation of
electromagnetic tracker accuracy in different clinical environments. The susceptibility of electromagnetic tracker
measurement accuracy is both highly dependent on nearby ferromagnetic interference sources and non-isotropic. These
inherent limitations combined with the various hardware components and assessment techniques used within different
studies makes the direct comparison of measurement accuracy between studies difficult. This paper presents a multicenter
study to evaluate electromagnetic devices in different clinical environments using a common hardware phantom
and assessment techniques so that results are directly comparable. Measurement accuracy has been shown to be in the
range of 0.79-6.67mm within a 180mm3 sub-volume of the Aurora measurement space in five different clinical
environments.
The purpose of this study was to quantify the effects of a computed tomography (CT) scanner environment on the
positional accuracy of an AC electromagnetic tracking system, the second generation NDI Aurora. A three-axis
positioning robot was used to move an electromagnetically tracked needle above the CT table throughout a 30cm by
30cm axial plane sampled in 2.5cm steps. The corresponding position data was captured from the Aurora and was
registered to the positioning system data using a rigid body transformation minimizing the least squares L2-norm. Data
was sampled at varying distances from the CT gantry (three feet, two feet, and one foot) and with the CT table in a
nominal position and lowered by 10cm. A coordinate system was defined with the x axis normal to the CT table and the
origin at the center of the CT table, and the z axis spanning the table in the lateral direction with the origin at the center
of the CT table. In this coordinate system, the positional relationships of each sampled point, the CT table, and the
Aurora field generator are clearly defined. This allows error maps to be displayed in accurate spatial relationship to the
CT scanner as well as to a representative patient anatomy. By quantifying the distortions in relation to the position of CT
scanner components and the Aurora field generator, the optimal working field of view and recommended guidelines for
operation can be determined such that targeting inside human anatomy can be done with reasonable expectations of
desired performance.
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