Effects of sensor orientation on AC electromagnetic tracking system accuracy in a CT scanner environment

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.