Quantification of AC electromagnetic tracking system accuracy in a CT scanner environment

Abstract

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.