EM Tracker Coil Characterization

EM Tracker Coil Characterization

When we test a tracker for accuracy, we generally check using point sets that make mechanical sense. For example, we may use a 3D robot to move the receiver all over the working volume (also called the motion box).

In 6DOF trackers, the coils must be precisely characterized for their electromagnetic properties: gains, non-orthogonalities, non-concentricities, and finite-size effects. Since characterization is measuring electromagnetic properties, the point set chosen should make electromagnetic sense rather than mechanical sense.

The coils can only be manufactured to so much precision. Characterization measures the actual properties of each coil, to improve the tracker accuracy.

The important electromagnetic property, is the boundary-condition property: If we know the magnetic field everywhere on the boundary of the working volume, we can calculate the field inside the working volume.

For coil characterization, the boundary-condition property suggests characterizing coils using only the the plane of the working volume closest to the transmitter. This could be done with a 2D robot or with a 3D robot. If a 3D robot is used, we can then use the remainder of the 3D robot points to check mechanical accuracy.

Pete believes (though has not verified) that the 2D robot can be replaced by the scribble-test data-collection method in this paper:

• C.A. Nafis, V. Jensen, L. Beauregard, P.T. Anderson, "Method for estimating dynamic EM tracking accuracy of Surgical Navigation tools", SPIE Medical Imaging Proceedings, 2006. Reports low-cost accuracy-measuring techniques and results for various trackers.

The scribble data-collection method involves mounting the receiver on a small flat slider, and slowly sliding the receiver around (and rotating the receiver) on a known-flat surface (such as a granite surface plate). We know that the points mechanically are all on a plane (though we do not know exactly what plane), and that the receiver orientations must all be the same, except for rotations about the axis perpendicular to the plane.