12
The zeroing method
This method must be implemented by the integrator and be
automatically run at the application startup and inform the
user on how to proceed.
The zeroing method is a procedure which checks that the fusionTrack device and the used
markers
are
correctly calibrated. A failure of this test does not allow to determine which element is faulty: it is a global
test. In case of a failure of this test, the navigation process is not allowed to start.
12.1
General concept
The principle of the zeroing method is simple: a known distance is checked to be measured correctly.
Most surgical applications use a pointer, the position of the pointer tip is therefore a good candidate to be
monitored. This is achieved by using the pointer and an additional
marker
, the latter is required to have a
cavity in which the tip of the pointer must fit whatever the pointer position and orientation (e.g. a cone or
a cylinder), as illustrated in Figure 12.1. Once the pointer tip has been inserted in the
marker
cavity, the
pointer should be rotated around its tip, which must remain in the
marker
cavity. The fusionTrack records
the position and orientation of the marker and the pointer, from which the position of the tip of the pointer
relative to the marker is extracted. This relative position should not vary within measurement uncertainties.
During this operation, the marker must lie around the centre of the working volume, as shown in Figure 12.2.
Figure 12.1: The tip of the pointer must lie in the cavity.
The integrator must specify a maximal value on the error of the position, above which the test is consid-
ered to fail. A failure of the test may be caused by (non-exhaustive list):
76
