Biomedical Engineering Reference
In-Depth Information
12.2.1.5
Active/Passive Optical Tracker
With three linear cameras or two 2D cameras, if a point can be located in
each view, the 3D position of the point relative to the cameras can be calcu-
lated. This is the basis of a number of tracking systems. The localized points
are either active (bright infrared-emitting diodes, IREDs) or passive (highly
reflecting spheres). In smaller camera systems such as the Polaris from
Northern Digital Inc. (Ontario, Canada) or Flashpoint from Image-Guided
Technologies (Colorado, U.S.), each IRED or reflecting sphere can be local-
ized with a FLE of 0.2 to 0.4 mm. With the Optotrak, a larger and more
expensive version from Northern Digital, accuracy is 0.1 to 0.2 mm. The
main difficulty with optical tracking is that line of sight between the cam-
eras and tracked objects must be maintained. An additional possible error
source with the reflecting sphere system is partial obscuration of the sphere
or contamination with blood, which may shift the measured location. Nev-
ertheless, the high accuracy and stability of these systems has meant that
optical tracking is now the technology of choice for most commercial
image-guided surgery systems.
12.2.2
Immobilization Techniques
Having defined a coordinate system for the room, we now want to describe
physical points inside the patient. To define accurate coordinates within a
patient, the subject must be rigidly fixed with respect either to a tracker or the
coordinate measurement device itself.
12.2.2.1
Stereotactic Frames
Stereotactic frames have been used in neurosurgery since the 1950s.
These devices are rigidly attached to the patient's skull prior to imaging.
High contrast imaging markers relate the frame to a trajectory and target
defined from the images. The therapeutic device, usually a biopsy needle
or electrode, is then mounted on the frame, for example via an isocentric
arc system. The angles on the arc are calculated to give the required tra-
jectory. Since the frame inherently defines a coordinate system, there is
no need for any of the tracking technologies described in the previous
section.
Stereotactic frames have the advantage that they have been used for many
years and are well understood by surgeons. They are also considered by
many clinicians to be the most accurate guidance method and have been
measured as providing registration accuracies of better than 1 mm.
14
There
have, however, been some suggestions that the accuracy is overstated.
15
The
disadvantages are that only a single point target and trajectory can be defined,
the frame is invasive and uncomfortable for the patient as the device has to
