Information Technology Reference
In-Depth Information
Table 1 Summary of advantages and disadvantages of optical and electromagnetic tracking
technologies
Optical tracking
Electromagnetic tracking
Advantages
Accuracy
*
1
-
0.1 mm
Does not depend on
objects in its environment
Large range (several
meters)
Wireless position markers
Can track without line of sight (inside
body)
Position sensors can be small to
tin
needles and catheters (
*
0.5 mm)
Disadvantages
Requires line of sight
Optical markers are
relatively large
2mm
Limited range (typically 20
-
60 cm)
Affected by ferromagnetic metals in its
environment
Wired position sensors
Accuracy
*
1
-
Common position tracking devices in medicine are using either optical or
electromagnetic technology (Table
1
). Optical tracking uses cameras and optical
position markers that the computer automatically detects on the camera images. The
main advantages of optical tracking are its accuracy and robustness. The main
disadvantage is that the position markers need to be relatively large and in the line
of sight of the cameras. An emerging alternative to optical tracking is electro-
magnetic tracking technology that uses an electromagnetic
field generator, and
wired position sensors that detect their position relative the
field generator. Elec-
tromagnetic trackers generate a known changing magnetic
field, and measure the
currents in sensor coils that are induced by the changing magnetic
field. A signature
of currents in the sensor is unique to its position relative to the field generator. The
main advantage of electromagnetic tracking is that it does not require line of sight,
although it is less accurate and is sensitive to certain metal objects, especially
electric devices in its environment.
Tracking the ultrasound transducer expands the possibilities in ultrasound-gui-
ded needle interventions. By attaching a position tracker to the ultrasound trans-
ducer and the needle, their relative positions can be computed and visualized, even
when the needle is not in the ultrasound imaging plane. Such a tracked system can
be further enhanced by attaching another position sensor to the patient. This allows
visualization of the needle not only relative to the ultrasound image, but relative to
pre-procedural CT, MRI, or other models of patient anatomy.
There are other technologies for needle tracking in ultrasound-guided inter-
ventions beyond optical and electromagnetic. The most simple and oldest way is
mechanical tracking is to attach a passive needle guide to the ultrasound transducer.
Ultrasound guidance methods for abdominal interventions use mechanical needle
guides, but they constrain the needle motion to a single line relative to the ultra-
sound imaging plane. This line is displayed on the ultrasound display, so the
operators see where the needle will be inserted relative to the image. The needle
target can be chosen by moving the transducer with the
fixed needle guide. But in
the spine, the target areas are only visible from a limited range of angles. And the
