Biomedical Engineering Reference
In-Depth Information
• The breakdown of assumptions inherent in the type of transforma-
tion used. Almost all image guidance assumes a six degree of free-
dom rigid-body transformation between images and physical space,
so any tissue deformation, voluntary or involuntary, by the patient,
or tissue distortion caused by the intervention itself will lead to error.
12.2.1.2
Mechanical Localizers
The first frameless neuronavigation device to be widely used was the Faro arm
(Faro Technologies Inc., Florida, U.S.), a mechanical device that is attached to
the side of the surgical table. Encoders on each of the axes of the arm enable cal-
culation of the tip position. Problems with such a device are that range of
movement is somewhat limited, and any movement of the head clamp
requires reregistration. Moreover, the inherent accuracy was found to be some-
what lower than other methods. Marketed as the ISG Viewing Wand, this
mechanical localizer started the regular use of frameless image guidance, find-
ing applications in ear, nose, and throat (ENT) surgery
6
7-10
and neurosurgery.
12.2.1.3
Ultrasound Transducers in Air
The first example of frameless navigation was the system developed by Roberts
et al.
11
This system used a microscope to register the images to the patient and
provide the guidance information. The localization system was based on
ultrasonic “spark-gap” transducers attached to the microscope. These emit a
very short ultrasound pulse which can be detected by three or more micro-
phones in the operating room. The time delay for the sound pulse to reach
each microphone gives a measure of distance and hence localizes the spark
gap. Others have developed this technology for conventional pointer-based
guidance,
12
where the transducers are attached to a wand. Some problems have
been encountered due to variations of the speed of sound with temperature
and air flow which led to a significant fiducial localization error of several
millimeters, but research is still being carried out to refine the method.
13
12.2.1.4
Radio Frequency Tracking
Another technology that has been proposed for surgical tracking is radio fre-
quency (RF) coils. Three large orthogonal coils are used to transmit a signal
that is picked up by three smaller orthogonal coils inside a tracker attached
to the patient or pointing device. The signal received by each of the receiver
coils can be used to calculate the position and orientation of the tracker.
These devices suffer from inaccuracies when brought close to metal due to
distortion of the RF field. This distortion can lead to FLEs of several centime-
ters, and there is usually no way of knowing that the accuracy has been com-
promised in this way. For neurosurgery, a wooden or plastic operating table
needs to be constructed. Interference with other devices in the operating the-
ater has also been reported.
