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tracking signal, which is inconvenient for the operating staff. Electromagnetic
trackers do not need line of sight, and—if
—
if the
field generator can be placed close
enough to the operating region
it is usually accurate enough.
When choosing an ultrasound machine for a tracked ultrasound system, we
should
—
first consider systems that are already integrated with position tracking, and
have research interface that provides real-time access to the ultrasound image and
tracking data streams. If tracking is not already available in the chosen ultrasound
machine, an external tracker needs to be attached to the transducer. Even if the
ultrasound machine does not offer digital access to the images and imaging
parameters, most ultrasound machines have a standard video output that can be
tapped into using a video grabber device.
Fixing the tracking sensor on the ultrasound transducer is not dif
cult using glue
or a rigid clip. If sterile environment is needed, the transducer along with the sensor
can be placed in a sterile bag. The reference position sensor needs to be
xed to the
patient as rigidly as possible. Since the reference sensor provides the link between
the patient and the navigation coordinate system, it makes the system more con-
venient to use if anatomical directions are marked on the reference sensor, so it can
be placed in the same orientation. A reference sensor holder can provide the ana-
tomical markers, along with an interface that can be
firmly attached to the skin
using an adhesive sheet (Fig.
2
). Tracking the needle is the most challenging task,
especially if the needle is thin (smaller than about 17 Ga) and bends during
insertion. A larger, more accurate sensor can be clipped to the needle using a
disposable plastic interface. But when the needle bends, a clipped sensor at the hub
will not give accurate information on the tip position. Smaller sensors can be
integrated in the needle stylet to provide direct tip tracking. Some companies offer
electromagnetically tracked stylets approved for clinical use. However, such small
sensors have a very limited (around 200 mm) usable range around the
field gen-
erator, which can make the system hard to set up around the patient.
Fig. 2 Reference sensor holder
