Biomedical Engineering Reference
In-Depth Information
to track the patient without the need for immobilization. This is only relevant
if we have a treatment machine capable of adjusting for small movements.
In the meantime, tracking methods may be able to detect patient movement
and abort treatment when necessary.
For surgery away from bone, it will be necessary to solve the problem of
nonrigid tissue movement. For surgery of the thorax or abdomen there will
be considerable movement of tissue, and this must be compensated for. The
use of surface registration techniques has been proposed to allow preopera-
tive CT images to help guide liver resections.
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Tracked ultrasound, by pro-
viding real time image data about underlying structures, may become a
significant tool for this purpose. Interventional MRI, which gives volumetric
data in close to real time, will also play a useful role as the cost of this emerg-
ing technology comes down.
12.6.1
Clinical Accuracy Requirements
It is important to consider what accuracy is required for a given application.
It has become the holy grail of neurosurgical guidance that the accuracy of
any system should be
1 mm. The accurate placement of a small cranio-
tomy, however, probably does not require accuracy much better than about
5 mm,
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and for some procedures in the thorax and abdomen it may be possi-
ble to tolerate even larger errors.
12.6.2
Visualization
Alignment of the preoperative images to the patient is not the end of the story
for image-guided therapy. The preoperative data still need to be presented to
the clinician in a useful way. In standard commercial neuronavigation sys-
tems, image guidance is provided using a tracked pointer or probe, and the
position of this is shown as orthogonal slices, or perhaps surface-rendered
view on a computer monitor (Figure 12.4a). This has the disadvantage that
FIGURE 12.4
Pointer based guidance (a) showing the need to look away from the surgical field, and
augmented reality guidance (b) with overlays directly on the surgical view.
