Biomedical Engineering Reference
In-Depth Information
(a)
(b)
FIGURE 12.4
(a) Electromagnetic tracking used to guide percutaneous biopsy of a lung tumor in the CT suite. Doughnut markers on back of
patient are used for paired point registration. (b) The physician is using a tracked biopsy needle to touch each of the doughnut markers for paired-
point registration. These needles have an electromagnetic sensor at their tips and the wire coming out of each needle connects to each control
station for the electromagnetic tracking system.
following the procedure is hard to gauge using CT or US, PET/
CT has been shown to provide an effective means of determin-
ing procedural efficacy (Israel et al. 2001). Therefore, image fusion
will most likely play an increasingly important role in conjunc-
tion with registration of multiple imaging modalities to complete
a staging, planning, and postprocedural assessment function.
instrument. Once the images have been acquired and regis-
tered and a path has been planned, the image-guided system
can provide a navigation view, where the tracked position of
the instrument is overlaid onto the patient anatomy. The navi-
gation view of an open source image-guided system developed
in our research group is shown in Figure 12.5. As described in
the figure caption, the navigation system provides a four-quad-
rant display that includes several views of the anatomy and the
planned path. The physician can then use this display to guide
the instrument to the target. The system was constructed using
the open source software package the Image-Guided Surgical
Toolkit (Enquobahrie et al. 2007), which is freely available at
igstk.org and can be used to prototype image-guided surgery
applications.
12.2.5 planning
The success of a percutaneous tumor treatment depends on the
choice of probe trajectory, maximal destruction of the tumor,
and minimal harm of healthy tissue. Planning, therefore, needs
to optimize these parameters. Typically, planning enables the
physician to preview the patient dataset and plan an optimal
path to the tumor while avoiding critical structures.
The tasks defined within the planning component have a signif-
icant dependence on the clinical procedure. Each therapy choice
has technological limitations based on the physics of its operation
and unique criteria that need to be optimized. With procedures
such as RFA and cryoablation, ablation of a margin of tissue sur-
rounding the tumor is critical to a successful clinical outcome.
More sophisticated planning algorithms incorporate models
that characterize tissue properties, ablation dynamics, and heat-
sink systems. For RFA applications, the Pennes bioheat equation
(Pennes 1948) determines the transfer of heat energy around an
RFA probe in tissue. The planning component of an image-guided
therapy application taking this into consideration can therefore
provide a representation of the heat-induced necrosis region
around the RFA probe (accounting for vasculature in the vicin-
ity and the subsequent result of heat-sink), overlay this informa-
tion on the preoperative dataset, and plan a trajectory to the target
site such that the full tumor is covered and critical structures are
avoided along the path of probe insertion.
FIGURE 12.5
Navigation view for image-guided lung biopsy. The
top-left window shows a true axial view, and it can be seen that a true
axial path would hit the rib. Therefore, the off-axial path in the bottom-
left window was chosen. The top-right window shows a sagittal view.
A birds-eye view, looking down the needle shaft, is shown in the bot-
tom-right window.
12.2.6 Navigation
Navigation refers to the process of using the virtual display
from an image-guided system to precisely place the therapeutic
