Biomedical Engineering Reference
In-Depth Information
The rapid improvement in computing capabilities, the devel-
opment of systems capable of tracking objects in space, and
improvements in tomographic imaging modalities, including
CT and magnetic resonance imaging (MRI), led to a paradigm
shift in the practice of image-guided surgery. As optical track-
ing systems became smaller and less expensive, their adoption in
clinical practice led to frameless stereotaxy as a means of local-
izing surgical tools without a bulky mechanical reference frame.
This progression enabled increasingly less invasive surgical
interventions while simultaneously achieving improved clinical
accuracy and outcome.
The shift toward minimally invasive surgery necessitates a
greater reliance on image guidance and preoperative planning.
Minimally invasive surgery can reduce patient trauma and lead
to faster recovery times compared to conventional open surgery.
The primary goal of image guidance and navigation is to assist
the physician in precisely localizing the anatomy of interest. This
goal is accomplished by overlaying the surgical tools on an aug-
mented reality view of the patient anatomy.
A typical navigated image-guided surgery system consists
of five key components as shown in Figure 12.1: (1) imaging,
(2) tracking, (3) registration, (4) planning, and (5) naviga-
tion. A medical image dataset serves as the anatomical map
upon which tracked surgical tools are displayed. Surgical
tools are tracked in three-dimensional space by a tracking
system. The image dataset and tracker information are ref-
erenced in different coordinate reference frames. The process
of registration aligns these two coordinate frames to a single
common frame. Once registered, the physician can manipu-
late the visualization and representation of the patient data.
Navigation and guidance is achieved by constantly updating
the position of the tracked surgical tools with respect to the
patient image. A brief overview of each of these components
is provided next.
12.2.2 Imaging
Within the context of an image-guided navigation system, the
role of the imaging modality is primarily to provide a three-
dimensional (3D) map of the patient anatomy. CT is the pre-
ferred modality for visualizing bony structures as well as lungs
and organs in the chest cavity. MRI, however, provides better
soft tissue contrast and is preferred for brain and spinal cord
imaging or visualizing tendons and ligaments. While CT has
the disadvantage of using ionizing radiation, the risk versus
reward to the patient must always be considered. Image-guided
navigation often reduces the number of CT images required to
precisely target the anatomy and, therefore, may lead to a net
reduction in total radiation dose when compared against con-
ventional imaging without the use of navigated image guidance.
Limitations with MRI include high cost, availability, and the
need for compatible ablation tools and devices. Ultrasound has
the advantages of being real time, low cost, and readily avail-
able, but the image discrimination of ultrasound for navigation
purposes can be poor and ultrasound is usually deployed as a
two-dimensional imaging modality.
12.2.3 tracking
Tracking devices are an essential component of a navigated
image-guided surgery system. These devices are used to track
the position of instruments relative to the patient anatomy.
Improvements in sophistication and accuracy of tracking sys-
tems in the last 10 years have been critical to the development of
image-guided systems. Early tracking systems were essentially
mechanical digitizers and proved to be too large and bulky for
use in a clinical operating environment. Optical tracking sys-
tems were soon adopted due to their high accuracy and relatively
large workspace. However, optical tracking systems require that
a line-of-sight be maintained between the tracking device and
Acquire pre-operative tomographic dataset
Imaging
Localize surgical instruments
Tracking
Register surgical tools relative to pre-operative data
coordinates
Registration
Visualize patient images and plan surgical
procedure
Planning
Real-time visualization of surgical tools relative to
patient anatomy
Navigation
FIGURE 12.1
Image-guided system workflow.
