Page 435 - Biomedical Engineering and Design Handbook Volume 2, Applications

COMPUTER-INTEGRATED SURGERY AND MEDICAL ROBOTICS 413

Navigation Systems. The purpose of intraoperative navigation systems is to provide surgeons with
up-to-date, real-time information about the location of surgical tools and selected anatomy during
surgery. The goal is to improve the surgeon’s hand/eye coordination and spatial perception, thereby
improving the accuracy of the surgical gestures. They support less invasive procedures, can shorten
surgery time, and can improve outcomes.
The basic elements of a navigation system are
1. A real-time tracking system to follow one or more moving objects (anatomy, surgical tools, or
implants)
2. Tracking-enabled tools and reference frames
3. A display showing the intraoperative situation
4. A computer to integrate the information (Fig. 14.2)
Since the patient is usually not immobilized, a dynamic reference frame is attached to the anatomy
to correct the relative position of the tools to the images.
What is displayed depends on the type of images that are available. The navigation systems can
be based on
• Preoperative images, such as CT or MRI augmented with CAD models of tools and implants
• Intraoperative images, such as fluoroscopic x-ray, ultrasound, or open MR images augmented
with projections of tool CAD models and implant axes
• Intraoperative video streams from an endoscopic camera or a surgical microscope, shown along-
side or fused with preoperative CT or MRI images
Navigation systems based on preoperative CT or MRI images are typically used as follows:
Shortly before surgery, a preoperative CT or MRI study of the anatomy of interest is acquired. In
some cases, fiducial markers that will be used for registration are attached to the patient skin or
implanted to the anatomy so that they appear in the images. The data are downloaded to a computer,
and a model of the anatomy is created. When there are fiducials, they are identified and their precise
relative spatial location is computed. The surgeon can visualize the data and elaborate the surgical
plan. Before the surgery starts, the preoperative data, model, and plan are downloaded to the com-
puter in the operating room. A dynamic reference frame is attached to the patient, and the intraoper-
ative situation is registered with the preoperative data by either touching the fiducials with a tracked
tool, or by acquiring a cloud of points on the surface of the anatomy. Once the registration has taken
place, a display showing the preoperative images and model with the CAD models of the tools super-
imposed is created on the basis of the current tool and anatomy position obtained from the tracker
(Fig. 14.14). Several commercial systems are currently available for a variety of procedures. Clinical
studies report millimetric accuracy on tool and implant positioning. These types of systems have
been applied extensively in orthopedics, (the spine, 58,81,82 ) pelvis, 83,84 fractures, 85–89 hip, 56,90,91 and
knee 57,92–94 neurosurgery, and craneofacial and maxillofacial surgery.
Navigation systems based on intraoperative images combine intraoperative images with position
data from surgical tools and implants to create augmented intraoperative views. An example of such
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system is the FluoroNav system, which uses fluoroscopic x-ray images. During surgery, a tracking
device is attached to the fluoroscopic C arm, and one or more images are acquired with it.
Projections of the tools are then superimposed on the original images and updated in real time as the
tools move (Fig. 14.15). Since the camera and the tools are tracked simultaneously, there is no need
for registration. The advantages of these systems are that they do not require a preoperative study
and that no registration is necessary. However, the views remain two dimensional, requiring the sur-
geon to mentally recreate the spatial intraoperative situation. Recent clinical studies show that these
systems are having excellent acceptance, since they are closest to current practice, and beginning to
be used successfully. 95
Other navigation systems combine video stream data obtained from endoscopic cameras or sur-
gical microscopes, with data from preoperative studies, such as CT or MRI. The camera is tracked,
so its position and orientation during surgery are known and can be shown, after registration,