Many surgical procedures require highly precise localization, often of deeply buried structures, in order for the surgeon to extract targeted tissue with minimal damage to nearby structures. Although methods such as MRI and CT are invaluable in imaging and displaying the internal 3D structure of the body, the surgeon still faces a key problem in applying that information to the actual procedure. Since he is limited to seeing exposed surfaces within the surgical opening, he cannot easily visualize paths to targets or positions of nearby, but hidden, critical structures. As well, the lack of visible landmarks within the surgical opening may inhibit his ability to determine his current position, and thus to navigate safe trajectories to other structures.

Because traditional clinical practice often only utilizes 2D slices of MR or CT imagery, the surgeon must mentally transform critical image information to the actual patient. Thus, there is a need for techniques to register a 3D reconstruction of internal anatomy with the surgical field. Such registered information would support image-guided surgery, by allowing the surgeon to directly visualize important structures, and plan and act accordingly. Visualization methods include "enhanced reality visualization" [14], in which rendered internal structures are overlaid on the surgeon's field-of-view, and instrument tracking, in which medical instruments acting on the patient are localized and visualized in the 3D MR or CT imagery. The benefits of image guided surgical methods include the following:

• Accelerated migration to minimally-invasive surgeries via improved hand-eye coordination and better transfer of a priori plans to the patient.

• Shorter procedures through increased visualization of the surgical field.

• Reduced risk of sensitive tissue damage.

• More accurate and complete tissue resection, ablation, or biopsy.

The key stages of an accurate, reliable, image-guided surgery system are as follows:

• Creating accurate, detailed, patient-specific models of relevant anatomy for the surgical procedure.

• Registering the models, and the corresponding imagery, to the patient.

• Maintaining the registration throughout the surgical procedure.

• Tracking medical instruments in the surgical field in order to visualize them in the context of the MR/CT imagery and the reconstructed models.

In this paper, we describe the registration process used to align preoperative imagery with the actual patient position, and the process by which a surgeon visualizes and navigates through the patient using that information. We do this using an example of a neurosurgical image guidance system, although the same issues arise in other areas as well.

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