mage fusion enhances visualization in neurosurgery

Chris Chinno

SAN DIEGO, CA--At the Virtual Reality Meets Medicine conference in January, neurosurgeons described the latest developments in neuronavigation. Such capability can hel¥surgeons to plan surgeries, and in the operating room, neuronavigation guides them through delicate brain tissues.

For example, Dr. Norbert Huewel of Johannes Gutenburg University (Germany) described a new stereomicroscope from Carl Zeiss (Thornwood, NY) that allows a surgeon to look into the microscope, focus on a particular tissue, and have the three-dimensional (3-D) coordinates of the focal point calculated automatically. This "localization" in space is used to register a corresponding magnetic-resonance (MR) or computed-tomography (CT) image and display it in the right ocular of the microscope. The diagnostic image can be faded in and out to allow the surgeon to interactively fuse the diagnostic image with the visible image in the microscope (see photo).

Neuronavigation systems are important for a number of reasons. Perhaps the most compelling is that with improved navigation of surgical instruments, less trauma and damage to tissue may result, leading to better outcomes and lower costs. Dr. Patrick J. Kelly at New York University Medical Center (New York, NY) has performed more than 1100 neurosurgical procedures with advanced guidance systems. He sees the main benefit of these systems as providing hel¥in delineating the boundaries of a tumor. Dr. Kelly noted that "even though tumorous tissue may be readily identifiable in MR data, the boundaries of the tumor may not be so obvious once the surgeon is looking at the tissue through an operating microscope. By overlaying the diagnostic data with the visible image of the tumor, the surgeon can remove just the right amount of tissue."

Image registration

Such systems are sometimes called enhanced-reality systems because they augment the surgeon`s visualization of the surgical field. But one of the most important requirements in any type of image-fusion approach is the need to register the two images. Diagnostic data are collected presurgically in one coordinate reference frame, while a new coordinate reference frame exists in the operating room (OR). What complicates matters even further is the need to do this registration in three dimensions. Most methods for accomplishing this task rely on using reference markers that are visible on the patient in the OR and also in the diagnostic data.

The Carl Zeiss system places the microscope at the end of an articulated arm that contains many optical encoders, permitting precise calculation of the focus coordinates in 3-D space. Registration is accomplished by focusing on reference marks on the patient to determine the coordinates of these points in the surgical space. A computer then aligns these points to the corresponding points in the MR or CT data.

A second registration approach involves the use of probes whose position is tracked in space. By simply touching registration reference marks, coordinates are read out and aligned to the MR/CT reference points. Alternatively, a team working at Brigham and Women`s Hospital (Boston, MA) has developed a prototype system that uses a laser to scan the patient`s face and to produce a topographical ma¥that is compared to a computer rendering of the MR data to align the two contour maps of the face.

Neuronavigation allows the surgeon to "bridge the gap" between preoperative surgical planning and actually performing the surgery in the OR. Because preoperative surgical planning is usually done with MR or CT data and what the surgeon sees in the OR is a visible image, these neuronavigation systems hel¥the surgeon to realize the plan. For example, at the beginning of the surgery, the surgeon must first determine the right entry point and the correct trajectory to arrive at the tumor site. With the Zeiss instrument, the surgeon moves the microscope around the patient`s head to find the right entry point and trajectory. By simply focusing the microscope forward, the surgeon zooms through the MR data to see if he or she has arrived at the tumor site.

This feature is also useful during surgery, allowing a preview of what the surgeon will encounter next, permitting mid-course corrections, and helping to avoid dangerous areas such as vascular structures. Today, navigation accuracy to 2-3 mm is possible, but developers hope to achieve submillimeter accuracy within a few years.

Other techniques

Other diagnostic image-fusion methods are likely to become available to surgeons in the near future. For example, Dr. James Zinreich of the Johns Hopkins University (Baltimore, MD) reported on the merging of MR and positron-emission-tomography data. This will allow surgeons to see the functional ma¥of the brain fused with soft-tissue images, allowing better guidance to the site of interest. In addition, other medical disciplines are beginning to evaluate advanced neuronavigation technologies for spinal and sinus surgery. Both processes require precise navigation to avoid potentially fatal or debilitating consequences.