St. Paul, MN--The five-year-old girl lies motionless beneath the blue drapes atop the operating table, her head held snugly in the jaws of a jig-like device known as a three-point pinion. At one end of the table, brain surgeon Mary Beth Dunn hefts a pneumatic saw and aims it at the child's head.
It's a simple procedure: The surgeon drills a series of holes in the skull, then uses the saw to connect the dots. Known as a craniotomy, it leaves an opening large enough for the surgeon to gain access to the brain. Dunn has performed the procedure countless times. Operating the saw at speeds up to 160,000 rpm, she quickly cuts a 2-inch-diameter hole in the child's temporal lobe, scattering bits of bone and blood on the blue drapes.
For visitors, the craniotomy is a reminder of the critical nature of the operation. There, lying on the table, is a five-year-old girl with a gaping wound--a two-inch-wide hole in her head. When the bone flap is removed, it reveals a brain that pulses visibly with each beat of the child's heart.
For Dr. Mary Beth Dunn, however, this two-inch hole has a very different meaning. When Dunn looks at it, she sees technological advancement. A few years ago, a typical craniotomy was wider than a grown man's fist. "We used to make the craniotomy about two or three times this size," she says, pointing to the opening.
Now, however, Dunn uses an image-guided procedure known as frameless stereotaxy. Incorporating MRI data taken the previous day, she can more easily pinpoint the location of the child's brain tumor before she opens the skull. As a result, the craniotomy can be far smaller.
Since Dunn started using the system, known as the Regulus Navigator, she says her patients recover faster. "These systems are so precise that there isn't much tissue damage," she explains. "The openings are smaller, so the recoveries are quicker."
Indeed, Dunn says that her patients' average length of stay for a craniotomy has dropped from more than six days to two. And intensive care time is shorter, too. "Our ICU stays used to be two days," Dunn says. "Now we measure them in hours."
Greater precision. Fast recovery, however, isn't the only reason for using the image-guided system. Surgeons say it also helps them tell good brain tissue from bad.
After the skull has been opened, Dunn penetrates the dura mater--the brain's protective lining--and begins the search for the child's tumor. Surprisingly, tumors aren't easily recognized. "I can't always use my native eye to distinguish," Dunn says. "The reason is that a lot of tumors arise from cells that are, by nature, supposed to be there. So they look like the good brain tissue right next to them."
Often, that neighboring brain tissue may be responsible for critical functions--speech, hearing, or various forms of comprehension. If the surgeon accidentally resects that tissue, the function may be lost.
To ensure that she does not damage good brain tissue, Dunn uses a hand-held probe. The probe, about the size of a tire pressure gauge, enables her to find the location of brain structures shown on the MRI taken a day earlier. Dunn touches the probe to the child's brain, then stares up at a computer monitor near the wall. Her probe can be seen on the monitor as a yellow crosshairs. As Dunn shifts the probe, the yellow crosshairs move precariously close to a white, walnut-sized lump on the MRI picture. That lump, Dunn knows, is the tumor. By looking back and forth from the monitor to the actual brain, she finds the boundary line between good brain tissue and bad.
Dunn could do all of this without the probe, of course, but the recovery times would be longer and the risks, greater. "A surgeon knows from years of experience when the brain tissue looks a little funny," Dunn says. "But about the time you start betting on that experience, you run into trouble."
Computer connection. Not long ago, however, many surgeons relied solely on their experience to find and recognize tumors. It wasn't until the early 1980s that a few began to consider computer technology as a potential operating room aid. One of those, Dr. Patrick Kelly of the Mayo Clinic, began to toy with the idea of combining computers with stereotactic systems--that is, mechanical systems that help position instruments.
As early as 1980, Kelly wrote that "without a dedicated operating room computer...volumetric stereotactic procedures were too time-consuming to be practical." By 1982, he and his associates had incorporated a computer in their work and had developed stereotactic software to go with it. Those efforts eventually led to the formation of Compass International Inc. (Rochester, MN), which later developed the Regulus Navigator.
Then, as now, much of Compass' work focused on the use of computers to store imaging (MRI or CT scan) data. Its early systems combined the computerized imagery with so-called stereotactic frames--bulky, halo-like structures that were fixed to the patient's head. The frames aided surgeons in their efforts to locate precise structures within the brain, but they were not without drawbacks. They were uncomfortable for patients because their carbon fiber pins needed to be inserted through the scalp and anchored in the skull.
That's where the new frameless system is different. The Regulus Navigator, designed by the company's staff of six engineers, does not need a mechanical connection to the patient. Instead of a frame, the Regulus employs stick-on radiographic reference markers made by I.Z.I. Medical Products Corp., Baltimore, MD. Surgeons place the adhesive-backed markers at eight locations around the patient's head before doing the CT scan or MRI. Then, when the "pictures" are taken, the radiographic markers are visible on the images. Images are then stored in a SPARCstation, made by Sun Microsystems Inc. (Mountain View, CA) which is an integral part of the Regulus planning console.
On the day of surgery, those images are re-displayed on a monitor in the operating room. When the neurosurgeon subsequently touches the patient's reference markers with the stereotactic probe, he or she begins the process of mapping the points in three-dimensional space. Using a so-called transformation matrix, the software in the SPARCstation matches the patient data from the markers to the image data from the MRI. Then, as the probe moves, it appears on the monitor with reference to various brain structures, including the tumor. "With the MRI data, we build a map," explains Nick Chapman, a neurosurgical specialist at United Hospital, (St. Paul, MN) where Dr. Dunn performed the operation. "Then we go to the operating room and orient the patient to that map."
The key to this real-time navigation is the creation of a magnetic field around the patient's head. Compass engineers accomplish that by mounting a magnetic field transmitter on the three-point pinion, or jig, that holds the head in place. The 144-Hz pulsed dc device, known as the Flock of Birds transmitter, is made by Ascension Technology Corp. (Burlington, VT). It emits a spherical magnetic field measuring about one cubic meter. The probe's position within that spherical magnetic field is recognized because it carries a small magnetic field receiver. "No matter where the probe is within that sphere, the system can always calculate distances," notes Jon Rousu, director of engineering for Compass International.
Economy and simplicity. Compass engineers say they could have used other means to create a frameless stereotactical system. Indeed, frameless infrared techniques are now available, as well as wands that employ an articulating arm.
Compass' goal for the product, however, was to keep costs low and promote ease of use. Unlike previous frameless systems, which have cost more than $200,000, the Regulus ranges between $100,000 and $120,000. The key to that, Rousu says, was the use of magnetic technology. "The magnetic technology is inherently less costly, so it seemed like the best fit," he explains. "With this magnetic system, you just bolt the transmitter to the clamp, touch a few points, and you're in the navigation mode." In contrast, he says, infrared systems require more set up and require line-of-sight accessibility to work. As a result, hospitals must install IR cameras somewhere above the operating table to make the systems work.
Ultimately, Compass engineers say they expect to extend the technology for use in frontal and endoscopic sinus surgery, as well as for spinal cases. They also hope to broaden its use in brain tumor surgeries, of which there are currently between 50,000 to 60,000 per year in the U.S.
Among neurosurgeons who have used the technology, however, there is little doubt of the its ultimate value. "This girl will recover fully and should live a long, healthy life," Dunn says, nodding toward the five-year-old patient on the operating table. "And the technology helps make that happen."
For more information
Call 1-800-828-6344 X011 and key in the specific Product Code below
Radiographic reference markers by I.Z.I. Medical Products Corp ; Product Code 4306
SPARCstations from Sun Microsystems ; Product Code 4307
Magnetic field transmitters from Ascension Technology Corp ;Product Code 4308