July/August 2008

A Messy Art

Continued from page 2

By Katrina S. Firlik

Once the patient is asleep, we can't actually start the operation until we've registered our navigation equipment with her anatomical data, carefully matching up her head images with her actual head. All told, equipment setup and registration can add up to a half-hour to the case.

Registration first requires immobilizing the patient's head in a three-point-fixation device that resembles a vice or ancient torture clamp. This part almost always makes a visitor squirm, and I agree it does seem brutal, but it's crucial. If the head moves even a little bit during the operation, all bets are off in terms of navigation accuracy. (I was impressed once when I saw doctors on Grey's Anatomy using what appeared to be an authentic, properly set up navigation system in a brain-tumor operation; but then I noticed that the patient's head wasn't stabilized in a clamp.) In some cases, an unsettling head wiggle can be detected partway through the surgery, and it's up to a nonsterile person in the room to peek under the sterile drapes and do some investigating while the surgeon pauses and feigns patience. Where is that damn wiggle coming from? The bed? One of the joints of the clamp? In an operation without navigation, we'll tolerate a little wiggle. In an operation with navigation, we can't afford to.

Once the head is immobilized, the surgeon touches the fine tip of the wand to the center of each fiducial marker and depresses a foot pedal. This correlates the location of the wand's tip with the image of the fiducial on the patient's MRI. One of many problems here is that five shiny metal balls attached to the butt of the wand must be visible to the large camera in the room in order for the system to accurately register the wand's location. Depending upon the patient's positioning, sometimes the camera can't see all the balls when the tip of the wand is at the center of a fiducial. We try to place the fiducials so that the balls won't end up hidden by a particular turn of the head, but we're not perfect.

Another source of fiddle factor is that certain parts of the scalp are mobile: consider how a sticker on your forehead moves if you wrinkle your brow. So the registered position of a given fiducial in the OR may be slightly different from the position recorded in the MRI scanner. Fiducials attached to the fixed bony prominence just behind the ear tend not to move as much--but they're particularly likely to be hidden from the camera if a patient's head is rotated. You can't win.

In a newer method of navigation registration, a handheld scanning device is moved slowly over the patient's face to register dozens of points along its topography, doing away with the fiducials altogether. But this system has its own kinks; for example, the tip of a patient's nose is sometimes "cut off" by the MRI scan. I've tried this scanning device but have not been able to get it to work well. Maybe I'll try it again at some point. But then I'd have to deal with the headache of using an unfamiliar technique.

Let's get back to our patient. We are able to capture eight of the ten fiducials, with an overall margin of error of 1.4 millimeters. Decent. I do a crude check of the system by placing the tip of the wand at the top of the bridge of the patient's nose, right in the center. I look up at the monitor, which displays the patient's MRI in three planes. The position of the dot on the images--representing the tip of my wand--assures me that the system can tell where I am. I do a similar check with the inner and outer corners of both eyes. Perfect.

Next is the fun part. Before the operation begins, before I even shave a path for the incision and prepare the head with an antibiotic solution, I test my own visual-spatial skills. Where do I think the tumor is? I know it's in a certain region of the left frontal lobe, but here's why this exercise is a bit of a challenge: the frontal lobe is large (the largest lobe of the brain), the tumor is small, and the head is round. I point to where I think the tumor is, mark the scalp with a surgical marker, and then reach for the wand. I run its tip around in the general vicinity of my mark and watch the corresponding MRI images as they appear on the screen. The images continually shift as I move the wand. When I reach the point that lies right above the middle of the tumor, I freeze my position. My original guess was about two centimeters off: not terrible, but certainly not dead on.