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we can point to a specific location on or in a patient's head, and
the system will show us---we hope---the corresponding spot on
a previously obtained MRI of the patient's brain. It works well
most of the time, but like almost every other technology we
surgeons use, it has a few kinks and causes a few headaches.
Most brain trauma cases don't require the navigation tech-
nology, for three reasons. First of all, if the case is urgent, we
don't have time to set up the equipment and do the necessary
scanning. Second, what we're after is usually large and can't be
missed, like a big blood clot. Third, in trauma cases we're less
concerned about the niceties that navigation helps us provide,
such as a minimal hair shave and a minimal incision.
A small tumor, on the other hand, is a perfect situation for
navigation. I'll walk you through a sample case, altering (in the
interest of patient privacy) a few unimportant details.
The patient is a 62-year-old woman who has had a seizure,
the first of her life. Upon visiting the hospital, she undergoes
a brain MRI, which picks up a round, two-centimeter tumor
in her left frontal lobe. She has been a smoker since age 20.
She has no previous history of cancer.
In a long-term smoker, a small, round tumor in the brain
certainly doesn't look good, but we always hedge our bets: "We
won't know what it is for sure until we actually get a piece of
it." In our line of work, it's not unusual to see a diagnosis of
lung cancer made only after the disease has metastasized to
the brain. The parent tumor may have lurked silently within
the lung for years.
The decision for surgery is made by the patient, her oncolo-
gist, and me. Such decisions take many variables into account,
but su ce it to say that medicine is often equal parts science
and art. As is often the case in neurosurgery, the best treatment
is not entirely obvious. Something has to be done, but that
something doesn't necessarily need to be surgery: the patient
could choose the noninvasive option of stereotactic radiosur-
gery, a focused form of radiation that can control or shrink
(but not necessarily get rid of) a tumor in the brain. This wom-
an's oncologist, however, strongly favors surgery. So now the
patient is about to go under the knife. I spend plenty of time
with her and her family, preparing them for the experience.
Just before surgery, my patient is required to undergo a
second MRI ("You want me to get another one of those?" she
asks me), this time with several fiducial markers (small, round
foam stickers with holes in their centers) applied to her head
to serve as reference points. This particular MRI is sliced even
finer than her original one, and the images will be downloaded
into our navigation equipment. We're aiming for millimeter-
Next, in the operating room, while waiting for the patient
to be put under general anesthesia and "lined up" (fitted with
various catheters, or lines), I speak with the circulating OR
nurse and my physician assistant about the navigation setup.
(Given that we'll use a lot of bulky equipment, we give thanks
if we're in one of the larger operating rooms.) Where will the
head of the bed be? Did the disc with the patient's MRI actually
make it up from radiology? Where do we position the monitor?
What about the camera that tracks the location of the pointer
probe? We don't want to move any major navigation equip-
ment to the other side of the room once everything is already
plugged in; that, we worry, could trigger a full-scale meltdown.
In reality, though, I believe the occasional meltdown occurs
randomly, just because the system is so complex.
Once the patient is asleep, we can't actually start the opera-
tion 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 o 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 sta-
bilized 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 navi-
gation, we'll tolerate a little wiggle. In an operation with navi-
gation, we can't a ord 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 reg-
ister the wand's location. Depending upon the patient's posi-
tioning, 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
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