For patients with brain tumors, surgical resection is often the
ideal first treatment; in some tumor types, such as meningiomas,
surgery may be curative. However, the proximity of vital brain structures
may limit the ideal goal of complete tumor removal with preservation of
function. Surgery may not be offered to patients who might benefit
from it on the assumption that their tumor is too close to so-called "eloquent"
brain, such as the areas responsible for controlling movement or speech.
Recent advances in imaging techniques allow for nonivasive brain
"mapping", by which the precise relationship of areas controlling brain
function to a nearby tumor can be determined. One such method is
functional MRI, or fMRI.
How can functional image-guided surgery be done?
When an fMRI scan is done, the patient is asked to perform certain
repetitive tasks, or "paradigms", such as finger tapping, reading a word
list, or even to think about certain types of objects. Areas of the
brain that control these functions will show "increased activation" on
the scan, which can be turned into an image showing the anatomical location
of interest. This functional scan is then "registered" or combined
directly with a conventional MRI scan, in which an injection of a contrast
medium is used to show the outline of a tumor.
Next, this combined scan is transferred to a surgical navigation
computer in the operating room. Using this device, neurosurgeons
routinely can guide incisions, skull openings, and brain tumor removal
by the use of a special pointer whose position on a patient’s head is matched
to the corresponding point on an MRI or CT scan. With functional
image-guided surgery, not only the location of the tumor can be noted,
but that of critical brain areas as well.
How accurate is it?
We have compared the predicted location of motor cortex (the part
of the brain where movement on the opposite side of the body is initiated)
with conventional techniques of brain mapping, wherein at the time of surgery
a grid is placed on the brain, and sensory stimulation on the opposite
arm or leg used to identify the location of the motor cortex. In
some cases the motor cortex was mapped by direct stimulation with a weak
electric current.
In every patient we have found the location of the motor cortex
to be predicted accurately by functional image guidance. Maximal
tumor resection was achieved, and no new neurological deficit resulted.
What are the advantages of this type of surgery?
Functional image guidance can give the neurosurgeon increased
confidence that total tumor removal may be accomplished without giving
a patient new neurological deficits, such as weakness on the opposite side
of the body or difficulty speaking. In some cases, it may prevent
inappropriately aggressive surgery that may injure a patient.
While the conventional methods of brain mapping described above
are widely available, functional image guidance with fMRI allows for accurate,
noninvasive preoperative assessment and planning for brain tumor surgery.
Who can benefit from functional image-guided surgery?
Any patient who has a tumor near a critical area of cerebral cortex,
especially areas involved in controlling movement, sensation, speech, or
vision. There are no additional risks, and no added inconvenience
except for the relatively minor one of the functional MRI scan, which takes
less than one hour to perform.
We have also begun to use functional image-guidance for patients
undergoing stereotactic radiosurgery. This allows for the mapping
of critical cortex on the radiosurgery planning computer, so that the delivery
of potentially dangerous radiation doses to eloquent brain will be avoided.
Patient no.1
This 35-year-old man had been diagnosed as having a low-grade
glioma in his right motor cortex. He was treated with radiation therapy,
and told that surgery would leave him paralyzed on left side. Functional
image-guided surgery demonstrated that his tumor was actually in front
of the motor cortex. After exposing the brain at surgery, the stereotactic
probe demonstrated the location of the motor cortex, as seen on the surgical
navigation computer (figures 1 and 2). The tumor was removed, as
seen on postoperative MRI (figure 3); the patient is neurologically intact.
