The New Awake Brain Mapping|
An Advance in Imaging Helps Surgeons Find the Safest Route to a Tumor
Operating on a brain tumor is a delicate business. Surgeons want to remove cancerous material without harming nearby tissue that controls vital functions such as vision, speech and muscle movement. One way to do that is to keep the patient conscious (though sedated) and stimulate the exposed brain during the surgical procedure.
This isn't new – it's been done for half a century. But it can now be done at Westchester Medical Center in a dramatically safer, noninvasive way, thanks to a pair of technologies: diffusion tensor imaging (DTI) and functional magnetic resonance imaging (fMRI).
The technologies work together to create a map showing the surgeon's safest path to the tumor, according to P. Charles Garell, M.D., Director of Functional Neurosurgery, whose team does about 15 operations every year that involve awake brain mapping.
"DTI has been studied for more than a decade," says neuroradiologist Hasit Mehta, M.D. "But it has only recently been applied to general clinical use."
Successfully navigating challenging procedures
The new tools were employed recently for a Yonkers woman named Taledia Hairston, who had already been through surgery, chemotherapy and radiation treatments for her lung cancer. This spring, she experienced difficulty in speaking, right arm weakness and a muscle droop on the right side of her face. A conventional MRI revealed a mass fairly deep within the left side of her brain, near centers that control speech, as well as arm and facial movement.
"We couldn't just go in and take the tumor out," says Dr. Garell, "because cutting right through might have left her with permanent deficits in speech and motor function." What he needed was a way to navigate around these critical areas to the tumor. That's where DTI came in.
As Dr. Mehta explains, DTI works by measuring the motion of water molecules, which are constantly on the move, spreading out and diffusing in different ways depending on the structures around them. Water in human tissues with a large number of fibers – such as skeletal muscle, cardiac muscle and brain tissue – diffuses fastest in the directions the fibers are pointing in, and slowest at right angles to it. In contrast, water diffuses in a spherical pattern in tissues that contain few fibers.
DTI thus can help locate fibers that carry important information, as well as more "empty" areas through which it is safer to cut. "It lets us see where the important tracks are in the brain, and where they may be infiltrated or distorted by the tumor," says Dr. Mehta. "With DTI the surgeon can see things he or she otherwise can't – the deep pathways below the surface of the brain."
Brain mapping in minutes
The DTI scan takes just five to 10 minutes, and is followed by the fMRI, which takes another 15 to 20 minutes. This scan, not quite as new as DTI but still of relatively recent origin, maps the changes in the brain related to the patient's activity.
"The machine scans the entire brain while I ask the patient to do certain things, such as speak or open and close a hand," says Dr. Mehta. "From small changes in the magnetic signal I can make a color map of the brain and show the surgeon where the hand or speech control is."
"We can follow all the important connections of the motor cortex with this map," says Dr. Garell. "We used it to map out an anatomical track to get me to the tumor."
Hairston had her head shaved and marked with incision locations. "All the while I was having a conversation with her," Dr. Garell says. "I'd seen her several times before, and we'd developed a rapport, but this was a stressful moment. It was important to reassure her."
Her head was immobilized with a clamp, and the surgeon applied a strong topical anesthetic to the scalp. He then opened the skin, talking to Hairston at all times. "I tell patients the next part is like having the dentist drill a tooth," says Dr. Garell. "You feel vibrations and hear the clanking of instruments, but that's normal."
The surgeon next removed a palm-size piece of skull, and then opened the leathery brain covering called the dura. Now the brain was exposed. Armed with his map, Dr. Garell could find the pathway to the tumor.
First, though, he confirmed what the DTI and fMRI predicted. He asked the patient to talk or open and close a hand, then placed a small electrical charge in the area that he thought controlled that function.
"I was looking for areas where the stimulation made no perceptible change in activity," he says. That told him it was safe to cut there. If the speech slurred or the hand contracted, he knew to avoid that area.
He put little 5-millimeter square tags of sterilized paper on the brain to mark the areas related to different functions. (They're peeled away when the path to the tumor has been identified.) Next, an ultrasound located the tumor several centimeters under the brain's surface.
"Then it was relatively simple to find the corridor to get to the tumor," Dr. Garell says.
Once the corridor was confirmed, the anesthesiologist fully sedated the patient for the tumor removal. Dr. Garell sent an instrument into the tumor that released ultrasound waves, which broke it up. He then aspirated – sucked out – the pieces. He sewed up the dura, replaced the skull piece and sent the patient to the ICU. Hairston spent one day there, and was discharged a few days later.
"I was surprised at how quickly she recovered," says Dr. Garell. "By the next morning she was wide awake and eating eggs."
And she had full movement in her right hand to do so, thanks to the DTI and fMRI scans. "Awake brain mapping told us to shift to a longer, less direct route," says the doctor, "so we were able to spare her any impairment of hand movement or other vital functions."