University Of Michigan team uses MRI scans to help surgeons avoid crucial "white matter" links, because the brain is a very complicated place.
From the outside, it may look like a gray lump of tissue, covered with ridges and bumps. But inside, an incredibly complex network of thread-like white fibers carries signals back and forth between areas of the brain and the spinal cord. Each fiber is crucial to a particular aspect of how our mind communicates with our body.
But until now, brain surgeons haven't been able to see those fibers, called white-matter tracts. Invisible to the naked eye, and impossible to see on normal brain scans, they fall victim to even the most careful surgeon's hand during operations to remove tumors or calm severe epilepsy. And the result can be permanent, unintended damage to the senses, movement, or thinking ability.
Now, advanced medical imaging is making it possible for surgeons to know where those tracts are – and even to see them in their field of vision while they operate.
A University of Michigan Health System team is one of the first in the world to offer this type of image-guided surgery. Already, it has helped them plan the operations of patients, and reduce their risk of damage. And the technique holds great promise for other uses as it is developed further.
"In the past, we had a pretty good idea of what different parts of the brain do, but we've never been able to see the direct connections from one part of the brain to another, or from one part of the brain to the spinal cord," says one of the team's leaders, Suresh Mukherji, M.D. "We can see those connections now, by looking at the sub-cellular level to see how the water molecules in the tissue move."
Mukherji directs the U-M Division of Neuroradiology – a team of brain-imaging specialists. They work closely with U-M neurosurgeons, who perform thousands of brain and spine operations a year, and with neurologists who diagnose and treat brain and nerve disorders ranging from epilepsy and multiple sclerosis to cancer.
Neurosurgeon Oren Sagher, M.D., says this teamwork makes it possible for him to operate with the best possible information about each patient's brain. "Thoroughly imaging the brain is one of the keys to successful brain surgery. We have to be able to see all the structures that we're going after, and all the structures that are in our way and need to be avoided," he says.
That's what the new imaging technique, called tractography, makes possible for the first time.
The technique relies on powerful MRI (magnetic resonance imaging) scanners, which create images of patients' brains, one thin slice at a time. U-M has several extremely powerful MRI machines, call 3T scanners.
Then, ultra-fast computers equipped with special software compile all of those slices into a three-dimensional image of the brain. Lastly, the neuroimaging team uses special techniques to see how water molecules are moving inside every area of that virtual 3-D brain.
It's that water-movement imaging that allows the white-matter tracts to come into view. Inside the tracts, water can only move in a lengthwise direction, back and forth along the length of the thin strand. But in the rest of the brain tissue, the water can move around more freely.
In fact, U-M brain imaging specialists already use this kind of information to tell them if cancer cells are dying in response to chemotherapy or radiation, because water can move faster in dead or dying areas of cancer tissue than it can in healthy brain cells.
In tractography imaging, Mukherji explains, "How the water molecules are oriented, how they move, is something we can now detect, and as a result we can now see parts of the brain that we were never able to see before."
Add that together with the ability to create 3-D maps of the brain, and the result is spectacular images that just show the entire network of white-matter tracts and the individual nerve fibers that they're made of.
These images become a roadmap for surgeons like Sagher, especially when they're superimposed on other images that show the specific areas of "gray matter" where epileptic seizures begin or where tumors lurk.
This cross-registration, as it is called, fuses the information about the areas of the brain that the surgeon needs to remove or destroy in order to treat the patient's condition, with the information about what that the surgeon needs to avoid in order to preserve a patient's vision, for instance, or her ability to move her right arm.
Surgeons like Sagher can use these images in the operating room, and even have them fed digitally into the special eyepiece that they use to perform surgery on very tiny areas of the brain.
"The computers can decipher the direction of the fibers, and then assign a color to them so we know that this group of fibers belongs to this tract, which has this function by virtue of where it is," explains Sagher, who directs the U-M Image-Guided Surgery Program. "It essentially makes the invisible visible."
He contrasts this kind of imaging with what was used even a decade ago. Back then, the neurosurgeon might be able to have a series of CT (computed tomography) scans showing the structure of the brain in individual slices. These transparent films would be hung on a lightbox in the operating room, and give the surgeon a sense of what to expect when he or she cut into the patient's head.
Since that time, scanners have gotten better and the computers needed to create 3-D images have gotten more powerful, which has allowed surgeons to see the gray matter better and allow the computer to guide their hands to some extent. But only tractography allows them to see the white-matter tracts.
Now, he and his colleagues use the images to plan their operations ahead of time – for instance, when operating on a patient with epilepsy who has decided to have surgery after medicines have failed to control their seizures.
If the origin of the seizures is in a part of the brain that controls memory or learning, for example, the tracts that lead in and out of that area are key connectors that, if cut, can change the patient's life forever. And if a tumor is deep within the brain, the operation needed to get to it can cut across many important areas. But the tractography images can let the surgeon see areas where there aren't any tracts, and plan the route he or she will take to get to the area of the problem.
Mukherji, Sagher and their colleagues predict that tractography will change brain surgery and the way we see the brain's function forever, just like the first CT and MRI scans of the brain changed the diagnosis and treatment of many disorders. The team is pursuing research to improve the technique and show how it can best be used – and how it helps spare patients from unintended consequences.
"Instead of imaging the brain, we're essentially able to image the mind," says Mukherji. "We're able to image how a person's thoughts and brain impulses travel, and this is just the beginning."
