Diffusion tensor imaging (DTI), which reveals the location of the delicate neural fibers that transmit signals in the brain, is providing insight into how the living brain shares information.
While magnetic resonance imaging (MRI) produces vivid pictures of large brain structures and functional magnetic resonance imaging (fMRI) reveals which areas are most active at a given moment, DTI reveals how the structures of the living brain communicate, a process that produces what we experience as consciousness.
“DTI is the best way we have so far to look at the wiring of the brain—at the connections among neurons—noninvasively,” says Gregory Sorensen, an associate professor of radiology at Harvard Medical School.
DTI works by monitoring the diffusion of water in the brain. Normally, water molecules diffuse randomly. When they do not, they must be bumping into something, such as the myelin-coated axons that transmit signals in the brain and spinal cord. By monitoring this nonrandom movement, DTI infers the location of those fibers and manufactures vivid images that depict their path.
From Strokes to Schizophrenia
By creating roadmaps of the brain’s information highways, DTI provides insight about strokes, tumors, and traumatic brain injury.
“DTI can show the effect of a stroke much sooner than other types of nuclear magnetic resonance imaging,” says Van Wedeen, also an associate professor of radiology at Harvard. “It can show which tissue is likely to die without prompt intervention. It provides a marker for how bad the stroke is, and it’s a hugely valuable guide to tracking the progression of stroke.”
For brain tumors, neurosurgeons often use MRI to map the patient’s brain so they can avoid cutting into vital structures, but soon they may use DTI as well to locate axons that must not be cut during surgery.
DTI also provides insights into psychiatric problems such as schizophrenia, which may be caused by faulty neural transmissions rather than conspicuous structural abnormalities. For example, Kelvin O. Lim, a psychiatry professor at the University of Minnesota Medical School, has traced the memory and cognitive problems of schizophrenia to axons near the hippocampus, a brain structure crucial for the creation of short-term memories.
“Standard anatomical MRI provides us with measures of macrostructure, the volume of anatomical structures or tissue in the brain,” Lim says. “DTI provides us with information about tissue microstructure, the organization of the tissue. [It also] provides us with information about the anatomical connectivity of the brain—the axons in the white matter.”
Insights and Limits
By providing insights into brain damage and disorders such as epilepsy and dementia, DTI can provide clues to recovery, Sorensen says.
“What we’d really like to understand is how to help brains heal,” he says. “Christopher Reeve showed that with enough effort, the brain can recover from tremendous insults, but we don’t know how he did it. What happened in his spinal cord and in his brain? What drugs or therapeutic approaches could we develop to enhance that?”
Recently DTI provided startling evidence that damaged neurons can regenerate to some extent. Nineteen years after suffering a severe head injury that left him in a minimally conscious state, Terry Wallis suddenly started speaking again at age 39. Scientists at Weill Medical College of Cornell University used DTI scans to demonstrate that axons in Wallis’s brain apparently had formed connections that restored certain abilities, including speech.
“We think Terry’s brain may have sought out new pathways to areas involved in speech and motor control,” says Nicholas Schiff, assistant professor of neurology and neuroscience at Weill, who investigated the cause of the man’s dramatic improvement [with funding from the Dana Foundation].
DTI still has significant limitations. Its resolution is relatively low, and axons that cross may appear as a single axon.
“Right now we can see only the big highways in the brain, but we want to see every street,” says Denis Le Bihan, who introduced diffusion magnetic resonance imaging in 1986 and is a leader in DTI development. “Even more important is to see information flowing on those streets.”
When DTI starts to provide that information, human consciousness itself may start to come into view, says Le Bihan, director of France’s NeuroSpin (a facility dedicated to ultra-high-field MRI) and a member of the French Academy of Sciences.
“We can use functional MRI to demonstrate which brain regions are activated by certain stimuli,” he says. “The question is, how are those different regions connected? Those regions exchange the information that makes you aware of what is going on. That’s why it’s so crucial to understand those connections, and the order in which brain regions are activated.”