More Precise Imaging Technology Lets Surgeons Peer Into Pediatric Brains
Every year, about 2,200 children and adolescents in the United States are diagnosed with a brain tumor. Some of them are malignant — the kind that spread aggressively and are often fatal. But even benign brain tumors are dangerous because of where they are located. They can still be fatal, if they continue to grow or if they put pressure on vital structures.
In many cases, benign brain tumors, such as juvenile pilocytic astrocytomas — a relatively common brain tumor in children — can be easily removed surgically, when they grow on the surface of the brain. But sometimes they develop deep within the brain’s vital structures, and in those cases, pediatric neurosurgeons have been anxious about operating on them. If they don’t, the tumors can continue to grow and do damage — but if they do operate, surgeons must navigate an uncertain path through the complex microstructures of the brain, doing their best not to damage important nerve centers such as those that control a child’s ability to move and speak.
But now, a new advance in magnetic resonance imaging (MRI) is allowing pediatric neurosurgeons to fine-tune their map of motor pathways inside the brain and, using stereotactic navigation, plan a surgical route that avoids these key regions. Stereotactic technology provides three-dimensional navigation during surgery — in much the same way a GPS system does for a car — to precisely direct the tip of a delicate instrument (such as a needle or beam of radiation) using coordinates provided by medical imaging to reach a very specific point.
Standard MRI can show radiologists things like the volume of certain structures in the brain. For example, when a person has Alzheimer’s disease, standard MRI may show a decline in the overall volume of the hippocampus, an area of the brain that plays a critical role in memory. But what standard MRI can’t do is show more detailed images of the brain’s microstructures, like its neurons and axons, their pathways, and damage that has been done to them.
That’s where DTI, or Diffusion Tensor Imaging, comes in. “When you superimpose DTI imaging on top of stereotactic imaging, you get almost real-time feedback,” said Dr. Jeffrey Wisoff, director of pediatric neurosurgery at New York University’s Langone Medical Center. So far his team has treated six children with brain tumors this way, all successfully. What exactly is DTI? It’s best explained by first looking at what traditional MRI can do.
“MRI in general looks at water,” explained Joseph Helpern, a professor of radiology, psychiatry, physiology and neuroscience at NYU. “The beautiful clinical images provided by MRI depict the biophysical parameters of water in the human brain. MRI contrast is based on how fast the water moves: the brighter the image, the slower the water is moving. The darker the image, the faster the water is moving.”
But it’s not always enough to look at just how much water there is, or how fast it’s moving. Another key parameter is called diffusion — the random process by which water molecules spread out and ultimately form a pattern. When water molecules diffuse in the brain, their pattern changes if they bump into different things — like tiny structures such as neurons and axons. Diffusion imaging allows scientists to ask important questions about the microstructure of the brain.
Why is that important? “Microstructure is very closely associated with function,” said Helpern. “If you understand the micro-architecture of the tissue, you can understand if the tissue is functioning properly. If you don’t have any structure, it’s just water moving freely — then there’s no function there. The more complex the structure, and the more intricate, probably the more sophisticated it is.”
DTI’s insights into the brain’s function — and dysfunction — are now being used to study a wide range of conditions, such as Alzheimer’s disease and other dementias, multiple sclerosis, schizophrenia, autism and even spinal cord injury. Ultimately, researchers hope they will help achieve new insights that, for example, may allow for the diagnosis of dementia at its earliest possible stages, before irreparable damage has been done to the brain’s neurons.
But that’s in the future. Right now, for some children with brain tumors, DTI is already changing their lives for the better. After the success of the first set of cases using DTI to guide stereotactic surgery for pediatric brain tumors, Dr. Wisoff is moving forward. The first cases specifically involved mapping and avoiding motor pathways. Next, he plans to use DTI to help map surgical routes around other pathways, such as sensory and visual processing centers. Ultimately, he hopes to apply DTI imaging to surgeries for malignant pediatric brain tumors, such as gliomas.
“In adults, gliomas are almost universally fatal,” Wisoff said. “But in children, you get about a 25 percent disease-free survival rate if you can achieve a near-total resection of the malignant glioma, then treat the rest with radiation and chemotherapy. But we’ve been concerned about doing more harm than good getting there. DTI may allow us to attack these tumors more aggressively.”
About the Author
Gina Shaw is the medical writer for The Washington Diplomat.