The Sciences

The Inside Story

By NONE FOUNDApr 27, 2010 12:00 AM


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Once regarded as fundamentally enigmatic—a black box—the human brain ?now yields its secrets in Technicolor. Early brain scans produced grainy pictures that revealed only the most severe injuries or diseases. Today’s imaging technologies are far richer, depicting the brain’s inner workings in all their complexity.

Dense thickets of neural fibers course through the brain, connecting each part to many others in an intricate latticework. Diffusion spectrum imaging (DSI) can portray the full splendor of this network for the first time, says neuroscientist and DSI inventor Van Wedeen of Massachusetts General Hospital.

DSI—like its predecessor, diffusion tensor imaging (DTI)—tracks the movement of water molecules as they slide down axons, the long fibers that carry impulses away from one neuron and toward the next. Each point, or “voxel,” in the brain has neural fibers running in and out in three dimensions. DTI could visualize only one? direction at a time, but DSI maps all the routes simultaneously, as seen here. Each line represents thousands of neural fibers. DSI can also tease apart the confusing jumble of connections that appear at neural crossroads; such images are crucial for unraveling how signals travel through the brain.

The colors in this image trace this kaleidoscopic crisscrossing and show the directions, in three dimensions, in which brain signals travel. Red reflects left to right connections linking the two hemispheres; green indicates front to back; blue refers to up and down, including links to the spinal cord. Wedeen says DSI is so new that most of his studies pose simple scientific questions about how the brain works. But once the technique is ready to be used on patients, it could help study conditions like autism, which could be a “disease of connectedness,” he says.

Suffering from headaches, nausea, and left-side facial weakness, an 8-year-old girl underwent neuroimaging at Stanford University in May 2009. Her diagnosis: a glioblastoma multiforme, the most common type of brain tumor in adults?. These images, which depict the long fibers that cross the brain, helped surgeons to plan the operation by revealing brain areas that had to be carefully avoided. “If certain fibers are cut, you may not be able to move your hands or feet,” says Stanford neuroradiologist Kristen Yeom, who imaged the girl. Viewing the brain from different perspectives, as seen here, allowed surgeons to remove the maximum amount of tumor while doing the minimum amount of damage.

This DTI image appears as if it were a photo taken from below. The girl’s eyes would be at the top, and her right hemisphere is on the left. Each color represents the direction of flow of a tract, or a bundle of nerve fibers. The long green tract running from front to back in the left hemisphere (right side) is thought to be involved in visual recognition. The brighter green pathway intersecting it may figure in semantic function. Normally the two hemispheres are mirror images, but the tumor severely distorts the tracts in her right hemisphere (left side). With the tumor so close to these fiber bundles, the Stanford team decided not to remove the entire mass. “Sometimes surgeons want to take out the tumor—and more—to avoid regrowth, in case there are some microscopic cells left behind, but they don’t know how much they can safely cut off around it,” Yeom says. “These maps give surgeons more confidence going into the procedure.”

Here, a DTI scan is superimposed atop magnetic resonance imaging (MRI) scans to highlight certain brain structures. Orange marks the corticospinal tract, which runs from the cortex to the spinal cord, allowing nerve impulses to trigger body movement.

From this perspective, the girl’s right hemisphere appears on the right, and the tumor can clearly be seen displacing and compressing a tract. Doctors worry that such distortion of a tract can cause functional disability. Several days after imaging, the patient underwent brain surgery; using the images as a guide, surgeons safely removed most of her tumor. Following weeks of chemotherapy, she is now doing well.

Vessel damage in the neck can spawn blood clots that travel to the brain, and in fact this patient suffered a stroke when such a clot lodged in her delicate brain vessels. For this image, radiologist Anders Persson at Linköping University in Sweden digitally processed computed tomography (CT) scans produced using two X-ray sources and two detectors. By tuning the X-rays to two distinct energy levels, Persson created 3-D imaging that can reveal subtle differences in soft tissues like muscles and arteries. This technique allows doctors to conduct “virtual autopsies,” adding or removing bone and tissue layers as needed. The views here are taken from one of these interactive exams. “You can rotate and zoom the images, manipulating huge amounts of data,” Persson says. The images helped the patient’s doctor decide where to implant a stent to protect against another stroke.

The same CT technology used to diagnose the stroke in the? patient seen on the opposite page was applied to this murder victim. The image, assembled from thousands of X-ray images, shows that it was not the knife through his skull that did this man in. The knife passed just under an eye and through the soft tissue of the cheek, missing the brain and major blood vessels. This virtual forensic autopsy helped guide the murder investigation, Persson says. The middle-aged man also had stab wounds in the neck and chest. Police initially thought the knife to his head killed him, but “we realized they needed to look for the person who stabbed him through the heart,” he says. Because it is difficult to stab a live person at this angle, it is likely that the head injury happened after death. Persson and colleagues have performed more than 350 such virtual autopsies.additional reporting by andrew moseman

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